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US7020165B2 - Apparatus and method for trellis encoding data for transmission in digital data transmission systems - Google PatentsApparatus and method for trellis encoding data for transmission in digital data transmission systems Download PDF
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Divisional of prior application Ser. No. 08/895,612 filed Jul. 16, 1997 now U.S. Pat. No. 6,307,868.
This application is a continuation-in-part application of U.S. patent application Ser. No. 08/684,243, filed Jul. 19, 1996, now U.S. Pat. No. 6,356,555, issued Mar. 12, 2002, invented by Shlomo Rakib and Yehuda Azenkot which was a continuation-in-part application of U.S. patent application Ser. No. 08/588,650, filed Jan. 19, 1996, now U.S. Pat. No. 5,793,759, issued Aug. 11, 1998, invented by Shlomo Rakib and Yehuda Azenkot, which was a continuation-in-part application of U.S. patent application Ser. No. 08/519,630, filed Aug. 25, 1995, invented by Shiomo Rakib and Yehuda Azenkot, now U.S. Pat. No. 5,768,269, issued Jun. 16, 1998, the contents of all of which are hereby incorporated by reference. This application is also a continuation-in-part application of a U.S. patent application entitled LOWER OVERHEAD METHOD FOR DATATRANSMISSION USING ATM AND SCDMA OVER HYBRID FIBER COAX CABLE PLANT, Ser. No. 08/760,412, filed Dec. 4, 1996, now U.S. Pat. No. 5,991,308, issued Nov. 23, 1999, invented by Amir Fuhrmann, Shlomo Rakib and Yehuda Azenkot, the entirety of which is hereby incorporated by reference.
The invention pertains to the field of bidirectional passband digital communication systems, and, more particularly to the field of improvements in head end or central office modems to remove the phase locked loops therefrom.
Digital data communication systems are well known in the art. Many treatises are available that describe them. Among these treatises are: Dixon, "Spread Spectrum Systems with Commercial Applications", Third Edition, 1994 (Wiley & Sons, New York) ISBN 0 471 59342-7; Stallings "Data and Computer Communications", 4th Ed. 1994 (Macmillan Publishing Co., New York) ISBN0-02-415441-5; Lee and Messerschmit, "Digital Communication, 2d Ed.", 1994 (Kluwer Academic Publishers, Boston), ISBN 0 7923 9391 0; Haykin, "Communication Systems" Third Edition 1994 (Wiley & Sons) ISBN 0 471 57176-8; Elliott, Handbook of Digital Signal Processing: Engineering Applications, (Academic Press, Inc. San Diego, 1987), ISBN 0-12-237075-9, all of which are hereby incorporated by reference. Generally, the problem which the invention is an attempt to solve is how to get rid of as many continuous tracking loops as possible in a bidirectional digital data communication system. The reasoning for this can be understood from the following discussion.
Digital data distributed communication systems can be baseband systems or passband systems. In baseband systems, the transmission media has the capability of transmitting digital pulses between widely separated transmitter and receiver locations. Passband systems require that the digital data be modulated onto a carrier frequency for transmission over the media.
Receivers for digital data passband systems can be either coherent or noncoherent. In coherent systems, the receiver has a local oscillator, usually taking the form of a phase locked loop (PLL) which is part of a continuous tracking loop and is maintained in constant phase lock with the phase and frequency of the carrier on which the received data is modulated. Coherent systems can make use of modulation schemes which alter either the phase, frequency or amplitude or any combination thereof of the carrier in accordance with the information content of the digital data to be transmitted. Incoherent systems do not have the local oscillator at the receiver phase locked to the carrier phase and frequency. In these systems, the designers have chosen to ignore the phase of the received signal at the expense of some degradation of the system performance and throughput.
Coherent systems can utilize binary or M-ary amplitude shift keying (ASK), phase shift keying (PSK) or frequency shift keying (FSK), as well as M-ary amplitude phase keying (APK) of which QAM (quadrature amplitude modulation) is a special case. Incoherent systems are limited to binary or M-ary ASK, FSK or differential phase shift keying (DPSK).
Coherent systems are higher performance systems because they have an additional degree of freedom for use in the modulation scheme which means more complex constellations of symbol sets can be used and more data bits can be encoded in each symbol in the constellation. This translates to greater throughput.
However, coherent systems are more complex since they require additional tracking loop circuitry at the receiver to recover the transmitted carrier and use the information so derived to steer the local oscillator so as to maintain its phase and frequency locked to the phase and frequency of the carrier. Usually the local oscillator being steered in the receiver is a PLL or has a voltage controlled oscillator negative feedback system in it (which is at the heart of almost every tracking loop). Carrier synchronization has been achieved by any one of a number of different ways in the prior art including use of PLLs where the carrier is not suppressed or Mth power tracking loops or Costas tracking Loops where the carrier is suppressed. Mth power and Costas tracking loops also contain voltage controlled oscillators as part of the tracking loop. The problem is that PLLs and negative feedback voltage controlled oscillators in tracking loops can and often do lose lock especially where there is rapid change in phase or frequency caused by conditions in the transmission media. When a PLL or other tracking loop loses lock, the system goes out of synchronization and fails to communicate data—its sole purpose in life.
All digital data communication also requires clock synchronization in the receiver to the clock in the transmitter because data is sent during discrete times. These discrete times are variously called chip times, bit times or symbol times in the prior art references. The importance of synchronization of the clock in the receiver to the clock in the transmitter is that in all forms of modulation, the amplitude, phase or frequency of the carrier (or some combination of these) must be sampled during each chip time so as to determine which symbol in the alphabet or code set in use was transmitted during that chip time based upon the phase, amplitude or frequency characteristics of the carrier during that chip time.
Receiver clock synchronization can be done on either a long term basis or a short term basis. Short term clock synchronization is called, amazingly enough, asynchronous transmission, but in fact the receiver clock is periodically synchronized to the transmitter clock at the beginning of transmission of each "character". A character is a collection of 5 to 8 symbols which are transmitted over a very short time (usually the symbols or binary bits that only have two states). The receiver clock resynchronizes during each character at the beginning thereof and need not resynchronize until the next character starts. Asynchronous transmission is cheap and less complex since timing synchronization problems caused by transmission of long uninterrupted streams of bits is avoided by sending the bits one character at a time and requiring synchronization between the receiver clock and transmitter clock only during that character.
The problem with asynchronous transmission is the high overhead. Each character of 5 to 8 bits must include a start bit, 1 or 2 stop bits and a parity bit. The start bit is used by the receiver to resynchronize its clock. The overhead of 2–3 bits per character of 5–8 bits makes asynchronous transmission inefficient to transmit large volumes of data. Asynchronous transmission can be extended to sending several characters grouped together with a preamble which is long enough for the receiver to synchronize to transmitted before every group of characters and a tracking loop to maintain the receiver clock in synchronization with the transmitter clock during the transmission of the group of characters. The concepts of the invention are applicable to asynchronous transmission where there is a tracking loop in the remote unit receiver but no tracking loop in the central unit receiver and only a periodic or occasional phase adjustment of the master clock and master carrier phase for use by the central unit receiver.
Synchronous transmission is a more efficient way of transmitting data since blocks of symbols or bits can be transmitted without start and stop codes. Sampling by the receiver during the middle of each bit or chip time is accomplished by keeping the receiver clock in synchronization with the transmitter clock. This maintenance of clock synchronization has been done in the prior art in many different ways. For example, a separate clock line can connect the transmitter and receiver, but this is impractical in many situations. A way of avoiding this is to embed the clock information in the data signal transmitted from the transmitter and recover the clock in the receiver.
Clock recovery has been done in a number of different ways in the prior art including transmitting the clock along with the data bearing signal in multiplexed form and using appropriate filtering of the modulated waveform to extract the clock. Another method is to use a noncoherent detector to first extract the clock and then processing the noncoherent detector output to recover the carrier. Where clock recovery follows carrier recovery, the clock is recovered from demodulated baseband signals. The early-late gate symbol synchronizer has also been used in the prior art to synchronize the receiver clock to the transmitter clock. This type clock recovery takes advantage of the fact that a matched filter output of a filter matched to a rectangular clock pulse is a triangular waveform which can be sampled early before the peak and late after the peak. By changing the timing of the sampling until the early and late samples have equal amplitude, the peak of the matched filter output signal can be found, and this will have a fixed phase relationship to the clock phase. This information is then used to steer a voltage controlled oscillator in a negative feedback system. Again, complicated circuitry centered around a voltage controlled oscillator is needed to recover the clock.
A technique called remote loopback or remote clock has been used in the prior art on, for example T1 type digital data communication phone lines. This technique is similar to the aspect of the invention involving having the remote unit local clock synchronized to the central unit master clock and using that local clock at the remote unit receiver for the remote unit transmitter. It is also similar to the aspect of the invention of using the central unit master clock, after adjustment in phase to synchronize it to the phase of the received clock from the remote unit transmitter, as the clock signal from the central unit receiver.
Since PLLs and tracking loops are not always reliable, and add complication and expense to receivers, it is desirable to be able to get rid of them wherever possible. Thus, a need has arisen for a bidirectional digital communication system where continuous tracking loops in the central unit receiver (or the receiver in the unit having the transmitter which transmits with the master clock and master carrier signals) have been eliminated.
A bidirectional digital data communication system according to the teachings of the invention will have: a central unit transmitter with any encoder to receive downstream data, encode it and drive any type of digital passband modulator with the encoder receiving a master clock signal from a master clock oscillator and the modulator receiving a master carrier oscillator; a remote unit receiver which has any compatible detector which receives a local carrier reference signal which is synchronized in frequency and phase to the master carrier signal and which is generated by any form of carrier recovery circuit, with the detector driving a decoder to decode the received data and output it, with the decoder receiving a local clock signal which has been synchronized with the transmitter master clock signal by any clock recovery circuit; a remote unit transmitter having any encoder type for receiving upstream data, encoding it and driving any digital passband modulator, the encoder receiving the local clock reference generated by the remote unit receiver clock recovery circuit and the modulator receiving the local carrier reference signal generated by the remote unit carrier recovery circuit; and a central unit receiver with any compatible coherent detector to detect the signal transmitted from the remote unit transmitter, with the central unit detector using the central unit master carrier from the master carrier oscillator in the transmitter but adjusted in phase to account for propagation delay from the remote unit, and with the decoder using the master clock signal from the central unit transmitter master clock oscillator but adjusted in phase for the propagation delay from the remote unit to the central unit. Thus, the central unit has no phase locked loops or other voltage controlled oscillator circuits for clock recovery or carrier recovery.
In the preferred embodiment, the master carrier and master clock are recovered in the RUs and used to transmit data upstream along with preamble data preceding payload data. The preamble data from each RU is used by the central unit transceiver to generate an amplitude and phase correction factor for that RU. The signals from that RU are then demodulated using the CU master carrier and demultiplexed and detected using the CU master clock. Phase and amplitude errors in the detection process caused by latency and channel impairments are eliminated or reduced by using the phase and amplitude correction factors developed for this RU from its preamble data. Thus, there is no need for continuous tracking loops in the CU receiver to recover the clock and carrier used by each RU to transmit its data. This single master carrier and master clock concept and the frame synchronization provided by ranging, and the improved throughput and lower error rates provided by the equalization and power alignment processes taught herein are useful in any form of bidirectional digital data distributed communication system regardless of the form of encoding, multiplexing or modulation used. Examples of the types of multiplexing that can be used in such systems are CDMA, TDMA, inverse Fourier, DMT or any other system where orthogonal signals are used to encode each separate channel of data from a source such as sine and cosine signals etc.
In the broadest embodiment of the invention involving no continuous tracking loops in the CU receiver to recover RU clock and carrier, the type of central unit transmitter and modulation scheme is not important nor is it important whether the central unit transmits a single channel of digital data downstream or multiplexes several channels. If the central unit transmitter is a multiplexing transmitter, the type of multiplexing is not important. Likewise, the type of detector used in the remote unit receiver is not important as long as it is compatible with the modulation scheme in use and it is not critical whether the central unit transmitter transmits the master carrier or suppresses it or transmits the master clock separate or embeds it in the data so long as the master clock and carrier phase information get transmitted somehow to the RUs such as embedded in the Barker code of the preferred embodiment. Likewise, the type of carrier recovery and clock recovery circuits used in the remote unit to synchronize the local clock and local carrier oscillators to the master clock and master carrier are not critical. Also, the type of decoder used in the remote unit receiver is not critical so long as it is compatible with the type of encoder used at the central unit transmitter. For the remote unit transmitter, any type of encoder and any type of modulator may be used for the upstream data, and the type of encoding and the type of multiplexing, if any, used for the upstream direction need not be the same as the downstream direction. The clock and carrier signals used by the remote unit transmitter are the same clock and carrier signals used by the remote unit receiver.
The central unit receiver can use any type of detector that is compatible with the modulation scheme used by the remote unit transmitter. Likewise, the type of decoder used in the central unit receiver is not critical so long as it is compatible with the remote unit transmitter encoder. The structure and operation of the central unit receiver phase detection and adjustment circuit is not critical to the invention. The only requirement on this circuit is that it be able to occasionally or periodically detect any phase differential between the central unit master carrier and the carrier used to transmit by the remote unit transmitter and detect any phase difference between the central unit master clock and the clock information used to transmit the received data. These phase differences are used by the central unit receiver to occasionally or periodically adjust the phase of the master clock and master carrier to match the phases of the carrier and clock signals used by the remote unit transmitter as received at the central unit receiver.
The invention is applicable to both asynchronous and synchronous methods of transmission, although synchronous transmission is much more efficient in terms of overhead consumed per unit of payload data delivered. Use of the invention in asynchronous transmission will be useful in asynchronous systems where tracking loops are used to maintain synchronization of the remote unit receiver local clock during transmission of one or more characters in a group.
In the preferred embodiment, the transmitters of the RU use synchronous code division multiplexing (SCDMA). SCDMA is defined as transmission of frames of spread spectrum signals with data from different channels spread using orthogonal pseudorandom spreading codes, said frames being synchronously transmitted from different RUs located at diverse locations such that all frames of corresponding frame number from all RUs arrive at the CU modem with their frame boundaries exactly aligned in time with the frame boundaries of the CU frame of the same frame number. The upstream data is then demultiplexed and decoding by the inverse code transformation that was used in the RU transmitter to spread the spectrum of the data using the orthogonal, pseudorandom spreading codes.
According to the most preferred embodiment, there is provided a code division multiplexing multiple access (CDMA) scheme using orthogonal codes to encode multiple channels of digital data for simultaneous transmission over a cable television media which is also carrying frequency division multiplexed cable television programming. Further, in this most preferred embodiment, alignment of multiple subscriber remote units at diverse locations on the cable television media to the same frame alignment reference is used to substantially reduce crosstalk between adjacent codes and allow multiple users to simultaneously share the same cable TV media for auxiliary services other than cable TV programming delivery. The ranging process described herein is useful for any digital communication system which delivers data from physically distributed transmitters to a central location in frames, but in the context of a CDMA system on a cable TV plant, it provides for synchronous CDMA which greatly increases system payload capacity. The use of synchronous CDMA coupled with frequency division multiplexing of upstream and downstream data on different frequencies than the cable TV programming provides a system whereby the entire bandwidth devoted to the digital auxiliary services may be simultaneously shared by multiple users who share a plurality of channels. Any of the known ways of achieving frame alignment may be used to achieve synchronous code division multiple access data transmission. In the preferred embodiment, frame alignment is achieved with the bulk of the processing done by the RUs with the CU only acting in a passive role as a sensor for deciding if a Barker code is in the gap, if there is more than one Barker code in the gap, asking for authentication and providing feedback for all of the above and for fine tuning processing to exactly center each RU's Barker code in the gap. This ranging process is done by alignment of ranging signals transmitted by remote units to guardbands or gaps between frames.
One inventive concept disclosed herein is to achieve high noise immunity by spreading the energy of the transmitted data out over time during transmission, and then compressing the energy again at the receiver to recover the data. Spreading the energy of the transmitted data out over time reduces susceptibility to burst errors and impulse noise. In addition to this spreading concept, the spectral efficiency of the system is enhanced by transmitting multiple separate channels of data over the same media without interference by using separate orthogonal codes to encode the data of each channel so that no interference results when all channels are simultaneously transmitted so long as proper frame alignment is maintained. In this way, the spectral efficiency, i.e., a measure of the amount of data that can be sent from one place to another over a given bandwidth, is enhanced without degradation of the data by crosstalk interference. The orthogonality of the codes used for each data stream, i.e., each channel or conversation, minimizes crosstalk between channels where the system is properly aligned, i.e., synchronized.
Using cyclic, orthogonal codes for SCDMA further enhances noise abatement by providing the ability to perform equalization using a subset of these codes. Equalization, as that term is used herein, refers to the process of determining the amount of crosstalk between adjacent codes resulting from minor errors of frame timing alignment and then generating phase and amplitude correction factors which can be used to negate the crosstalk. In the preferred embodiment, the orthogonal codes are cyclic codes.
In some species within the genus of the invention, code diversity is used to achieve further noise immunity. It has been found that some orthogonal codes are less immune to narrow band interference and other sources of noise than others. To avoid using such codes to spread the data from the same channel or timeslot all the time, code hopping is used in these preferred species of the inventive genus. Code diversity is achieved in several different ways, but, in the preferred embodiment, each transmitter uses a code shuffler circuit and each receiver uses a code deshuffler circuit. All shuffler and deshuffler circuits receive the same seed and generate the same sequence of pseudorandom numbers therefrom. These pseudorandom numbers are used to generate read pointers to a framer memory and write pointers to a buffer memory. The framer memory is where the information vectors or symbols are stored, and the read pointers generated by the shuffler circuits are used to read the timeslot data, i.e., symbol/information vector elements out in pseudorandom fashion and store them in a buffer in accordance with the write pointers generated by the code hopping shuffler circuit. The information vector elements thus stored in the buffer are used to do the matrix multiplication required by the code division multiplexing scheme. Alternatively, the symbol elements may be read out sequentially from the framer memory and stored pseudorandomly in the buffer.
The effect of this synchronous CDMA scheme is to "whiten" the noise sources such that no matter how complex the noise signals, the noise can be effectively managed using conventional error detection and correction bits in a forward error correction scheme. The digital data providing the interactive or bidirectional data communication is sent using a CDMA scheme, but for purposes of synchronization, the CDMA scheme is mixed with a TDMA scheme. More precisely, a guardband or gap which is free of data is added between frames of the CDMA signal. Digital data is transmitted in frames, each frame comprising 3 data symbols and a guardband. The guardband is used for non-data usage such as ranging, alignment and equalization.
The synchronous CDMA modulation scheme disclosed herein may be used with any shared transmission media and with any apparatus or method that can get all remote units synchronized to the frame timing of the central unit including the ranging/alignment scheme disclosed herein. Other possible methods of synchronizing to the same frame timing are for all remote units and the central unit to receive the same timing reference signals from some source such as internal atomic clocks or from an external source such as a Global Positioning System satellite from which all remote units and the central unit are effectively equidistant.
Likewise, the ranging/alignment scheme disclosed herein is useful for any other modulation scheme which transmits digital data in frames, requires frame synchronization and can insert a guardband between the frames.
Some species within the inventive genus use M-ary modulation code division multiplexing. Each remote unit receives a time division multiplexed stream of digital data. Each timeslot contains 9 bits of data. Each 9 bits is stored in a framer memory, and is divided into three tribits, each having 3 bits during readout of the memory. Each of the three symbols transmitted each frame is comprised of 144 of these tribits, one for each timeslot or channel. These tribits are encoded with a 4th bit prior to spreading by the code division multiplexing operation. The 4th bit is added by a Trellis forward error correction encoder to each tribit based upon the three bits of the tribit and based upon the previous state for this timeslot's data during the last frame. This 4th bit adds sufficient redundancy to enable a Viterbi Decoder in the central unit receiver to make a more error free determination of what data was actually sent in the presence of noise without requesting retransmission. The 4th bit also maps each tribit to a 16 point QAM (quadrature amplitude modulation) constellation by using the first two bits to represent the inphase or I axis amplitude and the last two bits to represent the quadrature or Q axis amplitude. Thus, M-ary modulation is used to achieve greater spectral efficiency.
With the system described herein, full 10 megabit/second traffic volume per each 6 MHz channel can be achieved in both the upstream and downstream direction over HFC. Unlike conventional CDMA, SCDMA transmission from transmitters like those disclosed herein stays within 6 MHz bands that do not interfere with or effect other adjacent channels. SCDMA has a number of other advantages over pure FDMA and TDMA systems in terms of capacity, scalability and bandwidth allocation. Standard IS-95 asynchronous Code Division Multiple Access spread spectrum systems are hindered by the capacity constraints of the 5–40 MHz upstream channel and the presence of a large amount of noise, and they often require 30 MHz wide channels which creates channel interference problems with neighboring services in the HFC spectrum. The biggest problem with asynchronous CDMA systems is self-generated noise because the RUs are not aligned with each other thereby losing orthogonality and creating a high degree of mutual interference. The higher self-generated noise raises the noise floor and reduces the capacity. SCDMA system insure that the RUs are in frame synchronization with each other and using orthogonal codes to minimize mutual interference as data is sent upstream. Preferably, SCDMA transmitters are also used to send data downstream. With the system described herein, multiple streams of digital data, each having a 64 kbps throughput can be simultaneously sent over a 6 MHz channel with a total 10 Mbps throughput. Each data stream is Trellis encoded, interleaved and spread over the entire 6 MHz using its own individual spreading code. Use of forward error correction and interleaving increases noise immunity to impulse noise, narrowband interference and Gaussian noise. The Trellis coding adds 4.8 dB coding gain, and interleaving enables withstanding long duration impulse noise of up to 100 microseconds without incurring errors. Use of spread spectrum technology adds another 22 dB processing gain. The combination of techniques yield a total 27 dB interference rejection allowing the system to operate in negative Carrier to Noise Plus Interference Ratio. The SCDMA transmitters are combined with TDMA payload data input streams which makes the system extremely scalable.
The high capacity of the SCDMA system disclosed herein is made possible by orthogonality which is made possible by the orthogonality of the spreading codes which is a result of the ranging process and the equalization process. The ranging process assures frame synchronization such that all codes arrive from distributed RUs arrive at the CU at the same time. The ranging process is carried out periodically to account for cable expansion/contraction with changing temperature, but the process is transparent to payload traffic in that it does not slow it down, stop it or cause errors. Re-ranging occurs upons certain error conditions and upon disconnect from the network and each powerup.
Equalization is achieved by measuring the channel response from each user to the CU and adjusting a precoder at the RU transmitter to "invert the channel", i.e., predistort the transmitted signal such that it arrives undistorted at the CU. Power alignment by each RU such that each RU transmission reaches the CU at approximately the same power level also helps to minimize mutual interference.
Dynamic bandwidth allocation allows as many 64 kbps streams or channels as necessary to be allocated to a particular service so that high demand applications such as video teleconferencing or high speed internet access can be supported simultaneously with low demand applications like telephony over the same HFC link. Bandwidth allocation is managed at the CU through an activity status table in each RU and the CU that indicates the status of each timeslot and code assignments. The CU updates the RU tables by downstream messages. Bandwidth can be guaranteed upon request while other services with more bursty traffic may contend for the remainder of the total 10 Mbps payload.
The advantages over TDMA systems include less need for fast acquisition and correspondingly lower sensitivity to narrowband interference. Further, below a certain SNR, TDMA systems may fail altogether. Contention for certain channels and contention affecting adjacent can cause amplifier overload in TDMA systems and can cause severe throughput and performance problems. FDMA systems where each user gets a narrow upstream frequency slice is very susceptible to narrowband noise which can wipe out an entire channel. FDMA systems often try to avoid this problem with frequency reallocation. This complicates and raises the cost of the system by requiring more intelligence. Throughput is also adversely affected as nothing is sent while frequencies are reallocated. Guardbands between channels waste bandwidth, and frequency misalignment degrades FDMA systems.
Any method or apparatus that uses these inventive concepts is within the teachings of the invention and is deemed to be equivalent to the apparatus and methods described herein.
Referring to
If the signal on line 14 from the message source 12 is analog, one of the functions of the encoder 16 is to sample and digitize it prior to assembly of the digital data into symbols or chips to be transmitted during each chip or bit time.
If message source 12, is digital, it emits one symbol every T seconds with the symbols belonging to an alphabet of M symbols comprised of symbols m1, m2 . . . . In binary systems, there are only two symbols, logic 1 and logic 0. In larger alphabets, each symbol may be represented by multiple bits so the output on line 14 is M-ary meaning there are M possible symbols in the alphabet, each of which can be made up of multiple bits. Generally, encoder 16 serves to produce a signal vector S made up of N elements with one such set for each of the M symbols in the source alphabet.
The signal vector S is passed on bus 18 to a passband modulator 20. The details of construction and operation of the modulator 20, like the details of the construction and operation of the encoder 16 are not critical to the invention, and they can be anything known in the prior art or later developed. The function of the passband modulator 20 is to construct a distinct signal si(t) of duration T for each symbol mi by modulating the master carrier signal during each time T using the bits of the symbol to guide the process of altering the phase, frequency or amplitude or some combination thereof in accordance with the selected modulation scheme.
Typically, the master clock oscillator 24 is a temperature compensated, crystal controlled oscillator and the clock signal thereof is fed to a master carrier frequency synthesizer 28, as symbolized by line 30, for use in synthesis of the master carrier signal. In alternative embodiments, the master carrier synthesizer can be a stand alone crystal controlled oscillator. In other alternative embodiments, the master clock signal need not be generated in the central unit transmitter, and, instead, the master clock signal is received from an external source such as the message source 12. Likewise, the master carrier can be received from an external source. In still other alternative embodiments, the master clock and master carrier synthesizer need not be crystal controlled and may even vary in frequency so long as they vary slowly enough that the tracking loops in the remote unit receiver can stay in lock. If variable sources for clock and carrier are used, the periodicity of resynchronization process carried out by a phase detect and adjust circuit 32 in the central unit receiver should be made to have a smaller period so resynchronization is done more frequently. More detail on the function of circuit 32 will be given later.
After modulation, the signal is transmitted on transmission media 40 to the remote unit or units. Transmission media can be a hybrid fiber coax cable plant, a cellular phone system, a landline telephone network or a local area or wide area network medium connecting computers and peripherals together. Transmission from the central unit to the remote units will be referred to as the downstream direction, while transmission from the remote units to the central unit will be referred to as the upstream direction. The carriers used for upstream and downstream transmission are usually in the RF range of frequencies and are separated in frequency although alternatives discussed below to separate upstream and downstream data may also be used.
The downstream RF signal is received by both a coherent detector 44 and a carrier and clock recovery circuit 42. Carrier and clock recovery circuit 42 is typically a phase locked loop or other type tracking loop. The functions of circuit 42 are: to generate a local clock signal and a local carrier signal internally, usually employing voltage controlled oscillators; detect the phase difference between the received master clock signal and the local clock signal and lock the local clock signal in phase and frequency with the received master clock signal and output the phase and frequency locked local clock signal on line 46; and detect the phase difference between the received master carrier signal and the local carrier signal and lock the phase and frequency of the local carrier signal to the phase and frequency of the received master carrier signal and output the phase and frequency locked local carrier signal on line 48. The manner in which the master carrier and clock signals are transmitted downstream are not critical to the invention, and any of the ways known in the prior art or later developed will suffice the practice the invention. For example, in multiplexed systems, one code, timeslot or frequency may be devoted solely to sending the master carrier or master clock or both. In the particular example of the invention in a synchronous code division multiplexed environment, the master carrier is sent as a pilot tone on a dedicated code, and the clock information is embedded in a unique Barker code which is transmitted during every guardband between frames of payload data. The details of the construction and operation of PLLs and tracking loops are well known in the art, and any of the prior art configurations that are compatible with the particular manner in which the master carrier and master clock signals are transmitted downstream will suffice to practice the invention. Examples of types of tracking loops other than PLLs that are also known in the prior art for use in carrier synchronization and tracking the phase and frequency of a master carrier are Mth power loops and Costas loops described at page 564–565 of the Haykin treatise incorporated by reference herein. The clock synchronization circuitry in circuit 42 can be as simple as an appropriate filter when the master clock is transmitted with the data bearing signal in multiplexed form. Another approach used in the prior art is to use a noncoherent detector to extract the clock signal embedded in the data. Clock recovery can be performed after carrier recovery to recover the clock from baseband signals output from the coherent detector. The preferred method of clock recovery is to use the early-late gating method to sample the output of a matched filter which has a transfer function matched to a particular signal bearing the clock information such as a unique Barker code or a rectangular clock pulse. Some of the prior art clock synchronization techniques are described at pages 566–7 of the Haykin treatise incorporated by reference herein.
The coherent demodulator/detector 44 functions to demodulate and detect the incoming symbols and can be any prior art design which is compatible with the modulation scheme used in the central unit transmitter. Although, the detector 44 is stated here to be a coherent detector, in embodiments such as the SCDMA (synchronous code division multiple access) example described below where a rotational amplifier is used to correct phase errors, the demodulator/detector 44 does not have to be coherent. In such embodiments, the demodulator portion does not have to receive a reference carrier which is phase locked with the master carrier so long as the rotational amplifier is used in the detector to correct the resulting phase errors. The same is true for the coherent detector 70 in the CU receiver.
The function of the demodulator/detector is to use the local carrier reference signal on line 48 to demodulate the payload data from the downstream RF signal, detect the transmitted constellation points and output a baseband signal on line 50.
The baseband signal is received and processed by any compatible decoder represented by block 52. The function of decoder 52 is to reverse the encoding and/or multiplexing process carried out by encoder 16 at the central unit transmitter and determine which symbol was sent during every chip time or bit time. The decoder 52 receives the local clock signal on line 46 and uses it to determine when the bit time or chip time boundaries are for purposes of sampling. The decoder 52 also functions to detect the frame boundaries of the downstream frames and reorganize the received data back into the frames organized by the encoder 16 for output on bus 54. To assist the decoder is doing the frame boundary recognition, a framing signal is generated on line 55 by a frame detector circuit 57. Any prior art decoder design that can perform this function for the particular encoding/multiplexing scheme selected for use by the central unit transmitter will suffice to practice the invention. The frame detector circuit 57 receives the downstream RF signal on line 59 (or is coupled to the baseband signal output from the detector 44) and looks for unique frame boundary signals in the stream of data transmitted from the central unit transmitter. Frame detectors are well known in the art, and there is such a circuit in every digital communication system that transmits data in frames. One method of frame detection used in the SCDMA examples presented below is separation of frames by a guardband, and transmission of a unique Barker code by the central unit transmitter during every guardband. This stream of incoming data at the remote receiver is passed through a filter having a transfer function matched to said Barker code and the correlation peak which results when the Barker code passes through the matched filter is used to mark the frame boundaries.
The upstream payload data is received from any message source 56 on line 58.
An encoder 60 receives this message signal (and digitizes it if necessary) and functions like encoder 16 to assemble the bits into symbols for transmission during each bit time or chip time defined by the local clock reference signal on line 46. As was the case for encoder 16, encoder 60 may also do the multiplexing of different channels of data onto different codes or into different timeslots in CDMA or TDMA systems, respectively. In the case of an FDMA system, encoder 60 assembles the message bits from one or more sources into separate bits streams which are supplied to a modulator 62 for use in modulating separate carriers received by the modulator from a synthesizer (not shown) which receives the local carrier reference signal on line 48 and generates a plurality of different carriers therefrom.
The encoder 60 may use a different form of encoding and/or multiplexing for the upstream direction than were used for the downstream direction. Any encoder design known in the prior art or later developed will suffice for purposes of practicing the invention. In fact, the encoder 60 can even use a clock signal at a different frequency from the master clock signal so long as the different clock signal is phase coherent with the master clock signal. Phase coherent means that there is periodic coincidence in time of clock edges between the two different clock frequencies. In such an embodiment, the local clock reference signal on line 46 (locked in phase and frequency to the master clock) would be supplied to a frequency synthesizer which would then generate the new clock frequency so as to be phase coherent with the master clock signal.
The upstream data output from the encoder 60 on line 64 is received by an adjustable delay circuit 65 which receives an adjustable delay value Td. This circuit is used when the system of
The output of the delay circuit 65 is sent to a modulator 62. Modulator 62 functions to guide modulation of the phase, frequency or amplitude of some combination thereof of one or more carriers. In a single carrier system, the carrier being modulated is the local carrier reference signal on line 48 which is locked in frequency and phase with the master carrier. In an alternative embodiment, the local carrier reference signal is supplied to a frequency synthesizer which generates a different frequency carrier which is phase coherent with master carrier signal. Phase coherent in this context is that there is periodic coincidence in time of zero crossings of the master carrier and the new carrier frequency generated by the synthesizer.
The particular structure and operation for the passband modulator 62 is not critical to the invention. Also, the particular modulation scheme used is not critical to the invention and need not be the same modulation scheme used in the downstream direction. Any prior art design for a modulator or a design subsequently developed will suffice to practice the invention so long as it is compatible with the type of encoding done by encoder 60.
Typically, the transmission in the upstream direction is done at a different frequency from the downstream transmission so as to share the transmission media 40 by frequency division multiplexing. However, other forms of multiplexing such as time division or code division multiplexing may also be used to separate the upstream and downstream data. Frequency translator 66 represents the circuitry needed to separate the downstream and upstream data and it assumes that the form of separation is FDM. If TDMA or CDMA is used to separate the upstream from the downstream data, the circuit 66 represents whatever circuitry is used to do the multiplexing. Such circuitry is well known in the art.
The upstream RF signal is transmitted across media 40 to a central unit receiver. The upstream RF is coupled in the central unit receiver to a phase detect and adjust circuit 32, a frame detector 68 and a coherent detector 70. The function of the phase detect and adjust circuit is: to occasionally or periodically extract the received clock signal and the received carrier signal from the upstream RF signal; determine the phase difference between the extracted clock and carrier signals and the master clock and master carrier signals, respectively; adjust the phase of the master clock and master carrier signals and apply the phase adjusted clock and carrier signals to the decoder 72 and coherent detector 70, respectively.
The design of the phase detect and adjust circuit 32 is not critical to the invention, and any circuit that can perform the function stated above will suffice to practice the invention. One example of a phase detect and adjust circuit would be a pair of delay lines through which the master clock and master carrier signals are transmitted, with the amount of delay set to equal the total turnaround time for transmission of the carrier and clock signals from the central unit to the remote unit and back to the central unit. If the total turnaround time is stable, this circuit will adjust the phase of the master clock and master carrier signals for phase coherence with the received clock and carrier signals and once the phase is adjusted, it does not have to be adjusted again. If the total turnaround time changes because of, for example, network expansion, the phase detect portion of the circuit can periodically or occasionally determines the phase differences in any one of a number of different ways known in the prior art. For example, phase differences can be determined by comparing the phase between preamble data and Barker code data encoding the carrier and clock data, respectively, said preamble data and Barker code data being transmitted by each RU occasionally or periodically such as at the beginning of each frame or on a dedicated code or in a dedicated timeslot. The phase information recovered from the Barker code and preamble data transmitted by the RU is compared to the phase of the master clock and carrier.
In other words, one way of transmitting the clock and carrier information for each RU to the CU is for the remote unit transmitter to generate a Barker code which is a copy of the CU Barker code but which encodes the local clock reference and which has good correlation properties such that it can be detected in the presence of noise or even if transmitted in the midst of payload data. This Barker code can be modulated onto the local carrier reference at the remote unit and transmitted to the CU. This Barker code can be transmitted in guardbands between upstream data frames where it is detected by the phase detect and adjust circuit and used to extract the phase of the local clock and local carrier signals used for upstream transmission. The extracted clock and carrier phase information is compared in phase to the master clock and master carrier phase in the CU for each RU, and the phase error for each RU's local clock and local carrier reference signals is applied to master clock and master carrier rotational amplifiers, respectively, when data from that RU is being received. These rotational amplifiers are coupled to receive the master clock and master carrier signals, respectively, and to receive the phase error signals and adjust the phase of each signal in accordance with their respective phase errors. The phase adjusted master clock and master carrier signals at the output of the rotational amplifiers are then applied to the decoder 72 and coherent detector 70 via lines 74 and 76, respectively. If no guardbands are used, the Barker codes for the upstream channel can be transmitted occasionally or periodically during the payload data, and the same process described above is carried out following extraction of the clock and carrier from the Barker code.
In alternative embodiments, only the clock is transmitted with the Barker code, and the remote unit carrier is transmitted as a separate sidetone pilot channel on a dedicated code, a dedicated timeslot or on its own frequency which is different than either of the upstream or downstream frequencies.
Each remote unit generates its own framing signals. These unique signals are detected by frame detector 68. The RU framing signals can be Barker codes transmitted by each RU indicating the start of its frame. The details of the design of the frame detector are not critical to the invention, and any frame detector from the prior art or later developed will suffice to practice the invention. A detailed design for a frame detector to detect a Barker code is presented later herein.
After the master carrier signal has had its phase adjusted to match the phase at the central unit of the carrier transmitted by remote unit, the phase adjusted carrier on line 76 is used by coherent detector 70 to detect the upstream RF and convert it to baseband observed data on line 80. Detector 70 can be any coherent prior art detector which is compatible with the, form of modulation employed on the upstream channel. The details of the detector design are not critical to the invention.
The baseband data output from the detector on line 80 is supplied to decoder 72. The function of the decoder 72, like the decoder 52, is to make a decision from the observed signal as to which symbol was transmitted during each bit time or chip time, and reverse the encoding process performed by encoder 60 by descrambling, de-interleaving and/or demultiplexing the data and reassembling it into the frames of data originally put together by the encoder 60. This process is done with the aid of the phase adjusted master clock signal on line 74 which defines the boundaries of the bit times or chip times and with the aid of the framing signal on line 82 which defines the frame boundaries. The reassembled data stream is output on line 84.
There follows some examples of specific systems that utilize the teachings of the invention. First an embodiment that uses synchronous code division multiple access is discussed. Synchronous code division multiple access in a distributed system requires that all frames from the remote units at different distributed locations arrive at the central unit receiver with their frame boundaries aligned in time. Accordingly, we start with a discussion of the ranging process which is carried out between each remote unit (hereafter RU) and the central unit (hereafter CU) so as to set a transmit frame timing delay for the remote unit which will result in proper frame alignment of that remote unit's frames.
Those skilled in the art will appreciate that the ranging processes described herein has applicability to any distributed data communication system which transmits data in frames regardless of the form of multiplexing or the form of modulation used. Likewise, the equalization processes and power alignment processes described herein all have applicability to any distributed digital data communication system having a near-far problem and having channel impairments that can cause phase and/or amplitude errors in the received points causing them to deviate on the constellation plane at the receiver end from their positions on the same constellation plane at the transmitter end.
In the SCDMA system to be described below, the upstream and downstream data is received at the transmitter in a time division multiple access (TDMA) stream with 8 payload bits in each timeslot. The digital data in the TDMA streams is re-arranged into symbols, as described briefly above, and is transmitted in frames, with three symbols plus one guard band or gap per frame. The guardband or gap is reserved for transmission of alignment Barker codes, and no other data is supposed to be transmitted during the gaps.
The concept in alignment is to adjust variable delays imposed at the site of each transmitter prior to transmission of a Barker code so as to compensate for different propagation delays from each transmitter site such that the Barker code from each subscriber transmitter trying to align arrives at the head end receiver during the same gap. When the variable delays at each subscriber transmitter are adjusted properly, each subscriber will be said to be in alignment so that the signals encoding the symbols that are simultaneously transmitted on the transmission media will all be transmitted with the same frame timing.
Alignment is important to obtain pure orthogonality so as to obtain low cross talk.
If the transmitters are not perfectly aligned, the signals transmitted can still be recovered, but there is some cross talk between channels which will limit the capacity of the SCDMA system to carry information.
This process of aligning all the delay circuits in the transmitters is sometimes alternatively called ranging herein and is broadly applicable to other types of multiple access digital data transmission systems also which suffer from different propagation times from different transmitter sites such as time division multiple access systems that form part of the prior art discussed above.
Referring to
One skilled in the art will appreciate that the construction of the information vector which will be used to generate each symbol by taking only some of the bits from each timeslot spreads the data from each timeslot out over time. This renders the data less susceptible to burst noise. The code division multiplexing allows multiple channels of digital data to be simultaneously transmitted in a 6 mHz channel without interference between channels. In addition, frequency division multiplexing may be utilized to transmit even more channels of digital data above and beyond the 144 channels transmitted in the first 6 mHz channel. In other words, another 144 different TDMA digital channels may be code division multiplexed and transmitted simultaneously with the first 144 digital channels but on a second 6 mHz channel. This second 6 mHz channel has a different center frequency than the first 6 mHz channel which is separated from the center frequency of the first 6 mHz channel sufficiently to not interfere therewith. Both the first and second 6 mHz channels have center frequencies which are separated sufficiently from the center frequencies of the cable television programming sharing the same media so as to not interfere therewith. In alternative embodiments, this scheme can be replicated with any number of symbols greater than 1, or with only one symbol if immunity to burst noise is not important.
The process of synchronization is the process wherein each RU has a variable delay in its transmitter set using feedback from the CU on one of the management and control channels such that the transmitted frame from each RU arrives at the CU with its frame boundaries exactly aligned with the frame boundaries of the frames from the other RUs. Alignment of all frames from all RUs results in the beginning of the gap 161 for each frame from each RU occurring at the same time at the location of the CU regardless of differences in propagation delays from the various RUs to the CU. In
Alignment of any Digital Data System that Sends Data Bits Collected as Frames
Referring to
The alignment process is started asynchronously by any RU that needs to align. The central unit transmits a Barker code during each frame at the same time in the frame. In the preferred embodiment, this Barker code is transmitted during the gap. This Barker code is received by each remote unit at a different time because of different propagation delays, but as to any particular RU, the Barker code is always received at the same time during every frame until the CU changes its delay (a concept to be discussed more fully below). The Barker code represents a trigger to any RU attempting to align and marks the receive frame timing reference for that RU. The time of receipt of the Barker code represents the start of the variable delay interval being adjusted by the RU during the alignment process.
The CU's "every frame" Barker code transmission during the frame shown in
Likewise, the RU #2 at position 169 uses zero delay and emits a Barker code transmission 182 immediately upon receipt of the Barker code trigger transmission 180 from the CU 170. This Barker code transmission 182 from RU # 2 also arrives during the middle of gap 171 thereby indicating that RU # 1 and RU # 2 are correctly aligned.
The alignment Barker code transmissions are typically short bursts having energy levels which are sufficient to make detection during gap 171 easy even though gap 171 also includes random noise energy and with good correlation properties and amplitudes not so high as to substantially interfere with data if the Barker code arrives at the CU in the middle of a symbol.
The alignment Barker code transmissions are detected during the gap by performing a correlation mathematical operation in the CU receiver between the Barker code that was transmitted and the received signal. If the received signal was the same Barker code that was transmitted by the CU, the correlation operation will output a signal that peaks at the time of maximum overlap between the Barker code transmitted by the CU and the received signal. The timing of this peak indicates the alignment state of the RU that transmitted the Barker code which resulted in the peak. Each symbol encoded in the code domain includes error detection and correction bits (ECC bits) such that any errors that occur can usually be detected and corrected when the symbols are re-constituted by the framer circuitry in the receiver.
Referring to
The RU #128, when located at position 192 uses no delay and immediately retransmits Barker code transmission 109 at time 102. Transmission 109 also arrives during gap 106 indicating that, at least at position 92, RU #128 is properly aligned.
Now suppose that the network physically expands such that RU #128 finds itself physically at position 193. In this position, RU #128 receives Barker code transmission 196 from the CU at time 103, and, because RU #128 is already using the minimum possible delay for retransmission of an alignment code, alignment transmission 110 is also transmitted at time 103. However, because of the physical expansion of the network, alignment transmission 110 reaches the CU at time 111 which is after the end of the gap 106 and sometime in the middle of the first symbol of frame #2.
When an RU properly hits the gap, it is authenticated, i.e., identified, and the CU tells it that alignment has been achieved thereby causing the RU to stop adjusting its delay by trial and error. Because RU #128 does not receive any acknowledgement from the CU that it is properly aligned, its starts incrementing its delay vector in a trial and error process. After several incrementations, the delay vector finally reaches the delay represented by vector 112. With this delay vector, an alignment transmission 114 is transmitted from RU #128 at time 113 which reaches gap 116 located at the end of frame 2. However, this means that RU #128 is synchronized with the wrong frame. It is required for proper operation of the system to have all RUs synchronized to the gap at the end of the same frame in which the Barker code transmission from the CU which triggered the RUs alignment transmissions occurred. If one or more RU aligns to the gap at the end of another frame, the results can be disastrous in terms of errors generated in the CU receiver in interpreting data transmitted by the RUs.
Referring to
The alignment transmission 196 from the CU reaches RU #128, the furthest RU, at time 120. Time 120 establishes the receive frame timing reference for RU #128 while at position 192. Thereafter, at time 125, the RU #128 transmits alignment transmission 128. This transmission arrives during the gap 106 at the end of the first frame thereby indicating that RU #128 is properly aligned at this position.
As in the case of RU #1, the delay between times 120 and 125 for RU #128 is predictable.
Now suppose that the network expands, and RU #128 finds itself at position 194. In this position, the CU alignment triggering transmission 196 arrives at time 127. In order to stay aligned, RU #128 will reduce its delay vector 126 to zero and immediately retransmit an alignment transmission 130 comprising the same Barker code which it received. The transmission 130 arrives during gap 106 thereby indicating that RU #128 is still aligned at its new position by cutting its delay vector to zero.
Now assume that the network further expands such that RU # 128 finds itself at position 196. In this new position, alignment transmission 196 from the CU would arrive at time 129. With a zero delay by RU #128, the resulting alignment transmission 131 would arrive at time 133 just after the end of the gap 106 thereby indicating the RU #128 had been taken out of alignment by the expansion of the network. RU #128 would then continue to adjust its delay vector until it aligned to the next gap following the end of frame #2 thereby causing errors.
To prevent this from happening, when the CU finds that an RU which was previously in alignment has gone out of alignment because of network expansion, the CU will reduce its initial delay from the delay represented by vector 116 to the delay represented by vector 132. With this new delay vector, a Barker code alignment triggering transmission 135 will be transmitted at time 137. This alignment triggering transmission 135 will arrive at the position of RU #1 at time 139 and will establish a new receive frame timing reference. If RU #1 has not adjusted its delay vector 122 in advance by one of the mechanisms to be described below, it will go out of alignment. It may then enter a realignment phase and will ultimately, by trial and error, adjust its delay vector to that represented by dashed vector 136. After so adjusting its delay, RU #1 will transmit an alignment transmission 124 at time 123 so as to again hit gap 106 thereby re-entering alignment.
The alignment triggering transmission 135 from the CU arrives at the position 196 of RU #128 at time 141. Using a zero delay vector, RU #128 transmits its alignment transmission 134. This alignment transmission 134 arrives during gap 106 thereby placing RU #128 again in alignment.
TTA=T cu +T ru+2×T p=constant=n×T F (1)
- TTA=the total turnaround time from the CU to the farthest RU;
- Tcu=the delay imposed by the CU illustrated by vector 116 in
- Tru=the delay imposed by the farthest RU illustrated by vector 126 in
FIG. 4(also called Tfar);
- 2×Tp=two times the propagation delay Tp from the CU to the farthest RU; and
- n×TF=an integer multiple of the frame interval TF.
Of course, when the network expands, there is a certain additional delay in the propagation delays which will be called Tu for the uncertainty of this additional propagation delay. Therefore, three additional requirements are imposed with respect to how much delay the CU and the RUs must be able to impose. Those additional requirements are given below in equations (2), (3) and (4):
T cu =[T d +T u] modulo TF (2)
- Td=the span of the network, i.e., equal to the quantity [TTA2−TTA1] where TTA2 equals the total turnaround propagation time for a signal to propagate from the CU to the farthest RU and back, and TTA1 equals the total turnaround propagation time for a signal to propagate from the CU to the nearest RU and back; and
- modulo TF=the remainder of [Td+Tu] divided by TF
Tfar>Tu (3)
- Tfar=the smallest possible Tru of the farthest RU and is equal to the smallest RU delay which can be imposed by the farthest RU;
T near <T F −T u (4)
- Tnear=the maximum possible Tru of the nearest RU.
What all this means in a practical sense is that to set up the delays in the network so that all RUs are aligned, the following steps are taken and the limitations on possible delays imposed by the CU and RUs given in equations (1) through (3) are imposed so that all RUs align to the same gap. The practical network to be aligned by the following procedure has a CU coupled by a fiber optic trunk line to an optical node. The optical node is located out in the area to be served and can be coupled to as many as 2000 homes by 2000 individual coaxial links. To align such a network, step 1 would be to bring an RU to the position of the optical node and fix its delay at Tnear=TF−Tu. With this delay, the nearest RU would not hit any gap except by shear luck. Assuming the nearest RU does not hit the gap with this delay, the second step would be to adjust the delay of the CU until the nearest RU hits a gap. When this occurs, the condition Tcu=[Td+Tu] modulo TF would be true meaning that the CU would have adequately compensated for the uncertainty of the propagation delay increment to Td caused by network expansion.
Referring to
Generally at the time of powerup of an RU, the RU first adjusts its AGC level to make full use of its analog to digital converter dynamic range. Next, the RU does frame detection to determine where the gaps in the CU broadcasts are in time by performing correlations in the RU receiver frame detector looking for the known Barker code which the CU transmits during every gap. Once the gap is located, the frame detector sets the time base generator to synchronize on that receive frame timing reference. Next, the RU performs chip clock synchronization and carrier recovery in the manner described below. Carrier recovery is done by examining slicer error on a known BPSK pilot carrier or pilot channel signal transmitted during a predetermined timeslot using a predetermined code (CU local oscillator signal samples in timeslot 0 spread with all 1s CDMA code and transmitted using BPSK in the preferred embodiment).
In this particular SCDMA embodiment, the pilot channel is the manner in which the master carrier signal from the CU is transmitted to the RUs so that they can synchronize their local oscillator PLLs to the master carrier for purposes of generating their local carrier reference signals. These local carrier reference signals are used by the RU receiver to detect the incoming downstream data and by the RU transmitter to transmit the upstream data. The pilot channel also carries the frame number data. In other words, the RU receiver slicer error on the pilot channel signal is used to synchronize the RU local oscillator to the phase of the CU master carrier local oscillator or other master carrier source.
Chip clock synchronization in the RUs to the chip clock, i.e., the master clock signal of the CU, is performed by the fine tuning circuitry of the frame detector in each RU. The frame detector in each RU synchronizes the RU chip clock to the master chip clock signal embedded in the Barker code sent by the CU during every gap. This is all the RU needs to do to set itself up for reception of CU data and messages.
The RU then starts listening to CU messages to determine if it tuned to the right CU and to determine when the CU solicits ranging activity by a message on one of the command and control channel. In some embodiments, the "clear to range" message can be eliminated, and the CU can watch for ranging Barker codes all the time, but it is preferred to allow the CU to throttle ranging activity. The RU then performs a ranging process described below and registers itself with the CU by sending an authentication sequence of Barker codes after frame synchronization has been achieved (discussed below). This is done by a CPU in the RU when it receives a message from the CU saying "I found one Barker code in the gap, please send your authentication code". The RU CPU then sends data to the RU transmitter telling it what authentication sequence of Barker codes to send back to the CU to identify this particular RU. The CU will then transmit a message indicating what authentication code it found and how many chips off center of the gap the Barker code it found landed. The CPU in the RU that is ranging then properly adjusts the transmit frame timing delay reference Td to center the Barker code in the gap. Other items of data the RU CPU sends to the ranging circuit in the RU transmitter is data indicating the power level to use for the ranging Barker codes and an RU/CU signal indicating to the ranging circuit whether it should follow the rules of ranging for an RU or CU.
The CU next instructs the RU to entering an equalization training interval to determine the coefficients to set into the RU transmitter's precode filter to predistort the RU signals to eliminate channel distortion and test the quality of the ranging result. The training algorithm is discussed below, but other ways of performing equalization which are known in the prior art can also be used. In addition, other ways of achieving frame synchronization known in the prior art can also be used and other ways of achieving synchronization of the RU local carrier oscillator and local clock oscillator to the master carrier and master chip clock signals, respectively, known in the prior art can also be used to practice the invention of eliminating tracking loops in the CU in the SCDMA environment.
Block 183 symbolizes the process wherein each RU trying to synchronize (the terms "synchronize", "ranging" and "alignment" all are used synonymously to mean the process of training an RU to set its delay vector properly to get its frame boundaries aligned with the CU frame boundaries) receives the Barker code trigger signal transmission from the CU using its frame detector and sets its receive frame timing and then sets a first trial and error delay value for its delay vector. Thereafter, the RU transmits the same Barker code it received from the CU towards the CU as an alignment transmission using the first trial and error delay value.
In block 185, the CU monitors the gap for receipt of a Barker code by performing a correlation between any received signal during the gap and the Barker code that was transmitted as the trigger signal. If a Barker code identical to the trigger signal is received during the gap, the correlation will result in a correlation peak being found in the gap. If a correlation peak is found, processing proceeds to the process symbolized by block 191. There, the CU broadcasts a message to all RUs indicating that it found activity in the gap. Then the process of block 192 is performed where each RU trying to synchronize sends its "signature", i.e., its RU identification code in the form of a Barker code transmission sequence. That is, in response to the broadcast from the CU indicating activity in the gap, each RU trying to synchronize sends its unique signature towards the CU in order to determine if that RU's Barker code is the Barker code the CU found in the gap and whether it is the only RU in the gap. This process is called authentication.
The process of block 193 symbolizes the start of the authentication process. Each RU has a unique signature which comprises the transmission and nontransmission of Barker codes during the gaps of a multiple frame authentication period. Specifically, the unique signature of each RU will involve transmitting the Barker code during some gaps of the authentication period but not during others in a sort of "Morse code". Each Barker code transmission results in a correlation peak during one of the chips in the gap. Each RU has a unique 16 bit RU ID, each bit being either the presence or absence of a Barker code correlation peak somewhere in the gap. Therefore, it takes 16 frames or 4 suprerframes to transmit the RU ID.
The number of gaps during which the Barker code is transmitted compared to the number of gaps during which the Barker code is not transmitted during the authentication period is such that if only one RU is aligned to the gap and is transmitting its authentication signature, activity will be found in the gaps of the authentication interval only 50% of the time. This scheme for authentication is chosen so that the CU can detect contentions, i.e., more than one RU in the same gap, in the manner described below.
After performing the process of block 193, the process of block 195 on
Next, the process of block 197 is performed. In this process, the CU counts the number of gaps in the authentication interval that have had activity detected therein, and then compares that number to the total number frames in the authentication interval to determine if the 50% activity level limit has been exceeded indicating that more than one RU is hitting the gap. The advantage of this method is that activity detection, contention detection and authentication are all combined into a single process thereby speeding up the process by more efficiency.
Returning to the consideration of the process of block 185, if the CU, while monitoring the alignment gap for activity, finds no peak resulted from the correlation calculation, then the process of block 186 is performed. In this process, the CU broadcasts a message to all RUs telling them to adjust their delays and to try again to hit the gap with their Barker code transmissions. Then, the process of block 188 is performed wherein each RU trying to synchronize increments its delay vector and retransmits the same Barker code as was received from the CU. Thereafter, the process of block 185 is performed again wherein the CU monitors the gap for activity. The loop comprising blocks 185, 186 and 188, taken together, comprise the trial and error process which causes all RUs trying to align themselves to continually increment their delay vectors until at least one of them hits the gap.
Returning to the consideration of block 197, if 50% activity level is detected during the authentication interval, it means that only one RU is in the gap. In such a case, the process of block 199 is performed. In this process, the CU identifies the RU whose Barker code transmissions are found in the gap from the unique signature sequence transmitted during the authentication interval. In other words, the CU examines exactly which gaps had correlation peaks therein and the sequence of these gaps and looks up this sequence in a lookup table listing the unique signature sequence for each RU in order to identify the particular RU that has successfully aligned itself. Block 199 is reached only if activity is detected in exactly 50% of the gaps.
After the CU identifies the RU, it broadcasts the identity so determined to all RUs as the last step of block 199.
Next, the process of block 200 is performed. In this process, the RU with the identity broadcast by the CU recognizes its identity in the broadcast message and enters a fine tuning mode. The purpose of the fine tuning mode is to cause the value of Td to be precisely adjusted so that the frame boundaries of frames transmitted by this RU arrive at the CU exactly aligned in time with the frame boundaries of the CU receive frames (which are offset in time from the CU transmit frame boundaries in some embodiments).
The fine tuning mode is represented by the process of block 202. In this process, the CU instructs the RU which has aligned itself in the gap on how to adjust its delay vector in order to center the correlation peak calculated by the CU to the exact middle of the gap. In the preferred embodiment, the gap is comprised of 16 chips which comprise 8 chips in the middle of the gap and then 4 chips on either side of this middle group of 8. It is desirable during the fine tuning mode to get the correlation peak centered in the middle of the middle 8 chips. As mentioned above, a chip is a small interval of time equal to the frame period of 125 microseconds divided by the 448 chips which comprise each frame. In other words, each chip is 279 nanoseconds in duration. The fine tuning process of block 202 involves sending messages back and forth between the CU and the RU which has been identified as having aligned itself in the gap. These messages are sent over the management and control channels. Since clock recovery and carrier recovery has already been accomplished in the RUs before ranging is started, receiving of these management and control messages is no problem and constellations involving phase information can be used. In some embodiments, the exchange involves only one instruction from the CU to the RU saying, for example, "Increase your delay vector by 2 chips" or, "Decrease your delay vector by 3 chips". In other embodiments, multiple trial and error adjustments are made. The RU then makes the instructed adjustment and retransmits the Barker code. The CU again calculates a correlation peak and examines where the peak occurs in the gap. If the peak occurs in a suitable position, the CU sends a message to the RU telling it to stop adjusting its delay vector as satisfactory alignment has been achieved. The RU then adjusts the coefficients of its precode equalization filters 563 in
Returning to the consideration of the process of block 197, if the CU determines that greater than 50% of the gaps during the authentication interval had correlation peaks therein, i.e., greater than 50% activity is detected, then the process of block 204 is reached. This process is only reached if more than one RU has aligned itself to the same gap. If this case, because each RU is transmitting its unique signature, and because each signature is a unique sequence with only 50% activity level, the result of two RU's being in the same gap will be that during more than 50% of the gaps of the authentication interval, correlation peaks will occur. It is impossible to find tune the RUs if more than one RU is trying to fine tune during the same gap. Therefore, the CU has to reduce the number of RUs that are in the gap to one, and it starts this process by performing the process of block 204. In this process, the CU broadcasts a message to all RUs instructing only the RUs attempting to synchronize to execute their collision resolution protocols.
Next, the process of block 206 is performed, to start the collision resolution protocol, wherein each RU attempting to synchronize executes a random decision whether to continue attempting to synchronize or to stop attempting to synchronize. Each RU will make this decision with a 50% probability of either outcome.
After all RUs make their random decisions whether to continue, the process of block 208 is performed. In this process, the RUs that have decided to continue to align retransmit their signature sequences without changing their timing, i.e., with the same timing as was used on the last iteration of the trial and error process. In other words, each RU that has decided to continue transmits its unique signature sequence (sometimes hereafter called a "dotted sequence") over another authentication interval using the same delay vectors that are currently set.
Next, the process of block 210 on
If the random decisions whether to continue or not result in no RUs transmitting their signatures, then no activity will be found in the gaps of the authentication interval. In this event, the process of block 212 will be performed wherein the CU broadcasts a message instructing all RUs to go back to the previous stage and to reexecute their decisions to continue or discontinue the ranging process.
The RUs then re-execute their decisions whether to continue or stop attempting to align themselves and retransmit their signatures during the authentication interval with the same delay timing used on the previous iteration, as symbolized by block 214.
Following the process of block 214, the process of block 216 is performed to determine if more than 10 attempts to get one RU in the gap have occurred. If so, the process of block 218 is performed to return to block 181 and restart the ranging process from the top. If fewer than 10 attempts have been made, processing returns to the process of block 210 wherein the CU again monitors the gaps of the authentication interval for activity.
If the process of block 210 finds only one RU in the gap, i.e., 50% activity level is detected during the authentication interval, then the process of block 222 is performed. The process of block 222 authenticates the RU by broadcasting the identity of the RU found in the gap and then the RU is fine tuned in the manner previously described with reference to block 202.
If the CU finds in the process of block 210 more than one RU is still in the gap, processing returns to block 204 where the CU broadcasts a message to all RUs instructing them to execute their collision resolutions protocols. This process is symbolized by block 220.
There are several alternative embodiments to the ranging process described in
Another alternative embodiment within the class where the RU determines the proper transmit frame timing delay by trial and error generally comprises the following steps. The RU precomputes an 8 of 16 temporary RU ID which is randomly selected. The CU solicits for ranging transmissions. Each RU which wishes to range, transmits its temporary RU ID as 8 Barker code transmissions in 8 gaps of the next 16 RU frames (selected to match the temporary RU ID sequence) with a first iteration of transmit frame timing delay value. The CU generates a ranging status data comprised of 16 bytes, each bit of each byte representing whether a correlation peak occurred during a corresponding chip of the middle 8 chips of a corresponding gap. The CU reorders the 16 bytes into eight 16 bit fields, and transmits this data to all RUs over 4 consecutive frames as a ranging status message which includes data regarding which superframe the ranging status data applies to and the superframe during which the next ranging transmissions are to be made. Each RU receives the status message and stores it in memory and informs the RU computer of the presence of the message. The RU computer parses and scans the ranging status message and interprets the data therein according to the ranging protocol as follows. If all entries are zero, then all ranging RU conclude they have missed the gap and set a new value for their transmit frame timing delays and retransmit their temporary IDs in the next iteration of 16 frames at an activation time specified in the downstream ranging status message. The new transmissions arrive at the CU, and one byte of raw ranging status data is stored in a FIFO memory in the CU. The CU controller initiates a DMA transfer of the FIFO data, and processes the raw ranging data into a new ranging status message and submits valid RU IDs to a training input queue. If the ranging status message analyzed by the RU controller indicates more than one pulse in some gaps, a collision has occurred. If an RU does not find its temporary ID in the status message, it assumes it was involved in the collision, and performs its contention resolution algorithm as described elsewhere herein. If an RU finds its temporary ID in the ranging status data, it is authenticated and in the gap. By looking at the positions of the pulses of its temporary ID in the gap, the RU determines how far off center it is from the middle of the middle 8 chips, and calculates its own offset and applies it to its transmit frame timing delay. The RU is now ready for equalization training. A variation of the above protocol is demand ranging where, after a power failure that would result in all RUs attempting to recover simultaneously thereby swamping the contention resolution mechanism, each RU is addressed individually by its RU ID and asked to begin ranging.
The other class of ranging embodiments involves the CU calculating the total turnaround time to each RU and instructing each RU as to how much transmit frame timing delay to use. In this class, the CU sends a marker signal which can be easily detectable by the RU receivers. Each RU trying to range, then immediately transmits back the same easily identifiable signal which can be detected by the CU receiver even if it arrives during the middle of a frame of payload data. Such a signal can be a chirp, a high-power, narrow pulse or a long sequence of chips that spreads out over one or more frames. The CU detects the correlation peak of the signal and compares it to the time of transmission of the original marker signal. The difference is the total turnaround time or TTA. The CU then sends a message to the RU to identify itself which can be done by the "Morse code" authentication sequence, or in one of the other ways identified above for the first class of ranging embodiments. Once the CU knows the RU's identity and its TTA, the CU can send a message to the RU instructing it as to how much transmit frame timing delay to use to achieve frame synchronization, and the RU sets this amount of delay for transmission of every frame.
Note that in these alternative embodiments of both classes where the ranging signal transmitted by the RU can be detected over the noise of payload data where it arrives at the CU during a frame such as in the embodiments using a large-amplitude, easily detectable pulse or a long sequence which stretches out over one or more frames and which can be detected by a correlator, there is no need for a gap in every frame. The only requirement in high, throughput SCDMA systems is that the RU frames arrive synchronously with correspondingly numbered frames from other RUs (lower throughput CDMA systems do not require frame synchronization). If that timing relationship can be achieved without a gap, then there is no need for a gap. For example, in the case of a narrow, large amplitude pulse, when the RU transmit frame timing delay is set so that this pulse arrives at the beginning of the correspondingly numbered frames from other RUs, then the RU has achieved frame synchronization. In the case of a long sequence that spreads out over, for example, two frames, where the correlation peak is found at the end of the second CU frame, this would mean that if the RU starts a frame transmission at the time it started transmission of the long sequence, that frame will arrive coincident with the CU frame boundaries and therefore, will also be coincident with the frame boundaries of other correspondingly numbered RU frames. Any methodology to achieve this frame synchronization is within the teachings of the invention.
The process of adjusting the delay vector used by the CU in transmitting its trigger signal Barker code can result in loss of synchronization by all RUs in the system unless something is done to prevent this before the CU changes its delay. That is, when the CU shortens its delay vector, the RUs closer to the CU than the furthest RU will all go out of alignment unless certain measures are taken to forewarn them of the coming change. There are 3 different embodiments of processes for realigning all of the RUs when the CU changes its delay vector. The preferred one of these embodiments is symbolized by the flow chart of
The first of these processes is shown in the flow chart of
As noted previously, because the CU uses the same delay vector during every frame in transmitting its Barker code trigger signal, the RUs have a predictable periodic signal from the CU upon which they can rely to measure the timing change made by the CU. In other words, the time of arrival of the Barker code from the CU during each frame is predictable to each RU, and when it changes, the RUs can measure by how much it changed. When the Barker code from the CU does not arrive at the predicted time, the RUs know that the CU has just altered its delay vector. The RUs then measure the deviation of the new receive frame timing reference, i.e., the time of arrival of the Barker code trigger signal from the CU, by measuring the difference between the old receive frame timing reference and the new receive frame timing reference. This process is symbolized by block 242.
Finally, each RU realigns itself in the process of block 244. In this process, each RU alters its delay vector by an amount equal to the change in the receive frame timing reference. Then each RU initiates a ranging process. The CU monitors the gap at the end of every frame so any RU can initiate ranging at any time.
Next the process of block 248 is performed wherein each RU receives the broadcast and alters its delay vector by an amount equal to the amount that the CU will be changing its delay vector at the specified time. That is, each RU alters its delay vector by the amount instructed by the CU at the time indicated in the message from the CU that the CU will alter its delay vector.
Finally, the process of block 250 is performed wherein each RU reinitiates a synchronization process.
Both of the embodiments of
The final embodiment for resychronizing after the CU changes its delay vector is for the CU simply to broadcast the message to all RUs saying, "You must all now realign as I have just changed my delay vector." Each RU then re-enters the alignment process symbolized by
Note that in the ranging process described above, it is the RUs that determine how far they are from the CU rather than the CU determining how far each RU is from it. The advantage of having the RUs doing the ranging is that the CU does not have to stop payload traffic on the various channels to perform ranging functions each time a new RU enters the system or an existing RU loses synchronization. In a system where the traffic may frequently include high demand applications such as real time video, stopping traffic flow for ranging is not a viable possibility because it would interrupt the flow of video information and disrupt the subscriber's video conference, movie etc. In the ranging system described herein in its various embodiments, there is no need to stop traffic since the ranging process is done out of band, i.e., in the gaps. Further, because the transmitted power of the Barker codes is low and correlation processes are used, the process can start blind with any trial and error timing value without interfering with channel traffic. That is, even if the Barker code transmitted back toward the CU by the RU has improper timing and lands somewhere outside the gap, its power level is low enough to not cause substantial interference, and even if some small amount of interference is caused, the chips of the symbols transmitted during the frame have enough redundancy with the Trellis encoded modulation to recover from the interference without an error. Because correlation to a known Barker code pattern (the same Barker code pattern the CU transmitted to the RUs during the previous gap) is used by the CU to determine whether it has or has not detected a Barker code from an RU in the gap, the RUs can transmit their Barker codes at very low power levels so as to avoid interfering with traffic and causing errors in the data of the various payload channels during the trial and error process of setting their transmit frame timing delay values Td so as to hit the gap.
Of course for embodiments where the ranging signals can be detected even when they arrive in the middle of the frame and do not interfere with payload data reception, traffic does not have to be stopped during ranging. As is apparent from the foregoing discussion, there is no need to preset an approximation of the correct transmit frame timing delay into the RUs before they start and then fine tune the delay since even a gross misalignment will not cause any appreciable errors in the payload data. Since Trellis coded modulation and a redundant bit are used in each tribit of payload data, any errors caused by misalignment can be detected and corrected by forward error correction without the need for retransmission. In other embodiments however, conventional ranging techniques could be used where the CU measures the range to the RUs to establish synchronous CDMA, and the particular ranging species initiated by the RUs described herein are not required to practice the invention of a system with a CU with no tracking loops.
In the high power pulse embodiments described above, the RUs act like transponders by sending a narrow, high amplitude pulse upon receipt of a trigger signal from the CU. The trigger signal from the CU could be a special pulse, a Barker code, etc. If the RU was misaligned, and the large amplitude pulse landed in the middle of the upstream payload data, the CU would ignore the particular chip which was "stepped on" by the high amplitude pulse. The payload data could still be recovered because the bandwidth of the payload data has been spread so widely using direct sequence CDMA spreading. Trellis code modulation is not needed for this scheme to work. After detecting the RU's pulse and comparing its timing with the position of the frame timing reference, the CU would ask the RU for its identity and the RU would send it by any conventional manner such as pulse position modulation, amplitude shift keying etc. The CU would then send a message to the RU instructing it to change its transmit frame timing delay in a direction to put the pulse closer to the fixed timing reference, and this process would continue until the RU hit the timing reference. Note in this method, that a gap or guardband is not needed in each frame.
Note that in the ranging embodiments described above, it is assumed that the "span" of the system, i.e., the difference between the TTA of the farthest RU and the TTA of the nearest RU, is smaller than one frame time. When this is true, all RUs can align to the same gap. When all the RUs are aligned to the same gap, and the CU knows the total turnaround time, dynamic code assignment can be used where the CU informs the RUs by downstream management and control messages what codes each is supposed to use. The CU will then know what codes to use and when to use them in decoding signals from each RU because both the RUs and the CU count frame numbers for the CU frames and all code assignments to the RUs are in terms of CU frame numbers.
In very large systems, the span may exceed the frame time, and to force the span to be less than the frame interval would unreasonably constrain the system size. When the span of the system is greater than the frame time, an accounting problem arises because not all the RUs can align to the same gap. This means that the CU will not know which codes each RU used to spread the spectrum of its payload data, unless it knows the total turnaround time to each RU. In other words, each frame transmitted by the CU downstream to the RUs is numbered by virtue of a kiloframe marker signal encoded in the pilot channel carrier tone. The RU receivers detect this kiloframe marker and count individual received frames and thus know what frame number each received frame from the CU is. If the span of the system is less than one frame interval and each RU is aligned to the same gap, each RU will know that when, for example, CU frame 99 is received, the next set of frames transmitted by the RUs all will arrive at the CU at the same time, i.e., the beginning of the next frame at the CU and all those RU frames will have frame number 100 assigned to them by the CU and will be despread and decoded together. In this situation, downstream instructions to RU #1927 to use codes #55 and 57 during frame 100 and to RU #3 to use code #3 during frame 100 make sense, and the CU can properly decode the data from each of these RUs because it knows which codes each used during frame 100. Suppose however that RU #1927 is aligned to the next sequential gap following the gap to which RU #3 is aligned. This means that when frame #99 is received from the CU, the frame transmitted by RU #3 in response to frame #99 (the downstream data of frame 99 if offloaded, new upstream data is loaded, and the frame is "retransmitted" back toward the CU) will be numbered 100 when it arrives at the CU.
However, the frame transmitted by RU #1927 in response to receipt of frame #99 will arrive at the beginning of CU frame #101 and will be treated by the CU as RU frame #101. If the CU does not know that RU #1927 is not aligned to the same gap as RU #3, it will assume that RU #1927 and RU #3 are both using the codes assigned to them for frame 100, when RU #1927 is actually using the codes assigned to it for frame #101.
One remedy for this accounting problem is for the CU to know the TTA or total turnaround propagation time for each RU and transmit that TTA for each particular RU to that RU. Each RU then uses its TTA time plus the kiloframe marker encoded in the pilot channel (or transmitted downstream in any other way) to keep track of what frame number each received CU frame is and what frame number will be assigned by the CU to the next RU frame transmitted in response to receipt of the CU frame. This allows the RU to use the proper assigned orthogonal, pseudorandom spreading codes assigned by the CU for each frame since the RU will know what frame number will be assigned by the CU to each of the RU's frames and knows that the code assignment messages from the CU are based upon the frame numbers assigned to RU frames by the CU.
The actual algorithm carried out in the CU to calculate TTA for each RU to support boundless ranging in this particular embodiment is quite simple. This algorithm happens after the RU whose TTA is being calculated has successfully completed the ranging process and is aligned with some gap. The CU sends a frame to the RU. The frame itself has no frame tag number, but the kiloframe markers in the pilot channel data allow the RU's to count received CU frames using a local counter. Meanwhile, as the CU sends frames, its frame count continues to rise. In response to the received frame, the RU sends a frame back to the CU along with a TTA_service_request which includes the RU frame tag number for the transmitted frame which is equal to the local counter value. In other words, the RU frame tag number sent back with the TTA_service_request matches the CU frame number of the frame just received as determined by the local counter value. When the RU's frame reaches the CU, the CU subtracts the RU frame tag number from the CU's current frame tag count. This difference times the frame interval is equal to the TTA for that RU. The multiplication times the frame interval is not actually necessary since the RU only needs to know how many complete frames behind the current CU frame count each one of the RU's transmitted frames will be in order to use the proper codes for each frame.
High Level Transceiver Block Diagram
Referring to
The circuitry of
The transmitter 401 of the transceiver uses a framer circuit 400. The function of the framer is to receive one or more streams of digital data via data path 399 from one or more sources and to organize this data into a plurality of frames, each frame comprised of one or more symbols. In the preferred embodiment, the framer circuit 400 composes the frames of data from a TDMA data stream on bus 399 where each timeslot corresponds to one channel, a channel being a logical construct indicating data on this channel will be kept separated from data on the other channels even though a shared transmission media is used and will appear to the user to have arrived on its own media dedicated solely to it. There are 128 payload data channels to share among all users and there are 16 management and control channels some of which are also shared for a total of 144 channels or timeslots. Each RU may be assigned one or more channels or timeslots depending upon the amount of bandwidth it has been awarded by the CU in response to requests for bandwidth from the RU. In addition, bandwidth may be reserved to the various RUs on a permanent basis in some embodiments, and in these embodiments, the channels or timeslots may be permanently assigned or the reserved number of channels may be assigned on a guaranteed basis each time the RU requests bandwidth.
It is not critical to the invention that the incoming data streams arrive in a TDMA stream on bus 399. The streams of data from peripheral devices or networks could, in alternative embodiments, arrive via FDMA on bus 399 or each source of data could be connected to the framer circuit 400 by a separate input bus.
The framer circuit 400 and its associated circuitry implement the variable delay that sets the variable transmit frame timing reference for each RU to achieve frame synchronization by the ranging process described above. This transmit frame timing reference establishes the timing of transmission of the orthogonally CDMA encoded chips of each frame such that all frames arrive from each of the physically distributed RUs at the CU at the same time and aligned with the CU frame boundaries. Although, the invention still works even if frame synchronization is not maintained because of the orthogonality of the CDMA codes which are used, it does not work as well since the maximum number of users which can be simultaneously sharing the available payload channels is limited. This is because there are higher levels of crosstalk between CDMA codes when frame timing synchronization between all RUs and the CU is not maintained. Therefore, each RU undergoes the ranging process described above after first powerup and from time to time thereafter to properly set its transmit frame timing delay to achieve frame synchronization. The transmit frame timing delay Td is learned by cooperation between the transmitter 401, the receiver 403 and the CPU 405 in the RU which is ranging and the counterpart devices in the CU by virtue of communication between the RU and CU on the management and control channels. The CPU changes the value of Td on line 499 until frame synchronization is achieved and thereafter maintains that value of Td until ranging is performed again.
The particular manner in which frame synchronization is achieved is not critical to the invention, and other processes can be used such as by trial and error correlation of a selected data string spread by a selected CDMA code transmitted at differing values of Td with the signal from another RU which is known to be in frame synchronization which comprises the same data string spread by the same selected CDMA code.
The framer circuit 400, in the preferred TDMA input bus embodiment, bridges the two time domains between the TDMA input data and the chip clock code domain (reading of the framer circuit is done at the chip clock rate and writing is done at the byte clock rate at which timeslots of data are written one 9-bit byte at a time). The output data stream from the framer circuit 400 comprises three arrays of tribits per frame, each array of tribits representing an information vector which, after encoding by the orthogonal multiplexer 408, is transformed into one symbol of chips. In the preferred embodiment, the orthogonal multiplexer 408 is a code division multiplexer which uses a plurality of orthogonal cyclic codes, each code being used to encode the data from a different channel. This is a so-called direct sequence type spread spectrum operation wherein the bandwidth of the baseband signals on buses 1068C and 1070C are spread across a broad spectrum by the CDMA codes using orthogonal code multiplexer 408 in
In an important class of alternative embodiments, the orthogonal encoding multiplexer 408 could be any encoder which encodes each channel with a different orthogonal waveform for purpose of multiplexing to keep data from different sources separate. For example, these orthogonal multiplexer could store digital samples that define a plurality of orthogonal sine and cosine waveforms, each at a different frequency. Any other set of orthogonal waveforms of different frequencies other than sines and cosines would also work to encode the various channel data samples. Each channel's data would then be multiplied by a different waveform's samples to generate new digital samples which define orthogonally encoded data on buses 417 and 419 for modulation onto the RF carrier frequencies. In such embodiments, the bandwidth of each channel's data is not spread as wide as in a CDMA system. In fact, each channel's data would be dumped into a narrow bandwidth frequency bin. In such systems, the orthogonal demultiplexer, i.e., decoder 462 in
Note that each information vector element in this embodiment always defines the magnitude of the same frequency component. In an alternative multitone system, the information vector elements can be pseudorandomly scrambled in the transmitters so that they define different frequency component magnitudes in each frame and then pseudorandomly descrambled in the same order in the receivers.
In SCDMA direct sequence spread spectrum transmitters of the preferred embodiment, the three information vectors output during each frame are converted by CDMA spreading to the three symbols that are transmitted during that frame. The data in each information vector spans the entire 144 timeslots in the sense that three bits from each timeslot or channel are present as the elements of the information vector as a tribit. This interleaving of data from each timeslot into each information vector is preferred but not critical to the invention. Likewise, the transmission of three symbols per frame is not critical to the invention and fewer or greater numbers of symbols could be transmitted.
In the preferred embodiment, the circuitry of the transceiver is virtually all digital, so the arrays of tribits are true arrays, the elements of which are used sequentially in the matrix multiplication to perform the CDMA spreading.
In analog embodiments, the arrays of tribits will be streams of tribits, with three separate streams per frame.
Before finishing the description of the rest of the transceiver circuitry in
The chip clock signal on line 348 of
A block diagram of the tracking loop 350 in the RU receivers is shown in
The details of the tracking loop are set forth in
The bit clock and chip clock signals are kept synchronized in RUs to the master clock signal by a phase detector 373 which compares the phase of the received signal on line 312 to the phase of the bit clock signal and outputs a signal which is coupled to the frequency control input 375 of the VCO through a low pass filter 397. The bit clock and chip clock signals in the CU modem are kept synchronized to the master clock signal on line 312 from a crystal control oscillator. In the RUs, the phase detector 373 takes the form of the clock recovery circuits in the frame detector described elsewhere herein coupled with a phase detection circuit that compares the phase of the recovered clock to the phase of the bit clock signal. The phase detector circuit 373 and low pass filter correspond to the frame detector circuit 513 in the block diagram of the RU transceiver shown in
The relationships between timing in the time domain and timing in the code domain are as follows:
- There are 144 total time slots or channels in the TDMA stream, of which 128 are payload time slots and 16 are management and control time slots;
- Each time slot or channel in the TDMA streams carries 9 bits of digital data synchronized with the bit clock;
- One time slot worth of data or 9 bits is stored in the framer for each cycle of the byte clock;
- 1 frame=144 times slots, each with 9 bits plus 16 chips for the alignment gap;
- 1 frame also equals 3 symbols plus the 16 chip periods of the alignment gap=448 chip periods;
- 1 symbol=144 chip periods;
- 1 gap=16 chip periods;
- For every 16 bit clock periods, there are 7 chip clock periods, and for every byte clock period, there are 9 bit clock periods.
To implement the delay necessary in each RU and CU transmit channel circuitry to maintain frame synchronization, consider the following with reference to
The 9 bit parallel format output of the register 314 is presented on 9-bit bus 318 to the write data input port of memory 300. Thus, a new 9-bit group of data from the TDMA stream is presented for storage on each cycle of the byte clock signal. Each 9-bit group of data from the TDMA stream is stored in a different memory location of memory 300 as will become clear from the discussion of the address generation circuitry described below.
Data is read out of memory 300 at the same rate at which it was stored, but starting at some programmable time after the data is stored, thereby implementing the variable delay needed to maintain frame synchronization with the CU frame timing. This programmable delay Td is set by the difference in addresses between the address stored in a receive frame counter (read pointer) and the address stored in a transmit frame counter 324 in
To illustrate this concept,
At time (0) in
The width along the X axis of each individual crosshatched block in
The blocks surrounded by dashed line 332 in the leftmost column represent 144 memory locations, each storing the 9 bits from one of the 144 time slots in one frame of data. The three crosshatched blocks 334, 336 and 338 represent the first symbol of the first frame, each symbol storing 48 tribits. Note in the middle column, after transmission of the first symbol in the frame, these three blocks are gone. Note also that the data of symbol 1 is read out of the memory "across time", i.e., along the y axis, thereby interleaving the data from the first tribits of individual channels in the time domain into different temporal relationships in the code domain and spreading out the energy of the time slot data over the entire frame interval. This is part of the teaching of code division, multiple access or CDMA modulation schemes, but is not critical to the invention. Interleaving of data improves the immunity of the data to burst noise. The three blocks within dashed box 334 in the middle column of
While the first and second symbols are being transmitted, another page of the memory 300 continues to fill up as the data from new timeslots is received. For example, while symbol 1 from page 1 of the memory is being transmitted during the first frame, the data in the three blocks encircled by dashed line 336 in the middle column is received in page 2 of the memory and stored. Thus, while one third of the data from page 1 is read and transmitted, one third of page 2 of the memory is filled with new data. Likewise, while the second symbol of page 1 is being transmitted, the data represented by the three blocks encircled by dashed line 338 in the right column is received and stored in page 2 of the memory.
The blocks encircled by dashed box 340 represents the delay Td implemented by modulo adder 326 in
The bit clock signal on line 377 in
The time delay Td necessary for hitting the CU gap with a Barker code transmission is added to the F0 signal on line 299′ by the modulo adder 326 to generate the F0′ signal on line 381. The value of Td is received from the CPU via bus 499 and changes by trial and error during ranging but is frozen at whatever delay centers the transmitter Barker code in the gap at the CU. The F0 signal on line 299′ also increments the page pointer 321 for the write pointer and simultaneously resets the write pointer 322 to zero at the end of each frame so as to cause a page swap and begin writing again at address 0 of the next page.
The delayed F0′ signal on line 381 increments the page pointer 323 of the read address circuitry to cause a page swap and simultaneously resets the read pointer counter 324 to zero so as to begin reading at address 0 of the next page at the end of the frame.
Returning to the consideration of
Line 352 in
A tribit select counter (which is not shown in
After the 144th incrementation, the read pointer counter 324 rolls over to zero and begins to count up to 143 again. At the 144th incrementation, the tribit select counter increments which causes the multiplexer 356 to select the middle column of tribits from symbol 2 in
Bus 360 in
Returning to the consideration of the transceiver block diagram of
The purpose of the forward error correction encoder 402 is to add one or more redundant bits to each tribit so as to improve the error rate for the energy per bit-to-noise power density ratio resulting from the chosen modulation scheme. In the preferred embodiment, the FEC encoder 402 is a Trellis encoder for a 16-QAM, Rate ¾ Trellis code having 16 states, a pi/4 rotational invariant, no parallel paths and an effective code length of 2. In yet another alternative embodiment, the forward error correction encoder 402 could be a Reed-Solomon Encoder which generates a first set of code words which are then further encoded in a Trellis encoder. An advantage of using Trellis encoded modulation either with or without Reed-Solomon coding is that it allows redundancy to be added to the payload data so as to enable forward error correction without increasing the symbol rate and the consumed bandwidth. This prevents the need for retransmission of garbled data since the errors can usually be eliminated by the Viterbi decoder using the redundant bits. Trellis encoded modulation uses redundant bits to map the payload data into a larger constellation of possible points (called signal space coding). The bandwidth required for transmission is not increased, nor is total noise admitted by the receive filter. Basically, Trellis encoding uses a channel coder to receive each k payload bits and convert them into n bits where n is greater than k and includes some redundant bits which contain information about the k payload bits. The n bit group is then processed by a modified line coder to produce symbols for transmission from a constellation having size 2n. Significant coding gains can be achieved in this way. For example, assuming a particular additive white Gaussian noise channel produces an acceptable probability of error without coding at some signal to noise ratio using a constellation of size M, using Trellis encoded modulation, the error probability can be reduced at the same signal to noise ratio or the signal to noise ratio can be reduced at the same error probability, and, per Ungerboeck, most of this theoretical reduction can be achieved using a constellation of 2M plus a channel coding scheme. As an example of the type of coding gain that can be achieved using Trellis coded modulation, consider the following. If only tribits were used without coding with redundant bits, and an 8-AM constellation were used, according to Ungerboeck transmission with 10−5 error probability with an SNR of 26 transmitting and 3 bits per symbol could be done. However, by the use of Trellis encoded modulation using a 16-AM constellation, it is possible to send 3 bits error free down to 18 db SNR. Therefore, using Trellis encoding, it possible to achieve a coding plus shaping gain of 26−18=8 db. In the invention, a coding gain of approximately 4 db is obtained. The main advantage of using Trellis coded modulation is the ability to reduce the error rate or increase the number of payload bits without increasing the symbol rate and bandwidth consumed. This can be done using a constellation no greater than 2M. More details about Trellis encoded modulation are contained in Lee and Messerschmit, Digital Communication, 2d Ed., 1994 (Kluwer Academic Publishers, Boston), ISBN 0 7923 9391 0, which is hereby incorporated by reference. Trellis encoded modulation is not required however to practice the invention of a CU with no tracking loops to constantly track the RU clock and carrier frequencies, and, therefore, the encoder 402 in
In the preferred embodiment, the forward error correction encoder 402 take the form of the Trellis encoder shown in
Mapper 1050 has a normal mode and several other modes including a fallback mode. In normal mode, the mapper takes the 16 combinations of the y0 through y3 bits and maps them to the 16-QAM constellation of
The I and Q bits on buses 1068 and 1070 are then stored as separate real and imaginary arrays for the information vector [b] in memory 406 in
Fallback mode and the other available modes are implemented in the Trellis encoder of
In embodiments where forward error correction is not used, encoder 402 is an ARQ encoder which simply adds enough ECC bits to allow the receiver to detect an error and request a retransmission. The retransmission request is made on one of the command and control channels. In some block code embodiments, the forward error correction encoder 402 uses cyclic codes where the sum of any two code words is a code word and any cyclic shift of a code word is also a code word. Note that the Viterbi decoder 468 discussed below in the description of the receiver is used only when the forward error correction encoder 402 is a convolutional or Trellis encoder.
Although the discussion of the forward error correction encoder 402 has not heretofore included any discussion of the modulation process carried out by modulator 410, Trellis-Coded Modulation (hereafter referred to as TCM) is preferred because of its lower error rate in the face of channel impairments. TCM modulation combines the forward error correction and modulation process by redefining the coding as the process of imposing certain patterns on the transmitted signal. This provide more effective utilization of band-limited channels as is the case for multiple access on HFC cable TV plants. Trellis-Coded Modulation is characterized by three basic features:
- (1) the number of signal points in the constellation used is larger than what is required for the modulation format of interest with the same data rate wherein the additional points allow redundancy for forward error control coding without sacrificing bandwidth;
- (2) convolutional encoding of the message data is used to introduce a certain dependency between successive signal points such that only certain patterns or sequences of signal points are permitted; and
- (3) soft-decision decoding is performed in the receiver, in which the permissible sequence of signals is modelled as a Trellis code.
The preferred form of the encoder 402 is the 16 state Trellis encoder shown in
In the preferred embodiment, the forward error correction encoder 402 has multiple modes which add different numbers of redundant bits while always maintaining the code word length at 4 bits. In a normal mode, one redundant bit is added per tribit. In a fallback mode when channel impairments are high, fewer payload bits are sent and more redundant bits are sent in each 4 bit code word.
The encoder 402 in the transmitter is a state machine which, in conjunction with state memory 404, receives the stream of tribits for each symbol and calculates a 4th redundancy bit for each tribit. This 4th bit provides redundancy for error detection and correction and for use by a Viterbi Decoder 468 in the receiver in ascertaining with greater accuracy the data that was actually sent despite the presence of noise. The 4th bit in each tribit is part of the Trellis modulation scheme and is generated by the convolutional encoder 402. A three bit constellation would normally have only 8 points. However, Trellis modulation adds redundant bits interspersed in the information stream of tribits and increases the size of the constellation to enable more spacing between constellation points thereby enabling better discrimination between points by the receiver and lowering the bit error rate without increasing the bandwidth. In noisy environments like CATV media, Trellis modulation is preferred, but some species of the invention will work without the redundant 4th bits and using a smaller constellation. In the preferred embodiment, the encoder is used to provide greater accuracy and better noise immunity. The encoder, in the preferred embodiment, is a state machine but it could also be a lookup table implemented in RAM or ROM etc. The implementation of the state machine is not critical as long as the implementation is fast enough to keep up with the chip clock data rate. For purposes of this discussion, it will be assumed that the convolutional encoder 402 is present.
M-ary Modulation in Code Division Multiple Access System
The output of the convolutional encoder 402 is an array of 4-bit digital numbers for each of symbols 1, 2 and 3 shown in
The stream of 4-bit symbol elements that are output from the encoder 402 are stored in memory 406 as three different linear arrays corresponding to symbols 1, 2 and 3 in
After passing the tribit stream from the framer 400 through the encoder, the resulting 4-bit data streams are stored as separate I and Q information vector arrays for each symbol in memory 406. Each symbol is comprised of two linear arrays of 2 bit numbers: one array contains multiple 2-bit elements defining the real or inphase "I" coordinates for all the elements of the symbol and the other array stores the 2-bit elements which define the imaginary or quadrature "Q" coordinate of each symbol element. The 144 array elements of each symbol define an information vector b for each symbol. The code division multiplexer 408 then spreads each information vector separately with a separate orthogonal code for each channel and combines the spread data into a single orthogonally coded data stream.
The encoding in CDMA MUX 408 spreads the energy of the symbols over time using orthogonal codes or orthogonal, cyclic codes. This is done in two steps. First, a linear array information vector of just real parts, i.e., inphase coordinates of the symbol to be transmitted, symbolized by array 405 in
Second, the same process is repeated for the imaginary coordinate linear array (not shown) for the same symbol the real coordinates of which were just processed. This results in another linear array comprising the imaginary or quadrature coordinates of the chips in the results array. This imaginary component array of the results array also is not shown in
The real component array, represented by linear array 409, is part of an overall result or "chips out" array which contains both the real and imaginary coordinates of a plurality of chips to be transmitted. These chips map to points in the result space, and the points in the result space map to whatever points in the input point space that are defined by the real and imaginary components in the information vector array b, of which array 405 is the real part. The mapping between the input point space and the results space is defined by the contents of the code matrix and the orthogonal codes.
Before performing the matrix multiplication, the 2's complement values of the real and imaginary components of the information vector b input array are converted to their decimal equivalents as shown in
Because each orthogonal code used in array 407 is also pseudorandom, and the rate of generation of the chips in the result vector (the chip rate) is much higher than the bandwidth of the input data represented by the information vector 405, the bandwidth of the resulting signals defined by the result vectors generated by this process is spread into an extremely broad spectrum. In fact, the bandwidth of the result vectors generated by this process extends to plus and minus infinity. The spread signal consists of replicas of the same power spectrum repeated end to end, so the signal can be recovered by the receiver even though only the portion within the passband of the amplifiers on the hybrid fiber coax channel and the transmitter and receiver filters is processed by the demodulation and despreading circuitry in the receiver.
The CDMA MUX 408 in
The resulting real and imaginary component linear arrays of the results or chips out array are stored in a memory within the CDMA Mux 408 which is not separately shown. The components of these two arrays are then output on separate I and Q buses to a modulator 410 where they are used to amplitude modulate the amplitudes of two RF carriers that are 90 degrees out of phase using a Trellis modulation scheme in one embodiment or are used to control modulation in a carrierless modulation scheme described elsewhere herein. The resulting two AM carriers are summed and output on the transmission media 412. This is done as illustrated in
Referring to
Because the RF signals that carry the information from the 144 channels must share the transmission media with other RF signals having adjacent frequencies, two optional digital passband Nyquist shaping filters 421 and 423 are used to limit the bandwidth of the signals on buses 417 and 419 to 6 Mhz to avoid interference with signals on neighboring frequencies. The digital signals on buses 417 and 419, when converted to their decimal equivalents usually have rapid transitions between levels in adjacent intervals. This is illustrated in
The modulator uses a local oscillator COS wave carrier signal from the master carrier synthesizer in the case of the CU transmitter. In the case of the RU transmitter, the carrier signal comes from the tracking loop carrier recovery circuit (515 in
The imaginary or quadrature component of each chip, after filtering, is input on bus 433 to another amplitude modulator 435. This modulator receives at a carrier input 437 a sine wave of the same frequency as the cosine wave on line 427, but shifted in phase by 90 degrees by phase shifter 439. In an alternative embodiment, these local oscillator SIN and COS signals on lines 427 and 437 are actually generated in the carrier recovery circuit 515 in
In some embodiments, the line 412 may be coupled to suitable interface circuitry to drive the signal on line 412 into a wireless or cellular system, a terrestrial microwave link, a coaxial cable of a cable TV, telephone or other system, a fiber optic link of a cable TV, telephone or other system, a local area or wide area network or any other media developed in the future for real time communication of data. Such interface circuitry is known and will not be described further herein.
As mentioned briefly above, in an alternative embodiment for purposes of carrier recovery by the RUs for downstream data, the master carrier signal is sent downstream as pilot channel data on a specific dedicated timeslot using a dedicated code. Referring to
In the transceiver of
Likewise, in the RU transmitter 401 of
The RU carrier recovery circuit 515 can be any conventional phase-locked loop clock recovery circuit, Mth power loop, Costas loop, suppressed carrier-tracking loop, etc. In the preferred embodiment, the carrier recovery circuit in the RU receivers takes the form shown in
The error computing circuit 521 also receives a timeslot number enable signal on line 531 in
An alternative carrier recovery arrangement is shown in
Referring to
To correct for each RU's phase and amplitude errors, the CU must determine the phase error and amplitude error for each RU and correct for them individually as the timeslot data for each RU is being received. The way this is done is for each RU to send known preamble data to the CU in the timeslots currently assigned to that RU before the block of payload data is sent. This is done each time the RU transitions between an idle state when no data is being sent and an active state when upstream data is being sent. The CPU in the CU assigns the timeslots to the various RUs and so informs them in management and control messages on the management and control channels. This is done in response to bandwidth requests from the RU. The process by which the CU adjusts the phase of the master clock and master carrier signals for each RU is shown in
Upstream Carrier Recovery Error Correction Factor Per Timeslot
Referring to
In response to the bandwidth request, in step 1508, the CU sends a downstream management and control message to the RU awarding bandwidth in the form of assignment of one or more timeslots. The RU responds to the bandwidth assignment by sending known preamble data to the CU during the assigned timeslots.
The CU knows what the preamble data is supposed to be and knows when the preamble data is being received by virtue of knowing when the timeslots assigned to the RU sending the preamble data are being received. When preamble data is being received, the CPU 405 in
Step 1514 in
The CU modem of
In the embodiment shown in
Line 1074, buses 1078 and 1080 and multiplexer 1076 are only present in the RU transmitters since the technique described here is used only in the upstream data to achieve proper synchronization, so these circuits are absent from the CU transceiver block diagram of
In the CU receiver shown in
where s(t) is the desired signal;
- a=the amplitude error caused by channel impairments and the near-far problem; and
- ejø=the phase error caused by channel impairments and the near-far problem.
The slicer detector 467 in
Specifically, the job of the CU receiver slicer detector 467 is to determine the correct 1/a and e−jø coefficients in a multiplication factor of the form:
(1/a)*e−jø (5)
where 1/a is the gain correction coefficient to solve the near-far problem and correct for channel impairments; and
- e−jø is the phase error correction coefficient to solve the near-far problem and correct for channel impairments and get the CU synchronized with each individual RU despite differing path lengths and differing channel impairments between the CU and each HU.
The near-far problem involves interference with reception of weak signals transmitted from a remote RU by strong signals transmitted by a near RU. In the prior art, this is often solved by time division multiplexing so that the two transmitters are never transmitting at the same time. In the SCDMA environment, this solution will not work since all RUs have to be able to transmit whenever they need to transmit if bandwidth is available. Therefore, in the SCDMA example described here, the amplitude levels of the signals transmitted by the RUs are controlled so that all signals arriving from the RUs at the CU should arrive at approximately the same amplitudes, and channel impairment effects are corrected by gain level adjustments in the CU receiver at a point before the baseband signal enters the slicer so as to minimize interpretation errors caused by amplitude errors (the G2 gain adjustment amplifier and rotational amplifier in slicer/detector 467 are located so as to receive the received signals before they get to the slicer). For a discussion of the iterative process carried out by this circuitry during the preamble for each timeslot to establish the values for the amplitude and phase error correction coefficients for use in receiving the payload data for that timeslot, see the discussion of the cooperation of G2 amplifier 788, rotational amplifier 765, slicer 800, control loop 781 and memory 796 in the detailed block diagram of the CU receiver discussed below.
Thus coherent modulation and detection is used for both upstream and downstream transmissions, but the coherent detection may be accomplished using rotational amplifiers.
The pilot channel data on timeslot 0 is spread with a dedicated CDMA code in CDMA multiplexer 408 for transmission on the timeslot 0 management and control channel as the pilot channel data which encodes the CU master carrier. Use of a pilot channel signal on one of the command and control channels is only one of the possibilities for distributing carrier frequency and phase information. Other possibilities are transmission of any modulated waveform which can be detected by the RU receivers in which the CU carrier frequency and phase information is encoded in the modulation. Note also that the RU can use another carrier frequency than the master carrier so long as phase coherence can be achieved in the CU receiver between the RU carrier and the master carrier.
The form of carrier recovery described above is only one way of achieving a coherent system with only one master clock and master carrier synthesizer. In
Another form of synchronization that is required is symbol or chip clock synchronization. The receiver must know the instants in time when the modulation can change its states from the amplitude and phase of one chip to that of the next. That is, the RU and CU receivers must know the start time and finish time of each chip in order to decipher what that chip was. This allows the receiver to determine when to sample and when to quench its product integrator or other chip state detection circuitry for purposes of starting the chip decoding process.
Symbol synchronization in the context of the SCDMA example herein is recovery of the CU chip clock in each RU. In the preferred embodiment, recovery of the CU master chip clock and master carrier is done by synchronization circuitry including frame detector 882, control loop 781, VCXO 784 and time base 886, VCXO 808 and frequency synthesizer 760. The master chip clock is recovered from the Barker code transmitted by the CU. The master carrier is synthesized by synthesizer 760 from the CU master chip clock in the preferred embodiment. There are at least two different ways of generating the master carrier in the RUs discussed herein and there are other ways known in the prior art. Any one of these ways will suffice to achiever carrier synchronization. One way to generate a synchronous local master carrier in the RUs is to control the phase of VCXO 808 in
In one embodiment represented by
The frame detector 513 in the RU receivers is only active during this window signal, so clock recovery in the RUs is actually periodic and not continuous in the SCDMA example given by
The time base circuit 886 in
The time base circuit 886 provides these signals which include receive frame timing reference information to any circuit in the receiver or transmitter that needs this information such as the receiver's orthogonal demultiplexer 462 in
The orthogonal code multiplexers in the RU and CU transmitters also get receive frame timing reference signals, but these frame timing reference signals establish the boundaries of the CU's frame timing reference since each RU transmitter times its transmissions and other processing so that frames transmitted therefrom arrive at the CU coincident with the CU frame boundaries. And of course the CU transmitter needs to transmit its frames in synchronism with the CU frame boundaries. To that end, the receive frame timing reference signal generated by the frame detector 882 in
The fine tuning circuitry in the frame detectors 513 and 882, in
The coarse tuning circuitry in the frame detectors 513 and 882 cooperates with a software process running in CPU 405 and the window signal generated by the time base under control of the CPU to help the frame detector locate the CU frame gaps. This is done using control and timing signals on bus 902 on the CPU and the real and imaginary data components on bus 904 output by the demodulator 460 in
Referring to
The ranging detector has an acquisition mode and a tracking mode. In acquisition mode, it is simply trying to rapidly find a known Barker code arriving in the collection of signals on bus 904 in
In acquisition mode, the interest is in quickly finding the gap by correlating the incoming signals with the known Barker code, but this can be done by simply looking at the sequence of signs of signals received since the Barker code is a known, unique sequence of chips of differing signs but constant amplitude. The Barker code can be located effectively in tracking mode by looking at only the sequence of differing signs in the received data. Therefore, in tracking mode, the CPU sends selection control signal ACQ on bus 902 to control the state of switches 906 and 908 so as to select the signals on buses 910 and 912. The signals on buses 910 and 912 are the outputs of circuits 914 and 916 which serve to compare the incoming signals on bus 904 to zero and output a first number if the sign of the incoming chip is + and output a second number if the sign of the incoming chip is −. When acq is not asserted, the raw data on buses 918 and 920 is selected for passing through switches 906 and 908. The acq signal also passes through OR gate 922 to gate the output signals from switches 906 and 908 through to finite impulse response filters 924 and 926 in acquisition mode for correlation. The OR gate 922 also receives a GAP a signal which is asserted by the CPU via bus 902 when the CPU thinks it is in the gap by virtue of signals from the frame detector. Therefore, the signals on buses 928 and 930 from switches 906 and 908 will be correlated by FIR filters all the time when the ranging detector is in acquisition mode and, while in tracking mode, only during the gap.
The FIR filters 924 and 926 have impulse response functions which are programmable and are set by the CPU 405 to match the Barker sequence which the receiver is looking for. The Barker sequence being sought is defined by data written by CPU 405 into register 932. When this exact sequence of + and − chips resides in either one of the FIR filters, the filter output will peak. Absolute value circuits 934 and 936 are coupled to the outputs of the FIR filters, and output the absolute values of the FIR output signals on buses 938 and 940. Circuit 942 has two different modes which are selected by the acq signal on line 943. In acquisition mode when the receiver is trying to initially locate the gap, circuit 942 selects the greater of the signals on buses 946 or 948 for output on bus 944. In tracking mode, the sum of the signals on buses 946 and 948 is output on bus 944.
Comparator 950 acts to set a minimum threshold above which the FIR output peaks must rise before they are counted as possible reception of the CU Barker code. Comparator 950 compares the signals on bus 944 to a threshold level on bus 945, and, if the threshold is exceeded, outputs a logic 1 on bus 951 during the interval when the threshold is exceeded. The threshold level is set by data written into register 952 by CPU 405 via bus 902 (bus 902 contains more signals lines than just the two lines shown in
Course tuning to find the gap is accomplished by the ranging detector as follows. The CPU starts with an estimate of when it thinks the gap will start. At that time, signal GAP_a on bus 902 is asserted during each frame interval. The CPU only wants to look at peaks during the gap in each frame interval, so it uses a sliding window to restrict the time during which it is looking for peaks. The sliding window is symbolized by bracket 962 in
Circuit 970 passes only the first peak on the output of the AND gate 968 which occurs after the GAP_a signal indicates the gap is thought to have started. A time base counter 972 counts chip clock signals on line 974 and is cleared by the GAP_a signal every frame. When circuit 970 passes a peak (actually a logic 1 level) through on bus 976, the current count of the time base counter 972 output on bus 980 is sampled and stored in register 978. The count value on bus 980 is also coupled to a comparison input of a greater than or equal to comparator 965, the other input of which is coupled to receive the output of the register 964. The output of the comparator 965 is the gating signal on line 966. Since the count of time base counter 972 will be reset to 0 at the moment the CPU thinks the gap is starting, the count stored in register 978 represents an offset error indicating how much later the gap may have actually started compared to the time the CPU thought the gap was starting.
During frame 1, shown on timeline A of
Because the noise pulse 990 was random, it does not occur again at time T1 in the second frame shown on timeline B of
The situation for frame 3 is shown on timeline C of
During frame 4, window 962 opens at time T4, but no pulse occurs again at relative time T5, but the Barker code pulse 992D occurs again at time T7. This Barker code pulse is gated through by circuit 970 and causes sample 4 to be taken. The gap acquisition process reads sample 4 and compares it to sample 3, and decides that pulse 996 was noise because pulse 992D did not occur at the same relative time. Accordingly, the gap acquisition process moves the position of window 962 again so as to open at a time T6 just before the occurrence of pulse 992D.
The situation during frame 5 is shown on timeline E of
The chip clock recovery process is carried out by early-late gate sampling circuitry in
The manner in which this clock recovery process is carried out by the circuitry of
In alternative embodiments, the chip clock could be recovered by transmitting the chip clock with the data bearing signal in multiplexed form and then using appropriate filtering or demultiplexing at the RU to extract the chip clock. Another possibility is to use a noncoherent detector to extract the chip clock taking advantage of the fact that the chip clock timing is more stable than the carrier phase. The carrier is then recovered by processing the detector output during every clocked interval. Another possibility where clock recovery follows carrier recovery, as is done in the preferred embodiment, is to extract the chip clock from demodulated baseband output from the CDMA demultiplexer.
In addition, all the RUs may possibly synchronize to a single common external time source such as GPS satellite time information although synchronization to within 1 microsecond may not be adequate accuracy in all applications. Any conventional methodology for achieving synchronization of the RU chip clocks and local oscillator signals to the corresponding signals in the CU will suffice for purposes of practicing the invention.
The ranging detector of
Returning to the consideration of
The results output from the demodulator on lines 469 and 471 are digital baseband data streams which basically defines the mix products comprised of a fundamental carrier frequency and upper and lower sidebands. Digital filters 473 and 475 filter out the desired sidebands that contain the real and imaginary parts of each chip or result point that was transmitted. The stream of quadrature or imaginary components of the received chips are output on bus 477. The stream of inphase or real components of the received chips are output on bus 479. Per the teachings of the invention, the recovered clock and carrier signals in the RU are then used for transmissions by the RU to the CU so that the CU can coherently communicate with the RU's without having to synchronize to different clock and carrier signals used by the RU's.
In alternative embodiments, the RUs can use their own clock and carrier signals which are unrelated to the CU's versions and the CU can contain its own phase lock loop circuitry to recover these signals and synchronize to them in order to demodulate and interpret the data transmitted by the RUs.
In some embodiments, the streams of real and imaginary components of the 144 chips of each symbol on buses 477 and 479 are stored in two linear arrays in CDMA Demultiplexer 462 in
The mapping by orthogonal code transformation from the constellation of possible input points shown in
The detector 466 outputs its preliminary determinations to a Viterbi Decoder 468 which performs the prior art Viterbi algorithm to determine the actual constellation point sent with each chip. The Viterbi Decoder uses the 4th bit in each chip of each symbol to detect and correct errors using the Viterbi algorithm to derive the most probable tribit path defined by the points actually sent from the path in the received chip space defined by the 4-bit components of the symbols actually received. The addition of the 4th bit to each tribit converts the input constellation from an 8 point to a 16 point constellation by addition of redundancy. The addition of this redundant 4th bit increases the distance between the path through a space defined by successive input constellations, one for each symbol time. The fact that the chip path is farther from the 3 bit path makes it easier for the receiver to divine from the noise corrupted received data what the actual tribits transmitted were. Viterbi Decoders are well known in the art of digital communications, and no further details will be given here. This Viterbi algorithm could be carried out by a programmed digital computer if slow speed is enough or by a dedicated hardware circuit if speed is important. Viterbi Decoder based systems are used by Qualcomm, Inc. in San Diego in cellular phone systems to combat noise in digital cellular phone transmissions, and the details of their patents and products are hereby incorporated by reference.
The output data points from the Viterbi Decoder are a stream of tribits. These tribits are stored in a memory in a deframer circuit 470 which functions to reassemble a replica of the TDMA data stream in the time domain from the incoming stream of chips or tribits comprising each symbol. This process is done by reversing the reading and writing processes described above in filling and emptying the framer memory 300 of
Fallback Mode
Fallback mode is entered when noise power gets too high. The noise power is detected by the CU, and when it reaches a predetermined threshold, the CU commands all RU modems to reduce the amount of payload in each symbol and add more redundancy. Fallback mode is implemented by a mode control signal on line 530 in
Code Diversity in CDMA To Improve Performance
Referring to
This code diversity concept can be used in any CDMA system. In CDMA systems where all the timeslot data is collected in one physical location, code diversity can be implemented using a shuffler 500 shown in
In systems like the CDMA CATV system disclosed herein where at each RU not all the timeslot data for all 144 timeslots is present at each location, the shuffler takes a different form and is located in the CU. In this embodiment, the inputs 502 represent requests for bandwidth relayed to the CU by all the RU's, and the outputs 504 represent code assignment transmissions to the RU's over the command and control channels where the code assignments could change every frame or even after transmission of each symbol. At the CU however, all the timeslot data of channels to be transmitted to the RU's is located in one place, so the shuffler can take the physical crossbar switch form previously discussed in the paragraph next above. The shuffler 500 can also take the form of a suitably programmed computer to shuffle the timeslots to different codes as well as perform the matrix multiplication.
The use of this shuffling technique spreads the weak codes around but the weak codes still cause errors. If the level of errors generated by this technique cannot be tolerated, forward error correction is used in conjunction with the code diversity to eliminate the errors. Forward error correction means sufficient redundant bits are inserted into the data stream by the encoder 526 in the CU and RU transmitters to allow any errors to be corrected without the need for retransmission of frames with errors. In the specific embodiments disclosed herein, Trellis modulation is used with a convolutional encoder in each RU and CU transmitter to calculate and add to each tribit a redundant 4th bit. These 4th bits are used by the receivers and Viterbi Decoders therein to correct errors by making judgments from the received data which points from the constellation of possible points were actually sent.
In the preferred embodiment for a transmitter described below with reference to
Preferred RU Transmitter Block Diagram
Referring to
Block 508 is the framer circuitry that implements the variable transmit frame timing delays needed to implement the ranging process to achieve the necessary frame synchronization and time alignment of the CDMA spread channel data for synchronous CDMA. The framer circuitry 508 is described in more detail in
Encoder 526 adds at least one bit to every tribit in the preferred embodiment to implement Trellis modulation. Some embodiments have no encoder, and some embodiments have an encoder which has no idle and/or no fallback mode.
The encoded bits are divided into real (or inphase) and imaginary groups by dividing each encoded tribit in half and outputting the first 2 bits as the real bits on bus 517 r and the last two bits on bus 517 i. Buses 517 r and 517 i are coupled to a switching circuit 544 which also receives as inputs real and imaginary components of access channel information on buses 542 r and 542 i. During normal payload transmission operations, switching circuit 544 selects the data on buses 517 r and 517 i for coupling on buses 546 r and 546 i to buffer memory 548. During access channel operations, switching circuit 544, under control of microprocessor 405 or other timing logic, selects the data on buses 542 r and 542 i for coupling on buses 546 r and 546 i, respectively. The real and imaginary components in each tribit on buses 546 r and 546 i are written into buffer 548 in the order dictated by write addresses on bus 533. Elsewhere herein, the manner in which the multiplexer 544 is operated to overlay media access control data on buses 542 r and 542 i with payload data on buses 517 r and 517 i in buffer 548 is described. Buffer 548, when fully written, during each symbol time has 144 4-bit elements comprising an information vector the order of which is randomly scrambled anew each symbol time in the preferred embodiment. In other embodiments, the codes may be assigned sequentially during each symbol for all active timeslots, or a rolling sequential assignment of codes to all active timeslots may be used.
Referring to
It is important in the embodiment of
Returning to the consideration of
In some embodiments, circuit 510 in
Block 514 on the left side of
Because the 4th bit to be added to each tribit depends upon the state of the tribit from this channel during the last symbol, a memory 528 is used to keep a record of the state of each channel's 4 bit chip state during the last symbol transmission. This information is supplied to the convolutional encoder via bus 530 as each channel's tribit is encoded during each symbol. The mode in which the diversity shuffler 506 operates is controlled by the diversity shuffler by a signal on bus 534.
Media Access Control
Block 540 represents circuitry to acquire an access channel and carry out media access control communications to implement ISO MAC layer protocols. Since there are only 4 access channels across which all message traffic requesting channel bandwidth and awarding same pass, contentions will occur when more than one RU simultaneously requests bandwidth on the same access channel. Therefore, access channels are acquired according to the following protocol. Each RU transmitter receives a seed number on bus 550 and pseudorandomly selects which access channel to attempt to use and pseudorandomly selects which 6 symbols of a superframe comprised of 12 symbols to send. The RU then sends an authentication code identifying itself in the form of the unique sequence of 6 of the 12 symbols of a superframe of 4 frames, said unique sequence pseudorandomly selected using the seed. All RUs use the same seed, so the likelihood of more than one picking the same authentication code is small. The 6 symbols sent can contain the RU's message telling the CU how many channels it needs, or a separate message can be sent after access is achieved. The CU listens on all access channels, and during each superframe determines if more than 6 symbols were sent. If so, the CU broadcasts a message on the control channel indicating there is a contention on a particular access channel. The RUs trying to gain access then do the contention resolution protocol described elsewhere herein used for ranging. If only 6 symbols are detected during the superframe, the CU broadcasts a message on the control channel indicating which 6 symbols were found. The CU can include in the broadcast message code assignments for the requested channels in reservation embodiments or, in another embodiment, can simply transmit updates to the timeslot activity table indicating which timeslots or channels have been awarded to the RU which gained access. The RU that sent these six symbols then knows that it has been awarded access, and updates its timeslot activity table which is maintained in the diversity shuffler 506. All RUs hear the timeslot activity update broadcast message and similarly update their timeslot activity tables.
Once an access channel is acquired, circuit 540 may, in some embodiments, present data on buses 542 r and 542 i to multiplexer 544 which comprise access control messages. Multiplexer 544 either selects these media access messages on buses 542 r and 542 i or the encoded chips from the convolutional Trellis encoder 526 to the code division multiplexer 527 via buses 546 r and 546 i and buffer 548. The multiplexer 544 is controlled by switching control signals from the CPU 405 to edit the contents of the buffer 548 to overlay the 4-bit groups of the access control symbols with the payload data on bus 507 so that the media access control 4-bit groups go into the right addresses of the buffer 548 so as to get spread by the CDMA codes assigned to the access channels.
The media access control messages constitute requests from RUs for bandwidth and awards of specific channels to the RUs by the CU in some embodiments. The awards of specific channels to specific RUs implement a reservations scheme and the awards can take many forms such as broadcasts on the control channel of timeslot activity table update messages or specific messages on the access channels in other embodiments. Also, other media access protocols other than the reservation scheme which are described elsewhere herein are also possible through various protocols some of which may require message traffic on the access channels. In an important alternative embodiment, all the different schemes for allocating channels to specific timeslots may be used or combinations of schemes for various groups of channels may be used. In this embodiment, the type of scheme used is programmable by the user, and in a variation of this embodiment, may be changed by the CU computer based upon traffic conditions and the number of contentions and efficiency considerations.
Because a reservation scheme is implemented in the preferred embodiment, no contentions occur on the 140 non media access control payload channels so no contention resolution protocols are carried out for these channels. However, contentions are expected to occur on the 4 access control channels shared between all the RUs so contention resolution will have to be carried out in the manner described elsewhere herein.
Spreading of the spectrum of the chips from the convolutional encoder 526 is done by orthogonal code multiplexer 527. This circuit or software routine performs code division multiplexing or orthogonal encoding of the data on each channel by matrix multiplication. It sets the amplitude of the output chips on buses 558 r and 558 i based upon matrix multiplication of the orthogonal codes times the elements of the input information vectors on buses 549 r and 549 i from buffer 548. Each of the information vectors on buses 549 r and 549 i is individually spread by the orthogonal code multiplexer to generate individual real or inphase and quadrature or imaginary result vectors 409 and 413 in
There is only one orthogonal, cyclic code that has 144 different codes. That code is used and is, in hexadecimal representation: 0218 A503 BA4E 889F 1D92 C1F3 AB29 8DF6 ADEF. Other codes can be used, but the above code is best. Although cyclic codes are the preferred embodiment for ease of implementation, any other orthogonal, noncyclic code set can also be used in alternative embodiments, or other orthogonal, cyclic codes can be used where fewer channels/timeslots are required. The cyclic code given above uses the convention that all logic 0's represent −1s and all logic 1s represent +1 in the orthogonal code spreading matrix. The first code of the 144 different codes in the code set will be all 1s regardless of the contents of the code given above. The second code in the code set is the code given above: 0218 A503 BA4E 889F 1D92 C1F3 AB29 8DF6 ADEF. The third code is obtained by shifting the code one binary place and taking the overflow bit that "falls off" the most significant bit position edge of the code in the second least significant bit position. The fourth code is obtained by repeating process done to obtain the 3rd code on the 3rd code.
The results of the matrix multiplication performed in the orthogonal code multiplexer 527 are coupled via buses 558 r and 558 i to one input of a switching circuit 556 switching of which is controlled by the CPU 405. The other input of the switching circuit 556 is coupled to buses 558 i and 558 r to receive the ranging data from ranging circuit 510. The switch 556 selects the data on buses 558 r and 558 i for coupling via buses 557 r and 557 i, respectively, to a precode FFE/DFE filter 563 during the three symbol transmission times of each frame when payload data is being sent. The switch 556 selects the ranging pulse data on bus 560 during the gap following transmission of the last symbol in each frame.
Equalization, as that term is used herein, is the process of compensating for distortions and noise that occur caused by noise in the channel between each RU and the CU. The precode filter 563 performs a measured predistortion at each RU transmitter so that the data arrives at the CU undistorted despite the channel impairments between that particular RU and the CU. The amount of the predistortion is calculated by each RU to substantially or exactly compensate for the current distortion conditions existing in the channel between it and the CU. The predistortion characteristic is implemented by setting the transfer function of the precode equalization filter 563 by changing the tap coefficients of the filter. This transfer function is controlled by the RU/CU Coefficient data input to the filter on bus 561. Each RU uses its own unique, measured RU/CU Coefficient data to establish a predistortion which is appropriate to its own signals for its position on the network so as to cause its signal to reach the CU with little or no distortion. More details on both upstream and downstream equalization are given in connection with the discussion of the training process symbolized by the flowcharts of
The CU transmitter of
The output of the precode filter on buses 562 r and 562 i is applied to a scaler amplifier 564 which scales the amplitude level of the digital numbers on buses 562 r and 562 i in accordance with a signal on bus 566 which indicates the activity level of the modem, i.e., how many timeslots are currently in use by this modem. The purpose of this scaling is to enhance performance by taking advantage of the full precision of a digital to analog converter 576 at the output of the transmitter. A digital to analog (D/A) converter has a dynamic range for its analog output. When few timeslots are active, the summation of the CDMA spreading matrix multiplication partial products does not lead to chip amplitudes which extend to the full limits of the D/A converter's dynamic range. As a result, the full precision of the D/A converter is not used, and the inherent noise of the D/A conversion process affects the transmitted signal more. To make use of the full precision of the D/A converter, scaler 564 "amplifies" the incoming signal based on the activity level such that the resulting swing in digital values going into the D/A converter 576 causes output analog signals which swing between the limits of the dynamic range of the D/A converter. These signals are later reduced in amplitude by a circuit (not shown) which limits the amplitude swings to prevent interfering with other signals sharing the media.
The output of the scaling circuit on buses 568 r and 568 i are coupled to shaping filter 570 which doubles to perform carrierless amplitude and phase modulation. There are two filters in the shaping filter which have transfer functions which are the Hilbert transform of each other and which have rolloff characteristics set to digitally filter the data on buses 568 r and 568 i to limit the bandwidth of the signal on each bus to the width and center frequency of the 6 mHz channel devoted to digital data communication on the coaxial cable or other media 24. The shaping filter has a squared raised cosine filter characteristic suitable to shape the outgoing chip pulses so as to satisfy Nyquist criteria in a known manner so as to provide optimal signal-to-noise enhancement and so as to minimize intersymbol interference. The filters in shaping filter/modulator 570 can have other transfer functions also which shape the chips to be transmitted such that the spectrum of the outgoing signals satisfy the Nyquist criteria. Any of these other pulse shapes will suffice to practice the invention. The coefficient data on bus 572 provide ability to set and change the filter characteristics of shaping filter/modulator 570. More details on the operation of the shaping filter/modulator 570 are given in connection with the discussion of
The output of the filter/modulator is coupled on bus 574 (the filter/modulator 570 sums the orthogonal real and imaginary signals after filtering to generate a single signal on bus 574) is coupled to the input of the digital to analog converter 576 for conversion to an analog signal for application to the input of an up/down frequency converter 577. The purpose of the up/down frequency converter is to convert the frequency of the transmitted signal to the frequency allocated for upstream or downstream transmissions as the case may be in accordance with the frequency plan for the shared transmission media. The up/down converter outputs its signal on the transmission media 412 such as coaxial cable, cellular system, satellite uplink etc.
Alternative Ranging, Contention Resolution and Authentication Processes Carried Out by RUs and CU
Referring to
The starting point of the ranging process is block 600 in the RU ranging process shown in
After the ranging pulse is transmitted, the CU receiver listens in the gap to determine if it finds a ranging pulse in the gap, and, if so, if only one ranging pulse is present. Block 604 vectors to test 606 after transmission of the ranging pulse in order to listen on the control channel. The CU will transmit an S on the control channel if a single pulse is found in the gap, and will transmit an E on the control channel if the gap is found to be empty. If test 606 hears an S on the control channel, processing is vectored to block 608 to start the authentication process. If block 606 hears an E on the control channel indicating the gap is empty, processing vectors to block 610 to move the ranging pulse plus 8 chips, and processing vectors back to block 604 to send a new ranging pulse. Processing then vectors back to test 606 to listen on the control channel again. This loop continues until either an S for single pulse is heard on the control channel or a C for collision is heard. The CU sends a C when it hears more than one ranging pulse in the gap.
When test 606 hears a C, processing is vectored to block 612 to start the contention resolution process which is then performed as symbolized by block 614. Contention resolution continues until only one pulse is found in the gap or no pulse is found in the gap. If, as a result of contention resolution, no pulse is found in the gap, the CU sends an E on the control channel, which vectors processing to block 616. Processing then vectors to block 610 to move the ranging pulse 8 chips forward, and the process repeats itself.
An Authentication Process
Authentication is started when the CU sends a message on the control channel that it has found a ranging pulse from a single RU in the gap. In both embodiments, the gaps of multiple frames are used to send an authentication code. Each RU that has been attempting to synchronize hears the "S" on the control channel in step 606 in
A Contention Resolution Process
Referring to
The determination of whether more than one ranging pulse is in the middle 8 chips is performed by test 622 which counts the ranging pulses in the middle 8 chips of the gap and determines their locations. If the count of the number of ranging pulses found in the middle 8 chips is greater than one, the CU broadcasts a C on the control channel indicating a collision state, which causes all RUs to vector processing to their contention resolution protocols, as symbolized by block 624. If test 622 determines that the pulse count is 0 or their is a position error in the position of the single pulse found in the middle 8 chips, test 626 is performed to determine if the number of retries exceeds the maximum allowable number. If not, the process of block 620 is performed again to send new course alignment data to the RUs on the control channel. If the number of retries found by test 626 is found to exceed the maximum, the process of block 628 is performed where the CU broadcasts an E on the control channel indicating the gap is empty. This causes all RUs trying to synchronize to return to their ranging processes and start over at block 600 in
Once test 622 determines that there is only a single Rus ranging pulses in the gap and it is within the middle 8 chips, processing is vectored to test 630 which determines if noise has caused detection of what was thought to be a ranging pulse but which was only noise. This test is performed by determining if at least two out of three ranging pulses were received when the ranging pulse was commanded to move to the extreme left edge, the extreme right edge and the center of the middle 8 chips of the gap. If ranging pulses were detected at at least two of these three positions, no false alarm exists, and processing is vectored to block 632. If a false alarm is detected, processing is vectored back to test 626 to start over in positioning the ranging pulse.
The process symbolized by block 632 is the process of the CU broadcasting an A on the control channel which signals all RUs that are attempting to synchronize to send their authentication codes. Therefore block 632 states State=Auth which means that the CU is broadcasting an implicit request for the authentication ID (AUID) of the RU whose pulse is in the gap. In response, all the RUs trying to synchronize send their AUIDs in the form of four ranging pulses during the gaps of each of the next four frames of a superframe, each ranging pulse being located in a specific one of the 8 chips positions of the middle 8 chips in the gap. The positions and sequence during these four gaps of the authentication superframe tell the CU which RU has hit the gap. This is the meaning of the language in block 632 "Look for one pulse in each gap [one SF, Pulse Position Becomes No. 1–7]" The steps following block 632 just check for errors in this process. Specifically, test 634 is performed after each frame to increment a pulse counter and determine if the pulse count has reached 4 by the end of the superframe. If the pulse count is 4 at the end of the authentication superframe, test 634 vectors processing to block 636 where the CU broadcasts an FAE message on the control channel indicating authentication is finished and sends the AUID code out on the control channel for recognition by the RU that sent it. The AUID will be a sequence of 4 numbers from 0–7 which indicate in which chip of the middle 8 of the gaps of the authentication superframe each ranging pulse was found. Each RU that is attempting to synchronize will compare this sequence of 4 numbers to the 4 numbers of its AUID. If there is a match, that RU will know that it successfully hit the gap and will freeze its transmit delay timing at the number that puts its ranging pulse in the center of the 8 middle chips of the gap. Step 638 is then reached indicating that authentication is complete.
If test 634 determines that the pulse count is less than 4 after any gap in the authentication superframe is complete, processing is vectored to test 640 to determine if the number of retries exceeds the maximum allowable number. Test 640 sends processing back to block 632 to look for pulses in the authentication superframe gaps and record their positions until the superframe is over and the pulse count is less than 4. Some number of superframes with the RUs sending their AUIDs can be allowed in some embodiments. Eventually, the number of retries exceeds the maximum, and processing is vectored by test 640 to block 642. In block 642, the CU broadcasts an E on the control channel and, in response, all the RUs attempting to synchronize will return to the ranging process.
Likewise, if at any time, the count determined by test 634 exceeds 4 during the authentication superframe or at the conclusion thereof, an error has occurred or another RU has moved its ranging pulse into the gap. If this happens, test 644 is performed to determine if the maximum number of retries has been exceeded. If not, processing returns to block 632. Typically, more than one authentication superframe will be permitted with the RUs sending their AUIDs during each superframe. Eventually, after several superframes, if block 636 is not reached, test 644 will trigger vectoring of processing to block 646 where the CU broadcasts a C on the control channel indicating a collision has occurred thereby causing the RUs to return to their contention resolution protocols.
Referring to
If test 654 determines that there is a ranging pulse in the gap, processing vectors to test 658 where the CU determines if there is more than one ranging pulse in the gap. If there is only one ranging pulse in the gap, step 660 is performed where the CU broadcasts an S on the control channel indicating to all RUs that are ranging to begin their authentication processes.
If more than one ranging pulse is found in the gap, step 662 is performed to broadcast a C on the control channel indicating to the RUs that there is a contention and forcing them to carry out their contention resolution protocols. The CU then checks the status of an iteration stack to see if it is full. The iteration stack is used to keep track of the rounds of ranging for purposes of contention resolution and more rapid ranging of all RUs attempting to synchronize in some embodiments. The stack is incremented by one, and tested in test 664 to determine if the maximum number of iterations has been reached. If not, processing returns to block 652 to again monitor the gap for ranging pulses transmitted during the next frame. If the maximum number of iterations has been reached, step 666 is performed to broadcast an R on the control channel thereby causing all RUs to reset and start the ranging process again.
Referring to
When RU receiver synchronization has been achieved, step 676 is performed to pick some arbitrary delay and send a ranging pulse using that delay. Test 678 is then performed to switch on the control channel signal and determine the state of the CU. If the CU did not find any ranging pulse in the gap, it broadcasts an E on the control channel. Each RU then changes its delay by adding 8 chip times, as symbolized by block 680, and transitions to step 676 to send another ranging pulse. This process continues until one or more RUs set their delays such that their ranging pulses arrive in the gap. If the CU detects a single pulse in the gap, it broadcasts an S on the control channel which the RUs interpret as an authentication command. Each RU then transitions to step 682 to begin the authentication process, which has been previously described. Basically, the authentication process involves the RU sending its identification code as either a unique sequence of ranging pulse positions in the middle 8 chips of the gaps of multiple frames or as a unique sequence of the presence and absence of ranging pulses in the gaps of multiple frames.
If multiple RUs hit the same gap, test 678 finds that the CU is broadcasting a C on the control channel indicating that the RUs need to perform their contention resolution protocols, as symbolized by block 684. As symbolized by test 686, each RU then "flips a coin" to determine if it should continue and examines the outcome. If an RU decides not to continue, processing in that RU transitions to test 688 where the RU determines the control channel signal type. If an E is being broadcast, it means that all RUs that were ranging decided to stop, and processing returns to step 686 to "flip the coin" again. If test 688 determines that any other signal is being received, processing returns to block 672 and the ranging process starts over for that RU.
If the coin toss results in the RU deciding to continue ranging, step 690 is performed to send another ranging pulse. Then test 692 is performed to listen to the control channel and determine what the CU state is. If the CU found no pulse in the gap, step 694 is performed to move the ranging pulse, i.e., adjust the transmit frame timing delay, and try again. Accordingly, processing transitions back to test 668 through step 672. If the CU is broadcasting a C, more than one pulse has been detected in the gap, and processing returns to step 686 to flip the coin again to decide whether to continue ranging. If test 692 determines that the CU is broadcasting the S or authentication command, processing transitions to step 682 to begin authentication. After authentication, the CU sends fine tuning commands over the control channel to the RU which just authenticated itself to adjust the position of its ranging pulse to the center of the gap.
Referring to
Returning to the consideration of step 702, if the original coin toss caused the RU to decide to continue ranging, step 712 is performed to send a ranging pulse. Then test 714 is performed to listen on the control channel to determine the CU status. If a C is being broadcast, more than one RU is in the gap, and processing returns to step 702 to flip the coin again. If an E is being broadcast, the gap is empty and the delay for the next ranging pulse is adjusted by moving the pulse +8 chips and restarting the ranging process in step 716 by transitioning to step 600 on
Preferred RU Receiver Block Diagram
Referring to
The RF signals arrives at the receiver on coaxial cable 412 or other media. An RF demodulator section 750 synchronously demodulates the RF signals in the case of the RU receivers using a detector like that shown in
The RF demodulator 750 outputs an analog signal on line 752 carrying the chip amplitude information for all time slots. The RF section 750 also includes a passband filter having a center frequency centered on the frequency of the 6 mHz wide band carrying the chip data and having a 6 mHz bandwidth. The RF section also includes a variable gain amplifier that has a gain control input coupled to line 758 coupled to automatic gain control circuit 756. The AGC circuit works over a fixed interval and counts the number of times the input signal is above a preset threshold and the number of times it is below it. A counter is preset to a negative value at the start of the interval. Each time the threshold is exceeded, the counter in incremented. If the counter has counted up to zero at the end of the interval, the AGC gain is set correctly. Positive values call for decreased gain, and negative values call for increased gain.
The signal on line 752 is converted to digital information by A/D converter 754 which performs IF sampling as is known in the prior art was first described by Colinberg, whose papers are hereby incorporated by reference. The sampling rate is 4 times the symbol period. The advantage of using IF sampling is that it allows the use of one A/D converter to sample both the sine and cosine carriers. In alternative embodiments, two A/D converters may be used, each having a sample rate substantially greater than the symbol period. IF sampling is not critical to the invention and other techniques of digitization which are compatible with the system may also be used.
The gain of the signal represented by the digital data output by the A/D converter 754 is examined by automatic gain control (AGC) 756, and if the amplitude is not high enough, the AGC circuit generates a signal on line 758 to increase the gain of the variable gain amplifier in the RF section.
Phase separation of the sine and cosine components of the QAM modulated data represented digitally on bus 760 is performed by matched filter 761. The matched filter has two filters which have filter characteristics that are the mirror image of the squared raised cosine filter characteristics of the filters 1134 and 1136 in the shaping filter/modulator 570 shown in
Next, despreading of the data and reassembly of the appropriate data into the corresponding timeslots to undo the code shuffling that happened in the transmitters is performed. The first step in this process is accomplished by CDMA MUX 766. This multiplexer multiplies the incoming data by the transpose code matrix CT of the code matrix used by CDMA MUX 527 in the transmitter that sent the data. The resulting despread data is stored in buffer memory 768 sequentially in the order of the individual code multiplications. The CDMA MUX 766 or control logic 1082 generates suitable read/write control signals to cause buffer 768 to sequentially store the despread data on bus 776 output by the CDMA MUX 766. A deshuffler circuit 770 receives the same seed number on bus 772 as was received by code diversity shufflers 506 in the transmitters. The seed number is sent on the control channel, and is relayed to circuit 770 by the CPU 405. The deshuffler uses the seed number to generate the same pseudorandom numbers as were generated from this seed during every symbol time by the transmitter. These pseudorandom numbers are used to generate read address pointers on address bus 774 which are coupled to the address port of buffer 768 along with suitable read/write control signals. The data stored at the addresses indicated by the read pointers is then output by the buffer on bus 795. This bus is coupled to one of two inputs of a switch/multiplexer 791. Because the address pointers are generated in the same sequence as in the transmitters when shuffling data, the data read out of the buffer 768 is read out in the correct sequence to put the despread data back into the sequential order of the timeslots.
Other data received by the code shuffling circuit 770 on bus 772 are the Tss data indicating which timeslots are currently being received, and RI indicating which codes are reserved and cannot be used by this RU or CU.
This deshuffling operation is not necessary if the receiver is located in an RU because the CU does not use code hopping for data it sends to the RUs. Therefore, in the preferred embodiment of RU receivers buffer 768 and deshuffler 770 do not exist. These circuits are present in
In some embodiments, the despread data on bus 776 is simultaneously read by a crosstalk detector which functions to determine the amount of interference between adjacent codes and also plays a role in clock recovery so that all RU and CU receivers and transmitters can be synchronized to the same clock. Crosstalk between channels encoded with adjacent cyclic, orthogonal codes always comes from adjacent channels and happens when the data encoded with adjacent cyclic CDMA codes do not arrive precisely aligned in time. In other words, to have zero crosstalk, the clock time at which the first chip of a symbol transmitted on one channel spread with a cyclic CDMA code arrives at the receiver must be exactly the same time as the clock time at which the first chip of a symbol transmitted on an adjacent channel spread with an adjacent cyclic code. This requires precise frame synchronization to minimize crosstalk between channels. A slippage of one chip clock means complete overlap and total crosstalk since adjacent cyclic codes are generated by shifting the code by one place to the right. A slippage or misalignment of less than one complete chip clock will mean that some crosstalk exists. The crosstalk detector in these alternative embodiments detects the amount of crosstalk affecting each chip of each channel by subtracting the amplitude of the chip of the channel currently being processed from the amplitude of the corresponding chip encoded on the immediately preceding channel.
In these alternative embodiments, the amount of crosstalk is sent as a clock tracking error to a control loop logic 781 which outputs a clock phase/frequency correction voltage on line 782 (RU receiver only—the following discussion applies only to the RU receiver clock tracking loop in this alternative embodiment). This signal 782 is coupled to the phase/frequency control input of a voltage controlled crystal oscillator 784 in the RU receiver which generates a chip clock reference signal on line 786. This chip clock reference signal is fed to one input of a switch 787, the other input of which is coupled to receive an external clock reference signal at 8.192 Mhz. A switching control signal on line 791 from the CPU 405 controls whether switch 787 selects which of the chip clock reference signals on lines 786 or 789 for output on bus 793 to the time base circuit 886. In some embodiments, the clock signal is multiplied in a PLL (not shown) by a factor of two so that two clock signals can be fed to the time base circuit 886. This PLL multiplies the clock reference signal on line 793 to generate two output signals at 114.688 Mhz and 57.344 Mhz which are supplied on bus 888 to a time base generator 886. The time base generator generates the various clock signals needed for synchronization of the system, and these clock signals are coupled to every circuit in the receiver and transmitter which need them.
In the preferred embodiment for an RU receiver however, clock recovery is performed in the RUs by frame detector 882 using the fine tuning circuitry shown in
Although a global automatic gain control adjustment was made by AGC 756, data is being received in the CU receiver from many different RUs located at many different positions on the network. To minimize errors in interpretation of the upstream received data caused by amplitude variance caused by differing path length losses from the various RUs and channel impairments, a separate gain control adjustment is desirable for each RU. This is done by transmitting from each RU a preamble of known data before the payload data for each timeslot assigned to that particular RU as mentioned above. A variable gain G2 amplifier 788 is employed in the CU to amplify each timeslot's data with an individual gain value established to overcome the near-far problem so that the data from all the RUs, regardless of their distance from the CU, have the same amplitude level at the slicer 800. The same G2 amplifier is employed in the RU, but the gain value is fixed at one value for all the timeslots from the CU so that the CU signals to be adequately strong to be detected in the slicer and Viterbi decoder. Thus, in the RU receiver of
During reception of preamble data, the control loop 781 cooperates with the slicer 800, the G2 amplifier 788 and the rotational amplifier 765 to carry out an iterative process to reduce the slicer error to as low a value as possible by adjusting the amplitude error and phase error coefficients in the Upstream Carrier Recovery Error Correction Factor equation (Equation (5)) given above. Specifically, the CPU 405 and control logic 1082 will signal the control loop 781 and slicer 800 when preamble data is being received. Notification to the slicer 800 in
The process described above regarding synchronization in the upstream to the preamble data gives upstream carrier recovery synchronization. Frame synchronization and chip clock synchronization are done in the CU for the upstream data by the frame detector 882 using the coarse and fine tuning circuitry of
After synchronization to the preamble in the upstream data, the CU receiver control loop 781 uses the information received on bus 792 regarding which timeslot's data is currently being received to generate an address pointer to that timeslot's amplitude (1/a) and phase error (e−jø ) correction coefficients in memory 796. The control loop 781 then sends the address pointer to memory 796 via bidirectional bus 794 along with suitable read/write control signals and receives from the memory the amplitude and phase error correction coefficients for the particular timeslot being received. The control loop then places the amplitude and phase error correction coefficients on buses 790 and 802, respectively, to control the digital amplification process carried out by the amplifier 788 and the phase error correction process carried out by the rotational amplifier 765.
The slicer 800 is of conventional design, and includes circuitry to measure both gain and phase error for each channel's data. These errors are measured by circuitry in the slicer which compares the amplitude and phase of a received chip to the amplitude and phase of the legitimate constellation point which the received chip is supposed to represent. Recall that the constellation of
The phase rotation amplifier 765 adjusts the amplified data on bus 789 representing each received chip so as to rotate the phase thereof to correct the phase error for that received chip. This is done by a matrix multiplication of the complex number representing each chip by cosine(ø)+j sine(ø) where ø is the amount of desired phase correction.
In the RU receiver of the embodiment of
In the preferred embodiment, the master clock and master carrier signals generated by the CU modem are generated from the same temperature compensated VCXO by using different multiplication factors to generate the high speed clock and the master carrier signals. Thus, both the master clock and master carrier phase and frequency information are inherently embedded in the Barker codes transmitted by the CU during every CU transmit frame gap. The RUs in the preferred embodiment use the early late gating circuitry in the frame detector 882 in
Once carrier recovery has been achieved, the kiloframe data encoded in the pilot channel is recovered to achieve kiloframe synchronization so that the RU modem registers and software can be initialized to beginning counting CU frames so as to be able to keep straight which assigned codes from CU messages are to be used during which transmitted RU frames. The RU receiver decodes the synchronization sequence data on the pilot channel using a bpsk constellation. The CU transmitter generates the pilot channel signal as pseudorandom synchronization sequence of bits which are taken one at a time, bpsk modulated and transmitted on channel 1, one bit per symbol or 3 bits per frame. The RU generates its own matching pseudorandom sequence locally in a manner to be described below. The RU receiver frame detector demodulates and decodes the incoming pilot channel bits using its own internal slicer and compares them to its own matching pseudorandom pilot channel sequence. Each bit has only 2 possible digital values which defines 2 points in the bpsk constellation. If the incoming points are rotated in phase from one of these 2 points, the rotation is a carrier phase error and is used to generate a carrier phase steering signal on line 900 (line 900 carries both carrier steering signals and clock steering signals from the early-late gate sampling circuit in the frame detector) in
A kiloframe is 1024 frames long. As the pilot channel bits are received, they are compared to the local pseudorandomly generated pilot channel sequence. If communications were perfect, and frame synchronization is perfect, the incoming bits of the synchronization sequence would match the locally generated sequence exactly. A state machine counts the number of mismatches, and, if it is less than a threshold, frame synchronization is assumed, and the errors attributed to noise on the channel. If the number of errors exceeds the threshold, an interrupt signalling loss of frame synchronization is generated, and re-initialization is started. The kiloframe marker is detected in the synchronization sequence when a 16 bit feedback shift register which is loaded with 16 bits of the incoming bit stream of the synchronization sequence reaches a state which it only reaches after 1024 frames of bits of the synchronization sequence have arrived.
Specifically, referring to
After 1024 frames of the foregoing activity, the state of the output lines 1346 will be all 1's. This is the kiloframe marker. This state causes AND gate 1348 to sent a K_F kiloframe marker 1 to training generator 1352. This circuit is used to coordinate frame tag number counting in the RU receiver. The RU counts incoming CU frames by virtue of a counter which counts the occurrences of the GAP_a signal from the CPU 405. In the preferred embodiment, this counter is located in the time base 886 in
Returning to the discussion of
The receiver of
The equalization process involves some interplay between these FFEs and DFE as will be described below in the section on equalization. Both of the FFEs function to eliminate or substantially reduce precursor intersymbol interference, and both DFEs function to reduce or eliminate post cursor intersymbol interference.
Precursor and postcursor ISI can be understood as follows. If a transmitter were to send an impulse signal on one symbol with adjacent symbols empty, the receivers in an ideal system would receive the impulse with zeroes on either side of it. However, because of channel impairments, the receivers will receive an impulse and there will be some nonzero data in symbols on either side of the impulse. The nonzero data in symbols that precede the impulse symbol in time are precursor intersymbol interference. The FFE circuits removes this interference. The nonzero data in symbols that follow the impulse symbol in time is postcursor interference which is removed by the DFE circuits. The DFE circuit 820 receives as one of its inputs the decision data output by slicer 800 on bus 836 and processes these signals in accordance with the filter transfer function established by the tap weight coefficients received on bus 842 from a least means square calculation circuit. The resulting signals are output on bus 846 to the subtraction input of difference calculation circuit 767. The DFE and difference calculation circuit combine to subtract out that portion of the intersymbol interference produced by previously detected symbols from the estimates of future samples.
All the DFE and FFE circuits are FIR filters with adaptive tap coefficients. There is one main tap designated tap 3 and three secondary taps designated taps 0 through 2. The DFE circuit 820 and the FFE circuit 765 (circuit 765 is an FFE only during the equalization training period and is a rotational amplifier during payload data reception after training) receive their adaptive tap coefficients on buses 842 and 838, respectively, from the least mean square calculation circuit 830. The FFE/DFE circuit 764 receives its tap coefficients via bus 844 from the least mean square calculation circuit 830. The FFE and DFE FIR filters are given initial values for their adaptive tap coefficients that are close enough to allow the adaptation process to proceed. These preset coefficients are supplied from the CPU 405 via buses 824, 821 and 822. Thereafter, the coefficients are adaptively altered by signals on buses 842, 838 and 844 by the least mean squared circuit 830 using a conventional precursor and post cursor ISI elimination tap coefficient calculation algorithm.
The least mean square (LMS) circuit 830 iteratively calculates the new tap coefficients in a conventional manner and interacts with the FFEs and DFEs in the manner described below in the equalization section. The LMS starts with the initial tap weights and iteratively calculates the convolution sum between the tap input signals (input signals to each stage of the tapped delay lines) within the FFE 765 and the DFE 820 and the tap coefficients of the FFE 765 and DFE 820, all of which are obtained via bidirectional buses 842 and 838. The LMS then receives error signals on bus 831 calculated by difference calculation circuit 832 defined as the differences between the desired data points on bus 836 and the received data points on bus 834. The LMS then calculates new tap weights by multiplying the error signals times the corresponding tap input signals used to calculate the convolution sum times a predetermined step size which sets the rate of convergence to a stable value, and the result is added to the old tap weights to arrive at the new tap weights. These new tap weights are then sent to the FFE 765 and DFE 820 for use during the next iteration.
The LMS circuit implements a calculation which is based upon the fact that the needed change in the adaptive coefficients to the adaptive FIR filters 764 and 820 is proportional to the error on bus 831 times the conjugate of the data being input to the filters. In other words, the error is multiplied by complex numbers representing the received chips which have had the signs of their Q or imaginary components inverted.
The DFE filter eliminates or reduces post cursor interference by supplying a subtraction value on bus 846 to subtractor 767. The data sent by the DFE filter on bus 846 is subtracted from the data on bus 769 output by the FFE filter 765 during the equalization training interval. Eliminating the precursor interference and post cursor interference from the data on the bus 834 allows the slicer 800 and a Viterbi Decoder 850 to make better decisions about what chips were actually sent despite the channel impairments. The LMS, DFE and FFE circuits can be eliminated in some simple embodiments with, for example, only 4 points in their constellations. But to get more data throughput, more complex constellations are needed, and in such a situation, the points are closer together and ISI interference makes decisional discrimination between the constellation points more difficult. This creates a need for the above described ISI elimination circuitry.
After correction for ISI interference, the corrected data is passed via bus 834 to slicer 800. The purpose of the slicer is to make instantaneous decisions regarding which point in the constellation each chip represents for purposes of generating the gain and phase errors needed by the control loop and for purposes of generating the desired data on bus 836. The slicer does not make use of the 4th redundant bit in each chip for this purpose, and, as a result, makes errors in interpreting chips. It is up to the Viterbi Decoder 850 to correct these errors of interpretation.
Viterbi Decoders are well known in the art, and any Viterbi decoder algorithm will suffice for purposes of practicing the invention. The particular Viterbi algorithm used in the preferred embodiment is given below. Basically, Viterbi Decoder 850 and memory 852 keep track of the present and last state for each timeslot for purposes of tracing a path through a three dimensional space defined by the constellation of permissible input points stretched out over a third axis representing time which is orthogonal to the I and Q axes. There is one of these three dimensional spaces for each timeslot. By making use of the redundant bit or bits in each chip, and examining the path the states of each timeslot take through the appropriate 3-D space over time, the Viterbi Decoder makes a better informed decision as to which legitimate point in the constellation of permissible points each received code represents. The information on bus 792 to the Viterbi Decoder from the deshuffler tells the Viterbi Decoder which timeslot during which each code received on bus 836 was transmitted. The Viterbi Decoder uses this information to generate an address pointer to memory 852 pointing to the state information for that timeslot. This allows memory 852 to output the state information which is used by the Viterbi Decoder to make its analysis.
In the preferred embodiment, the following Viterbi algorithm is used.
% number of states
% for trace back in one symbol time.
% (if trace back is 1/3 in a symbol time C=2+3)
Dd = 12
% Decision Delay
for every input_symbol
for present_state 0:N−1
for i = 0:7
% loop on previous states
previous_state = f(present_state, i);
tx = f(previous_state,present_state); % possible transmitted signal
bm = f(tx,r);
% branch metric
pm_tmp(i) = pm(previous_state) + bm; % ADD, find path metric
% ADD with limiter (no overflow)
% pm_new(present_state) can be computed here by
minimum of 2 values
% previous_state_min can be computed here with
[pm_new(present_state),previous_state_min] = min(pm_tmp); % Compare &
% Select (find min & index), (can be computed in loop of i)
survivor(present_state,survivor_pointer) = previous_state_min; % update
% i (3 bits) can be saved instead of previous_state_min (4 bits).
pm = pm_new; % update path metric, or switch path metric memory
if mod(symbol,3) = 0, do: % begin trace back when 1'st symbol of frame is received.
surv_rd_add_l = min(pm) ; % survivor RD address
surv_rd_add_h = survivor_pointer;
start TRACE_BACK;
% While trace back is employed continue the process.
% circular increment survivor_pointer
% TRACE_BACK (Two options:
% a. Trace back all the survivor memory and output 3 symbols in one symbol time.
% b. For each symbol trace back 1/3 of the survivor memory.
% the trace back is employed while the ACS is employed too.
surv_rd_add = [surv_rd_add_h, surv_rd_add_l]; % RD ADD of survivor memory
for k = 1:Dd + 2
% trace back loop,
surv_rd_add_l_old = surv_rd_add_l; % save old address
surv_rd_add_l = survivor(surv_rd_add); % read survivor memory
surv_rd_add_h = dec(surv_rd_add_h); % circular decrement
if k >= Dd,
% Get 3 output symbols
out(0:2) = f(surv_rd_add_l, surv_rd_add_l_old) % output 3 bit symbols
The branch metrics are calculated after one symbol in normal mode and after two symbols in fallback mode, and then are stored in memory. The precomputed branch metrics are then used to calculate the path metrics. In fallback mode, the branch metrics of the two symbols are computed by summing the two square distances to each QPSK symbol. The branch metrics of the decoded symbols are summed to obtain one branch metric as in normal mode.
The particular Trellis code selected for implementation in the invention is rotational invariant with no parallel paths and 16 states.
After the Viterbi Decoder 850 outputs the correct data for each timeslot on bus 854, deframer 856 reassembles the data into the time division multiplexed timeslots in which these same data originally arrived at the framer circuit of the transmitter for encoding and CDMA spreading. The deframer 856 also descrambles the data to undo the effects of the scrambling carried out by the scrambler 524. The resulting TDMA stream of 9-bit bytes is output on serial data format bus 858. Each 9-bit byte in this data stream is comprised of the deshuffled, descrambled three tribits into which it was originally broken in the framer of the transmitter to form the three symbols of the frame during which this 9-bit byte was transmitted.
The output bus 854 from the Viterbi Decoder 854 is also coupled to a command and control channel circuit 860 which stores and/or processes codes sent on the command and control channels in the downstream data. Some switching or multiplexing function to select the command and control codes out of the stream of data on bus 854 is provided but is not shown. Codes sent on the access channel in the upstream or downstream data are stored and/or processed by an access channel circuit 862 which receives these codes from the output of the Viterbi Decoder 850 via bus 854. The command control code data is input to C3 circuit 860 from the Viterbi Decoder via bus 854. The CPU 405 accesses the command and control data and access channel communications from the C3 circuit 860 and the access channel circuit 862 via bus 1096. The processing of the command and control channel codes and access channel codes may also occur in circuits 860 and 862, respectively, in alternative embodiments without interaction with the CPU, or the codes may simply be buffered in circuits 860 and 862 until they can be read by a management and control process performed in the CPU 405.
The ranging process in its various embodiments described earlier herein is aided by the R/Tng circuit 763. This circuit receives an RU/CU signal on line 759 from the CPU 405 which tells the circuit whether it is performing its function in an RU or a CU. In the preferred embodiment, circuit 763 is simply a DMA FIFO which stores status information regarding positioning of the Barker codes in the guardbands during the ranging and initial frame synchronization process. This status information is received from the frame detector 882 via bus 883. This data is relayed to the CPU 405 via DMA transfers over bus 755 to a memory (not shown) coupled to the CPU 405. If it is performing its function in an RU, circuit 763 stores status data generated by the frame detector circuitry in implementing any of the functions indicated for any selected one of the embodiments of the RU in the ranging, contention resolution and authentication flow charts. This data may include data as to how many ranging pulses appeared in the gap and data to be sent to the ranging circuit 510 in the transmitter via bus 757 for purposes of setting transmit frame timing delay. These messages to the transmitter on bus 757 include data telling the transmitter ranging circuit 510 when the Barker code or other signal from the CU has been received in each frame thereby establishing the receive frame timing reference, whether to transmit another ranging pulse after contention resolution, and how to adjust the delay factor that establishes the transmit frame timing reference before sending each ranging pulse or Barker code, and, in some embodiments, what Barker code to transmit.
In the preferred embodiment, command, communication and control (C3) circuit 860 receives message traffic involved in the ranging, authentication and media access control processes as detailed in the flow charts and transmits this data to CPU 405 via bus 1096. Such data includes data from the CU indicating when authentication is desired and data regarding when to start sending that particular RUs authentication code. Circuit 860 also receives the authentication code broadcast by the CU after an authentication interval to determine if it is the RU that hit the gap. If so, circuit 860 sends a message to the transmitter via CPU 405 to freeze its current value for the transmit frame timing reference delay at the value last used for transmission of the ranging pulses in the authentication code sequence. The circuit 860 also monitors the control channel for instructions from the CU on how to adjust its transmit frame timing reference delay to exactly center the ranging pulse in the center of the gap.
If the signal on line 759 indicates the receiver of
Referring to
Training is performed immediately after ranging and periodically thereafter. If the insertion loss, phase response and group delay were known for the channel and the effects of dispersion on the pulse shapes were known, intersymbol interference could be effectively controlled by the matched filters 761 in the CU receiver of
The equalization training process occurs in every RU as part of its startup sequence. The prechannel equalization process starts with establishment by the RU controller of default precoder coefficients, a default transmit power level (input on line 566 to the scaler amplifier 564 in
In the preferred embodiment, the CU has an array of 8 correlators each of which correlates the received data using one of the first 8 orthogonal cyclic spreading codes. This arrangement is used for maximum speed. In other embodiments, a single correlator can be used on the buffered received data with the first 8 orthogonal cyclic codes being supplied during successive correlation intervals. In other alternative embodiments, the correlation can be done in serial or parallel in software.
The equalization process also is used for power alignment. Power alignment of all the RUs is the process of setting their transmit powers so that their transmissions all arrive at the CU at approximately the same power level. This is important in preventing interference between the signals from different RUs as well as in allowing the CU receiver's detectors to properly interpret the QAM 16 constellation points which are distinguished from each other in part by their amplitude levels. This process is started with step 1108 in which the RU transmitter causes the gain of scaling amplifier 564 to be set to one. The CU receiver control circuitry then causes the initial gain level for code 4 to be retrieved from memory 796 and transmitted through control loop 781 to the gain control input 790 of G2 amplifier 788. This initial gain level set into G2 amplifier 788 is an approximation of the proper gain level needed for this amplifier to allow slicer 800 to make proper decisions. Next, in step 1110, the CU waits for its adaptive gain control circuitry to settle in at a gain level needed for low or no error interpretation of the BPSK modulated data being sent during the training interval. The adaptive gain control circuit is comprised of slicer 800 which outputs amplitude error numbers on bus 798 in
Since the overall gain of the system for code 4 is the gain of the RU transmitter amplifier times the gain of the CU receiver amplifier, the overall gain of the system does not change by swapping the gains. This power alignment process happens only for the RUs. Each RU, when it powers up, has its gain level aligned in this manner and will use that gain level for subsequent operation sending payload data until the power alignment is subsequently performed again.
Processing now moves on to the equalization process for both the upstream and downstream data path equalizers. The idea in downstream equalization is to set the tap coefficients of the FFE equalization filters in the RU receiver to values which equalize for channel impairments based upon errors observed in training data sent via the 8 training codes by the CU to the RU. The idea in upstream equalization is to set the tap coefficients of the precode filter in the RU transmitter to values which equalize channel impairments based upon information received from the CU receiver after training data is sent by the RU to the CU using the 8 training codes. The 8 training codes are the first 8 orthogonal, cyclic codes. They can and are used simultaneously in both the upstream and downstream directions (as are the rest of the codes) because the upstream transmissions are on a different frequency from the downstream transmissions. Although the flow chart of
The preferred embodiment is discussed later herein.
The first steps in the upstream equalization process are symbolized by steps 1114 and 1116 wherein, in step 1114, the CU sends a message to the RU telling it to send some equalization training data (any data but preferably a pseudorandom PN sequence) to the CU using all 8 of the first 8 orthogonal, cyclic codes. In the embodiment shown in
Step 1118 represents the process performed in the CU of receiving the equalization training data and using FFE equalizer 765, DFE equalizer 820 and LMS circuit 830 to perform one iteration of tap weight (adaptive coefficient) adjustment for the tap weights of the FFE and DFE. Step 1120 represents the process of continuing to make tap weight adjustments on subsequent transmissions of equalization training data using the same first 8 orthogonal spreading codes until convergence is achieved when the error signals computed by difference calculation circuit 832 in
After convergence, step 1122 is performed wherein the CU sends the final tap weight coefficients from FFE 765 and DFE 820 to the RU. This can be done by the CPU 405 reading the tap weights from shared memory in which LMS stores them via bus 833 and transmitting them to the RU on the command and control channels. To calculate the new coefficients for the precode equalization filter 563 in the RU transmitter of
A block diagram of the structure of the precode equalization filter 563 in the RU transmitter of
Finally, the CU, in step 1126, sets the main tap of said FFE equalizer 921 in
In some embodiments, after the CU sends its tap weight coefficients to the RU in step 1122, the process of steps 1114, 1116, 1118 are performed again for several iterations. The coefficients extracted from the CU adaptive equalizer on the second and subsequent iterations cannot be used directly in the precoder, since they were not produced from default precoder values. Instead, the new coefficients for use by the RU are computed as the convolution of the old precoder coefficients with the new adaptive equalizer coefficients. At each iteration, the CU evaluates the coefficients extracted from the adaptive equalizer, and when the errors have dropped below a predetermined threshold indicating that the RU is transmitting with adequate quality, then the training process is completed except for downstream training.
Regardless of which upstream training embodiment is used, processing now proceeds to the downstream equalization training process. This downstream equalization process starts with step 1128 wherein the CU send equalization training data to the RU using all 8 training codes. Specifically, the CU sends a PN sequence simultaneously on 8 channels, each channel spread by one of the first 8 orthogonal, cyclic codes modulated using BPSK. Step 1130 symbolizes the process of the RU receiver receiving the equalization training data in multiple iterations and using the LMS circuit 830, the FFE equalizer 765, the DFE equalizer 820 and the difference calculating circuit 832 in
The iterations of the processes symbolized by
After equalization training, the RU link to the CU is active, and the RU can begin receiving messages after receiving a hello message from the CU. The hello message gives the RU the CU's software revision number and the superframe offset number. The revision number allows the RU to check its software revision number for compatibility, and the superframe offset number is set into a SFDOR register in the RU receiver time base for use in correctly reproducing an external time division multiplex stream superframe signal at the appropriate spot in the data stream so that external devices that depend upon the superframe signal can correctly interpret the TDM data.
Referring to
This same result can be achieved in a substantially different way by using Hilbert transform filters and carrierless amplitude and phase modulation. In the preferred form of modulator 507 shown in
The Fourier spectrum of the baseband, orthogonally code division multiplexed data on bus 568 r is shown as a constant amplitude spectrum 1138 of amplitude Ar on the real axis in
Filter 1136 also has a "squared-raised cosine" passband filter characteristic 1144, but its filter characteristic is located in the imaginary plane of the frequency domain shown in
When the baseband spectra of
Referring to
The CU modem transmits data in the downstream direction toward the RU modems using a transmitter 1170 that uses digital data to modulate one or more radio frequency carriers that are transmitted over the media 1162 after frequency conversion by up/down frequency converter 1174 to the proper assigned downstream channel frequency. The transmitter can use any modulation and any multiplexing scheme which can effectively transmit a master clock reference and a master carrier reference signal as well as payload data to the RU modems. The clock and carrier references may be transmitted either in-band or out-of-band. Data is transmitted in frames which the RU receiver detects. The RU transmitter achieves frame synchronization by the ranging processes described elsewhere herein or by any other means. Examples of multiplexing schemes that will work for the downstream direction CU transmitter are TDMA, synchronous TDMA, FDMA, Inverse Fourier, SCDMA or DMT (digital multitone transmitter). Any compatible modulation scheme can be used. Any of the conventional transmitters described in the treatises incorporated by reference herein will suffice for the CU transmitter, but an SCDMA transmitter is preferred. Non-SCDMA multiplexing schemes can be used in the downstream direction because the noise and interference problems are less severe than in the upstream direction.
The definition of "in-band" transmission of the clock and carrier is that one or more channels which would otherwise be used to transmit payload data are dedicated to transmitting the clock and carrier signals. The definition of "out-of-band transmission is that a separate carrier or some other subchannel/sideband etc. modulation scheme is used to transmit the clock and carrier information so that no timeslot or packets that could be used to send payload data is used to send clock and carrier information. In the embodiment of
The CU transmitter has a framing/addressing/packetization circuit 1166 which functions to receive payload data at an input 1168 and organizes said data into frames and addresses the data to the proper destination RU modem and the proper peripheral device coupled to that RU. The manner in which this is done is not critical to the invention so long as the downstream data is organized into frames since the upstream data is transmitted by SCDMA. The CU transmitter's framing addressing circuit 1166 can have the structure and operation of the framing circuit 400 in
The CU receives upstream radio frequency signals using an SCDMA receiver 1172. The function of the SCDMA receiver is to extract the payload data from the upstream RF signals. In the preferred embodiment, the SCDMA CU receiver receives the master clock and master carrier signals on lines 1181 and 1180 and periodically recovers the clock and carrier from the RU signals using the preamble data and Barker codes transmitted by the RU in the manner previously described. This upstream payload data is multiplexed by an SCDMA transmitter in the RU modem using orthogonal, pseudorandom spreading codes. Then a suitable modulation scheme such as QAM is used to control one or more characteristics of one or more RF carriers to generate the upstream RF signal. The CU receiver 1172 can have the structure of the receivers of
The RU modem 1164 has the following structure. A receiver 1190, having a demodulator and detector compatible with the type of modulation performed in the CU transmitter, is coupled to the transmission media 1162. The function of the RU receiver is to receive downstream RF signals transmitted by the CU transmitter, recover the master clock and master carrier of the CU, and synchronously extract the downstream payload data transmitted by the CU. The RU receiver also extracts any management and control data transmitted by the CU to coordinate the process of achieving frame synchronization and coordinates with the CPU 1204 and the RU SCDMA transmitter 1210 to carry out the ranging process or whatever other process is used to achieve frame synchronization. The RU receiver recovers the master clock and the carrier used by the CU transmitter in the manner described above or in any other way known in the prior art. The recovered master clock signal is distributed on bus 1214 to all RU circuits that need it including the SCDMA transmitter 1210 in embodiments that eliminate the tracking loops in the CU receiver. The recovered carrier signal is distributed by receiver 1190 on bus 1216 to all circuits that need it including the SCDMA transmitter 1210 in embodiments that eliminate the tracking loops in the CU receiver. The preferred method of recovering the clock in the RU is by encoding the clock into Barker codes sent during every gap by the CU and using a gap monitor/frame detector circuit like that shown in
The RU receiver 1190 can have the structure of the receivers described in
An RU transmitter 1210 receives payload data on bus 1220 from the peripheral devices or processes and organizes that data into frames of the same size as the CU frames. The data so framed then has its Fourier spectrum spread by the transmitter over a bandwidth much larger than said data originally had by orthogonal code division multiple access encoding or by performing an inverse Fourier transform operation. In some embodiments RU transmitter 1210 can be a synchronous TDMA transmitter. If code division multiple access is used, the spread spectrum data is then modulated onto one or more radio frequency carrier signals using a suitable modulation scheme such as QAM16 as described elsewhere herein. The process of organizing the upstream data into frames, spreading the spectrum of each frame of data and using the spread spectrum data to modulate one or more RF carriers is done synchronously with the CU using the master clock and master carrier signals recovered by receiver 1190 and output on buses 1214 and 1216, respectively. In some embodiments the RU local carrier reference signal on line 1216 is a different frequency but phase coherent with the master carrier. The resulting RF signals are output on line 1224 to an up/down frequency converter 1226 where the frequency is converted to the designated frequency of an upstream frequency band, usually 6 mHz in width, and then the resulting signals are output on line 1228 to the transmission media 1162. Therefore, frequency division multiplexing for the upstream and downstream traffic is employed to share the transmission media between upstream and downstream channels. Those skilled in the art will appreciate that the system of the invention uses a combination of time division multiplexing, frequency division multiplexing and code division multiplexing to achieve high-performance, multiple-user, multiple-source bidirectional digital data traffic in a distributed communication system.
Frame synchronization is achieved, in the preferred embodiment, by the trial and error process of adjusting the transmit frame timing delay described elsewhere herein and then transmitting a unique code such as a Barker code which the CU receiver can detect using that transmit frame timing delay following detection of the CU gap by frame detector in RU receiver 1190. The CU modem includes a gap monitor circuit 1192 (shown separately but which could be inside SCDMA receiver 1172) that functions to monitor the CU guardband or other interval included in each frame to which the RU transmitters are trying to synchronize to determine if one or more RU Barker codes have been received. The gap monitor circuit can have the structure shown in
The RU receiver 1190 receives these management and control messages and passes them on bus 1202 to a computer 1204 which uses the management and control data to control the ranging process carried out by said SCDMA transmitter 1210 and Ranging Generator 1206 and for other purposes. The gap monitor circuit in receiver 1129 supports the CU gap acquisition process by locating the time of each CU frame gap. This gap monitor circuit listens for Barker code data transmitted by the CU during every gap, usually by correlating received energy against the known Barker code data pattern, and sends gap acquisition data detailing the receipt of correlation pulses and the relative times of their occurrence to computer 1204 via bus 1202. In some embodiments, the gap monitor circuit is structured like the frame detector of 34 and uses the Barker code transmitted by the CU to recover the CU master clock by an early-late gating technique.
Computer 1204 or other control circuitry uses this gap acquisition data to determine the time of receipt of the Barker code thereby establishing a frame boundary reference for the receiver to aid it in demodulating, decoding and deframing the received data and a reference time from which to start the transmit frame timing delay. Specifically, the computer 1204 uses the receive frame timing reference during the ranging process to establish the starting time against which to measure a trial and error value for the transmit frame timing delay value Td, and then sends this transmit frame timing delay value Td on bus 1212 to the RU transmitter 1210 to control the delay between the time when a frame of downstream data arrives from the CU transmitter, and the time the RU transmitter 1210 sends the same frame back to the CU receiver with new upstream data therein. During the ranging process, the value of Td is varied experimentally during successive Barker code transmissions until management and control data is received by the RU indicating that the Barker code has been centered in the CU frame gap thereby achieving frame synchronization. Once frame synchronization has been achieved, the computer 1204 freezes the value for Td thereby causing the SCDMA transmitter to send its frames in frame synchronization with the CU frames and frames transmitted by all other RUs.
The frames all have numbers and can be visualized in the following manner. A downstream frame travelling from the CU transmitter to the RU receiver is like a bus with a load of particular people, the people being the payload and management and control data in the frame. When that bus arrives at the RU, the people are unloaded, and a new set of people get on representing the payload and management and control data the RU wants to sent to the CU. After delay Td, the bus leaves the RU and travels back to the CU. The different channels of data can be visualized as different buses each destined for a different RU although in reality, they are data from different timeslots in the same frame which is received by all RUs. Frame synchronization is the process of setting the value of Td properly in each RU after a trial and error process so that the buses from each RU travelling toward the CU all arrive at the same time.
As a further operation in achieving frame synchronization, the computer 1204 also enables a ranging generator circuit 1206 via signals on a bus 1208 and passes messages to the ranging generator to control its operation. The ranging generator 1206 functions to generate and send to the RU transmitter 1210 data defining the Barker code for transmission during a ranging process, the power level for transmission and the unique on-off morse code authentication signature sequence that is used to identify each particular RU during the ranging process.
Computer 1204 also generates and sends management and control data to the RU SCDMA transmitter 1210 via bus 1212. This management and control data can include requests to start ranging, requests for more bandwidth, messages relinquishing bandwidth etc for various species within the broad genus of the invention.
Referring to
The signals on bus 1240 are corrupted by noise and impairments that degrade the upstream channel. In addition, the phase and amplitude errors, for each RU need to be removed. Accordingly, the detector 1246 includes a G2/rotational amplifier and a memory for storing gain and phase adjustment numbers, as well as a slicer. When preamble data is being received, the CPU so informs the detector by signals on bus 1241. This bus is also used to inform the detector which timeslot is being received so that the detection can retrieve the proper gain and phase connection factors. The corrupted phase and gain adjusted information signals can then be processed by a conventional decoder in detector 1246 to determine the actual constellation points that were sent. In the preferred embodiment, the decoder is a Viterbi decoder and FFE and DFE equalization is optionally used along with Trellis modulation at the RU transmitter to improve throughput, decrease error rate and improve signal to noise performance.
The information vectors resulting from the decoding process are then output on bus 1248 to a deframer circuit 1250 which functions to reassemble the original payload data streams, ATM cells, LAN packets or TCP/IP packets and management and control messages from the information vectors in each frame. The payload data is output on bus 1252 to the peripherals and interfaces to the phone network, internet etc. The management and control data are output on bus 1254 to the CPU 1194 for use in processing such as assigning channels and dynamic bandwidth management in species that implement these functions as opposed to fixed channel assignments.
An RU SCDMA receiver could be structured like the receiver of
Referring to
The framer circuit 1260 outputs information vectors on bus 1272 to one input of a switch 1274. The other input bus 1276 of this switch is coupled to the computer 405 and carries management and control data generated by the computer. The switch 1274 has a single output bus 1276 which is coupled to the data bus input of a buffer memory 1278. The buffer memory serves to store the final information vectors which will be input on bus 1280 to the spectrum spreading multiplexer 1282. The multiplexer 1282 functions to spread the Fourier spectrum of the data on bus 1280 over a much larger bandwidth than the data originally had. In the preferred embodiment, the multiplexer 1282 is a direct sequence code division multiplexer like those described elsewhere herein which carries out matrix multiplication between the information vectors and a plurality of pseudorandom, orthogonal codes, which are preferably cyclic codes. At least one code is assigned to each RU that has payload data to send, and in the preferred embodiment, multiple codes may be assigned when more bandwidth is needed by an RU. The number of codes assigned to an RU can be dynamically varied by exchanges of messages between the RU and CU via management and control channels. The computer 405 receives code assignment messages from the CU as well as other management and control data which supports, for example, the ranging process, via bus 1288 from the RU receiver.
The computer 405 carries out the assignment of codes per instruction from the CU (or frequency components in FFT and DMT embodiments) by controlling read pointer addresses on bus 1284 coupled to framer circuit 1260 and write pointers on bus 1286 coupled to buffer 1278. The read pointers control the addresses in the framer circuit from which data is read for output on bus 1272. The write pointers control the addresses to which data on bus 1276 is written into buffer 1278. Since the contents of any particular address in buffer 1278 always get multiplied by the same code, by controlling these write pointers, the computer can implement the code assignments and put management and control data in the correct channels if specific channels are assigned for management and control data. The embodiment of
The transmitter of
Whatever the spectrum spreading process carried out by block 1282, the resulting data is output on bus 1290 to one input of a switch 1292. The other input bus to this switch is coupled to receive Barker code data on bus 1294 from a Barker code generator/authentication sequence generator 1296. The switch functions to selectively couple the data on bus 1294 to the input of a modulator 1298 via bus 1300 when the CPU changes the state of the switch via control line 1302 at time delay Td after the CU gap is detected during ranging. Generally, the switch is controlled to send the Barker code data on bus 1294 to the modulator 1298 during the ranging process and to send payload data on bus 1290 to the modulator during normal operation after frame synchronization has been achieved. Computer 405 controls the Barker code generator 1296 via data on bus 1304.
The payload data extraction process is done synchronously in the CU and RU modem receivers. "Synchronously" as that word is used in the claims means the following forms of synchronization are practiced in the RU receiver, CU SCDMA receiver and the RU SCDMA transmitter. The RU transmitter uses the recovered master clock and master carrier reference signals recovered by the RU receiver to drive its digital circuitry and modulator in synchronism with the CU master clock and master carrier. Coherent detection is performed in the CU SCDMA receiver using the master carrier signal on line 1180 and a rotational amplifier or using a recovered carrier from either an in-band source like the pilot channel described elsewhere herein or some out-of-band source and driving the demodulator with the recovered carrier. In the preferred embodiment, the CU's SCDMA receiver uses its own master clock and master carrier without recovering either from the signals transmitted by the RU. These signals plus information derived from each RU's preamble data provides knowledge in the CU SCDMA receiver of the RU's SCDMA transmitter carrier phase and frequency. An RU SCDMA or other type of receiver recovers the master carrier reference from, for example, the pilot channel transmitted by the CU and recovers the master clock reference from the Barker codes sent by the CU during the gaps of every CU frame. Those recovered master clock and master carrier signals are used for synchronous demodulation in the CU and to synchronize the detector in the RU receiver and are also used by the RU SCDMA transmitter. Frame synchronization is also part of the synchronization implied by the term "synchronously" in the claims.
Referring to
The modulated RF signals are output on line 1394 to an up/down frequency converter 1396 which converts the frequency thereof to a downstream frequency which will not interfere with other services sharing the transmission media 1398 such as cable TV programming fed into the media from bus 1400. The frequency converted signals are output on line 1402. Frequency conversion is optional if the master carrier in the CU modem can generate a carrier at the desired downstream frequency and the upstream channel can be some frequency which can be synchronized to the downstream frequency such as a harmonic. An RU modem 1404 receives the downstream data on line 1408. A TDMA receiver 1406 coupled to line 1408 recovers the master clock and master carrier reference signals using any conventional circuitry or the circuitry and methods disclosed earlier herein in the STDMA embodiments. The TDMA receiver 1406 outputs the recovered clock signal on line 1410 and outputs the recovered carrier signal on line 1412. The recovered payload data is reassembled into a TDMA data stream and output on bus 1414 to peripherals or other interface processes.
Those peripherals or other interface processes also supply a TDMA input data stream on bus 1416 to an RU synchronous TDMA transmitter 1418. This transmitter receives the recovered clock and recovered carrier signals on lines 1410 and 1412, respectively, and synchronously organizes the TDMA input data on bus 1416 into timeslots in TDMA frames having the same duration as the CU frames. These frames are then modulated onto one or more carrier signals using the same (or different) modulation scheme used by the CU transmitter, and the frames of modulated RF signals are transmitted to the CU in frame synchronization with the CU. That is, the RU frames are transmitted from the RU transmitter with a transmit frame timing delay set for this particular RU's position in the system relative to the CU such that the frames transmitted by the RU arrived at the CU aligned with the CU frame boundaries. All RU modems in the system have their transmit frame timing delays set for their particular positions on the network so that all their frames arrive at the CU aligned with the CU frame boundaries. All RU's also send preamble data prior to sending payload data for use by the CU in determining the phase error for that particular RU. This preamble data is used in the manner described above for the SCDMA embodiments to find the phase error.
The modulated RF data output by RU TDMA transmitter 1418 is coupled on line 1420 to an up/down frequency converter 1422 that functions to change the frequency of the upstream channel to a frequency that is far enough removed from the downstream channel frequency and from the cable TV programming so as to not interfere therewith.
The upstream data is then transmitted via line 1424 and the transmission media to a CU TDMA receiver 1426. This receiver receives a master clock signal on line 1428 from the master clock oscillator 1384 and receives the master carrier signal on line 1430 from the CU's master carrier reference oscillator. The CU TDMA receiver 1426 also receives a PREAMBLE signal on line 1441 from the CU CPU 1442. This signal is activated when preamble data is being received. The CPU also supplies receiver 1426 with an RU ID signal on line 1439. This signal tells the receiver 1426 which RU's data is being received in the timeslot currently being received. The CPU keeps track of which RU's data is being received with the help of data received on line 1440 from receiver 1426. The receiver 1426 includes a frame detector which detects the Barker codes transmitted during the ranging process by the RU's. The TDMA receiver's frame detector can have the same structure as the frame detector previously described herein. The data on line 1440 tells the CPU when each RU has achieved frame synchronization. From that point, the CPU knows that the RU frames are coincident with the CU frame boundaries. Line 1440 also carries data received from the RU's requesting bandwidth. In response to these requests, the CPU would assign one or more timeslots to the requesting RU. These assignments would be conveyed to the RU's by management and control messages generated by the CPU 1442 and sent to the CU transmitter via bus 1444. The CPU keeps track of which timeslot is being received with the help of master clock data on line 1443. The CPU then looks up the RU assigned to each timeslot and sends that information to the TDMA receiver 1426 as the RU ID signal on line 1439.
The CU TDMA receiver 1426 recovers the payload data from the modulated upstream signals and reassembles the payload data into a TDMA output data stream on bus 1432.
The TDMA transmitters and receivers in this system can be conventional, but the RU TDMA transmitter must be able to delay transmission of its frames by a variable transmit frame timing delay so that its frames arrive in frame synchronization with the frame boundaries of the CU. Any ranging process described herein or any other known ranging process can be used to achieve this frame synchronization. If any of the trial and error class of ranging processes described herein is used, computer 1434 in the RU modem sets an initial transmit frame timing delay either at its own initiative or upon receipt of a ranging solicitation message from the CU via a management and control data path 1436 from the receiver 1406. This initial delay value is sent to the RU transmitter via bus 1438. The CU receiver assists in the ranging process by sending data regarding what signals from the RUs it found in the frame gaps if gaps are used or what RU ranging signals were detected over the frame interval via bus 1440 to CU computer 1442. The CU computer 1442 sends feedback ranging data to the RU via bus 1444 coupled to the CU transmitter 1386.
In the class of embodiments where the CU does the ranging process for the RU by determining how much the RU must move its ranging pulse to achieve frame synchronization and so instructing the RU, bus 1440 still carries data regarding what ranging pulses the CU receiver saw. However, computer 1442 then figures out how much delay the RU needs to add to or subtract from its transmit frame timing delay by calculating the total turnaround time and then sends a message via bus 1444 to the RU so instructing it. This message reaches RU computer 1434 via bus 1436, and the computer 1434 sets the instructed delay via bus 1438. Any other ranging process that can achieve frame synchronization other than the ones described herein will also suffice to practice this particular embodiment.
All of the transmitter embodiments disclosed herein can utilize an active bandwidth management process carried out by bidirectional message traffic between the remote units and central unit over the management and control channels. Remote units can request more or less bandwidth on a first-come, first-served basis or the RU's can request reserved bandwidth, i.e., bandwidth that has been reserved to each RU but which can be loaned to other RU's until the RU for which the bandwidth is reserved need it. The central unit can evaluate RU privileges for bandwidth reservation, privileges, etc., and arbitrate conflicting requests for reserved bandwidth or more bandwidth and then award bandwidth in accordance with the results. The CU then sends downstream management and control messages telling each remote unit which codes have been assigned to carry its traffic during specified frames.
Upstream Time Alignment Algorithm
Section 1.1 Time Alignment Procedure
Time alignment is the procedure by which fine delay adjustments are made to provide exact frame synchronization of all RU's to exactly center the RU Barker codes in the gaps. (Coarse frame synchronization is accomplished by ranging). Every RU undergoes an initial upstream training process to achieve coarse frame synchronization, exact centering of its Barker code in time alignment, power alignment to solve the near-far problem, and equalization to predistort transmissions to minimize the effects of channel impairments. Time alignment fits into the overall upstream training procedure as follows:
For initial training:
1. Ranging by any of the processes previously described
2. Time alignment
3. Power alignment
4. Time alignment
5. Power alignment
6. Equalization
7. Repeat steps 4–6 N times
For periodic training:
1. Time alignment
2. Power alignment
3. Equalization
Section 1.2 presents the requirements for one embodiment of time alignment, with input requirements dictated by ranging and output requirements dictated by system performance. Section 1.3 gives a detailed description of the time alignment algorithm of one embodiment with Section 1.4 giving alternative enhanced embodiments to the algorithm of Section 1.3.
1.2 Time Alignment Requirements
The ranging procedure is capable of aligning the upstream Barker code transmission to within +/−1 chips of the center of the gap and aligning the power to within a number of dB which is acceptable for the system performance criteria. Time alignment must operate within these constraints for this embodiment.
The demultiplexer and equalizer circuits require fine time alignment to within +/−1 high-speed clock in this embodiment.
Initial time alignment must be accomplished within a number of seconds in this embodiment which is acceptable for the system performance criteria.
Periodic time alignment must be accomplished within a number of seconds in this embodiment which is acceptable for the system performance criteria.
Time alignment phase shifts must be compensated in this embodiment.
1.3 Algorithm Description
The time alignment algorithm has two main components: coarse time alignment and fine time alignment. Coarse alignment can begin with offsets of up to +/−8 chips and align to within +/−½ chip. Fine alignment begins with offsets of up to +/−1 chip and aligns to within +/−1 high-speed clock.
1.3.1 Coarse Alignment Algorithm
Coarse alignment uses the two ASIC ranging registers (the modem transceivers described herein are typically implemented in ASICs), RGSRH and RGSRL. These registers contain information regarding the location of the Barker correlation peak. The registers are each 16 bits, with each bit representing a ½ chip spacing such that the alignment window is +/−8 chips. Referring to
Coarse alignment tries to align the RGSRH and RGSRL register to equal 0x0000 0x8000 or 0x0000 0x C000 for the 1B ASIC and 0x0001 0x0000 or 0x0001 0x8000for the 2A ASIC. The coarse alignment algorithm is summarized in the pseudocode algorithm given below (the actual code in the appendices hereto varies somewhat in that it is structured differently and it must handle both versions of the ASIC).
Coarse Time Alignment Algorithm Pseudocode Fragment
% Acquisition mode
iterations = 0;
while ((iterations)++ < TIMEOUT) { % TIMEOUT = 10
if (iterations > 4)
fine_shift = 1; % Gear shift to avoid oscillation
fine_shift = 2;
read non-zero value of RGSRH and RGSRL; % try up to 8 times
if (RGSRH == 0 && RGSRL == 0) return (TIME_ALIGN_ERROR);
if (peak 1/2 chip to right of center)
shift_value = fine_shift;
else if (peak is > 1/2 chip to right of center)
shift_value = 4;
else if (peak is 1/2 chip to left of center)
shift_value = -fine_shift;
else if (peak is < − 1/2 chip to left of center)
shift_value = −4;
else return (TIME_ALIGN_DONE)
SendPowerCommand (shift_value); % Send to RU
}% end while loop
if (iterations == TIMEOUT) return(TIME_ALIGN_ERROR);
The time required for coarse time alignment is dominated by the number of iterations required to converge since each iteration involves the CU having to send a command to the RU via the command and control channels. With a worst-case timing offset of +/−2 chips, the coarse alignment will take at most 10 iterations to converge.
With each command taking 64 frames, this amounts to 640 frames or 80 msec.
1.3.2 Fine Alignment Algorithm
The fine alignment algorithm uses the ClocK Recovery Error Register in
The fine time alignment algorithm of the embodiment currently being discussed is given below as a pseudocode fragment. The algorithm consists of reading N CKRER samples and averaging them. This averaged value is used to determine if time alignment had been accomplished. If it is not, the sign of the averaged CKRER value in the CU frame detector of
Fine Time Alignment Pseudocode Fragment
while ((iterations) ++ < TIMEOUT) { % TIMEOUT = 8
Read CKRER N times and take abs value of ave =>abs_ave_ckrer;
/* Check if below threshold or zero crossing */
if (abs_ave_ckrer < threshold) ||
(sign(abs ave_ckrer) == sign(previous_abs_ave_ckrer))
return (TIME ALIGN_DONE);
else {
if (abs_ave_ckrer < 0)
shift_value = −1;
shift_value = 1;
}% if
} % while
The time required for fine time alignment is dominated by the time required to read the 240 CKRER values. Because of software constraints, 8 CKRER samples are read each 64 frames. So to read 240 CKRER samples each for 8 iterations requires 64*30*8=15360 frames or 1.92 seconds.
1.3.3 Time Alignment Phase Shift Compensation
Time shifts at the RUs result in phase shifts at the CU. These phase shifts will cause bursts of errors in periodic training and therefore must be compensated. Each high-speed clock (57.344 MHz) offset results in a 22.5 degrees phase offset of the IF-1 (3.584 MHz) carrier frequency. The RU transmitters transmit at IF-10 (35.84 MHz), so the IF-1 carrier is multiplied by a factor of 10. This means that the phase offset is also multiplied by a factor of 10 so that for each high-speed clock offset, the upstream signal is phase shifted by 225 degrees.
The precode equalization filters 563 in the RU transmitters can be used to compensate for these changes. By multiplying all four feed-forward coefficients by the negative of the phase shift caused by a timing offset, the phase shift is exactly offset, thereby enacting only the desired time shift of the fine tuning process.
1.4 Performance Enhancements and Optimization
1.4.1 Coarse Alignment Optimization
Worst-case, the coarse alignment algorithm takes 80 msec to achieve alignment. In order to reduce this time, the number of alignment iterations which are required to converge would have to be reduced. Reducing the number of iterations, makes the algorithm more vulnerable to noise. Given that course alignment procedure is effectively only accomplished during initial training, it is not sped up in the preferred embodiment.
1.4.2 Fine Alignment Optimization
Worst-case, the fine alignment algorithm takes 1.92 seconds to achieve precise alignment. This worst case will likely only be reached in initial training. In periodic training, the system usually performs only one or two iterations for fine alignment. If the CKRER register is read in two iterations (one to detect an offset and another to verify convergence), this gives a fine alignment time of 0.48 seconds. If there are 2000 RU's in the system, this equates to a minimum training period for each modem of 16 minutes ignoring all of the other aspects of training. This fine alignment execution time can be reduced in some embodiments where such a delay is not acceptable.
The simplest option for reducing this time which is implemented in some embodiments is performing less averaging of the CKRER register. In the embodiment implemented by the software appendices appended hereto, 240 CKRER samples are averaged. Preliminary laboratory results have shown that averaging over only CKRER samples yields comparable results. In alternative embodiments where such a reduction is implemented, execution time is reduced by a factor of 8. This gives a worst-case number of 240 msec for initial alignment and a worst-case typical number of 60 msec for periodic alignment.
Another embodiment which applies only to 2A modems (and above) uses the demultiplexer memory instead of the CKRER register to sense fine timing alignment status. The demultiplexer memory stores the symbols from all 144 codes output from the demultiplexer. This memory can be used for time alignment by having the RU send only code 4 in BPSK mode. If the RU is perfectly aligned (and ignoring ISI), the CU will see energy only in code 4, with all of the other codes equal to zero. However, if there is a timing misalignment, this will result in the code "spilling" into adjacent codes. By analyzing the direction of spilling, the direction of the timing offset can be determined.
In order to make the timing measurements independent of phase (since the phase of the received code is not known), the absolute value of the received codes is taken. However, a blind absolute value will rectify the noise so that it cannot be averaged away.
This problem is eliminated by using the soft decisions to take the absolute value. If the RU is aligned within +/−½ chip, the phase of code 4 will equal the phase of the received symbol. This phase can be used to rectify that transmitted symbol. Since the noise is uncorrelated to the sign of the symbol, it will be reversed in sign. Subsequent averaging of these absolute value symbols will result in an averaging of the noise, but will not average away the transmitted symbols (which is what would happen if averaging was performed before a blind absolute value).
Define dmm(3), dmm(4) and dmm(5) as the complex symbols from the demultiplexer memory relating to codes 3, 4, and 5, respectively. Then the time alignment error is given by Equation 6, below, with the absolute value function being performed by multiplying the difference of the adjacent codes by the conjugate of code 4 and then taking the real part of the result.
error=real[(dmm(3)−dmm(5)) ((dmm)*(4))]/ (abs (dmm(4)))) (6)
This error is averaged N times and used in exactly the same manner as the averaged CKRER is used in these alternative embodiments as described above in subsection 1.3.2.
In order for Equation 6 to be valid, dmm(3), dmm(4) and dmm(5) must all correspond to the same symbol.
If the equalization process has stabilized, step 1515 is performed to normalize the symbol equalizer coefficients by dividing them by the value of the main tap SECFF(3). Then step 1519 is performed to convolve the old chip equalizer coefficients and the normalized symbol equalizer coefficients divided by two to derive the new chip equalizer coefficients for downstream or the new precoder coefficients for the upstream. The main tap coefficient of the SE feed forward equalization filter is then set to one and the side tap coefficients of the SE feed forward and decision feedback equalization filters are set to zero for receipt of payload data. Then test 1522 is performed to determine if the equalization process has converged. The equalization convergence test process symbolized by step 1522 is shown in more detail in the flow chart of
The process of
Rotational amplifiers which work on QAM16 constellations can lock improperly on false minima in the error surface of the constellations and cause improper decisions to be made by the slicer. The purpose of the process symbolized by the flow chart of
Referring to
Power Alignment Procedures
The near-far problem in upstream transmission is solved partially by the ranging process which does a coarse power alignment. In the preferred embodiment, a fine tuning of the power level of each RU is also performed so that the power of each RU's transmission as received at the CU is approximately the same. The power level of each RU at the CU is detected by a gain detector which, in the preferred embodiment, if the main tap amplitude correction factor of the SE. The RU transmitter power of each RU is adjusted with the help of information from the CU gain detector. The CU expects specific received power levels from each RU (0 dBmv). The RU transmitter power ranges from 32 dBmv to 52 dBmv. The power alignment fine tuning process is accomplished as follows and as depicted in the flow chart of
- 1) CU asks RU to transmit all training codes. These are the 8 training codes used for the equalization training process.
- 2) CU run the SE equalization process for N frames (N=80) (when the training codes are being sent, the CPU in the CU enables the SE in the CU receiver to iterate for 80 frames) with
SEKR=0x0855H (SEKR is a register in the LMS 830 in
SECFF(3) enabled and all other taps disabled (only the main tap of the FFE filter in the SE is enabled for fine tuning of the power alignment so as to act as the gain detector, and all side taps are disabled, as symbolized by step 1602).
- 3) Calculate delta1=(SECFF(3)^2−1FFFH)/k1, where k1=64 (this is the process of calculating the amplitude of the coefficient of the FFE filter main tap in the SE as the sum of the squares of the real and imaginary components I and Q; this amplitude has subtracted from it the expected value of the expected main tap value, 1FFF hex, when power alignment has been achieved; the difference is then divided by the constant 64, which in this embodiment is set equal to 64; the result is called delta1—all as symbolized by step 1604). The power alignment process of
FIG. 65differs from the power alignment process of steps 1108 through 1112 of FIG. 53Ain that the process of FIG. 65uses the main tap of the SE filter as the gain sensor in the CU after convergence by the SE whereas in the process of FIG. 53A, the gain control number for power alignment is derived by convergence of an adaptive gain control circuit comprising the slicer, slicer error signal, control loop 781 and the variable gain amplifier 788 in CU receiver. Further, there is no necessity in the power alignment process of FIG. 65to set the RU gain level at one before transmitting the training data.
- 4) If abs(delta1)<TH, Power alignment is done (this is the process of comparing deltal to a small threshold; if deltal is below the threshold, power alignment is completed and processing vectors to step 1608; if not, a message is sent to the RU to lower its power in the next step—all as symbolized by step 1606)
- 5) If not, send to RU a power adjustment factor equal to delta2=delta1 *2^(−ki). The factor delta2 is a power adjustment factor telling the RU a correction factor by which to adjust its power, with the correction factor larger for larger differences over the threshold than for smaller differences; the correction factor is equal to delta1 times a constant—this process is symbolized by step 1610 in
- 6) RU updates TXLVLR=TXLVLR+delta2 (this is the process symbolized by step 1612 wherein the RU updates the value in register TXLVLR which controls the power level of the RU transmissions by adding the value of delta2 to the register contents) Steps 1612, 1614, 1616 and 1618 in
FIG. 65are the steps which determine if the number of desired iterations of the power alignment process have been achieved.
Boundless Ranging Preferred Embodiment Previous Ranging Scheme
With the exception of the boundless ranging scheme wherein the CU calculates the total turnaround time for each RU and sends that data to the RU, the previously described ranging schemes do not deal with boundless distance ranging. Boundless ranging schemes are especially useful in large systems where there is an RU at the head-end in addition to many RU modems distributed throughout a system coupled by multiple fiber nodes, a situation depicted in
In addition, the previously described ranging scheme requires a calibration procedure with the modem at the optical node in order to find the latency from the head-end to the optical node. The other ranging schemes described herein (with the exception of the alternative boundless ranging scheme mentioned briefly above, required that all RUs be close enough (16 kilometers) of the CU such that their TTA was less than one frame (125 microseconds). In these embodiments, it was assumed that there was an RU at the optical node and the CU receive window and gap was offset from the CU transmit gap by the amount of the TTA to the first optical node such that if the RU at the optical node sent back the CU barker code immediately without delay, it would arrive at the CU later by the TTA to the optical node. This required a calibration procedure to determine the TTA to the optical node so as to offset the CU receive window properly. This was inconvenient, but is unnecessary in the boundless ranging embodiment disclosed here.
In addition, the previously described ranging schemes uses a sequence of 8 pulses during all the ranging stages which adds a lot of interference to the data portion, and requires a longer time. In other words, the old ranging scheme had the RUs continuously sending their ranging IDs which had 8 barker codes. If these 8 barker codes were misaligned, they landed on payload data and could cause errors. The boundless ranging scheme described here uses only one barker code transmission at a time until confirmation is received from the CU that it landed in the gap.
The preferred boundless ranging process has all RUs adjusting their delays to hit the gap after the frame number from receipt of the Barker code from the CU which the farthest away RU can hit (currently the 13th frame for a system which spans 100 miles). Each RU hits the same gap using an offset number plus a transmit frame timing delay value Td. The offset number is the total turnaround time (TTA) from the CU to that RU and back in frames. Any fraction of the TTA that is less than a complete frame is the value of Td. Note that it is only necessary for all the RUs to align to the same gap and keep track of CU frame numbers where the codes are being time shared. In embodiments where the codes are not time shared such as where each RU always transmits on the same code or codes, it is only required that the frame boundaries be aligned, and this is only necessary in order to minimize ISI. If other methods of limiting, eliminating or compensating for ISI are available, even this frame synchronization can be eliminated and regular CDMA used in the upstream channel.
This can best be understood by joint reference to
Proper accounting for assigned frames is the reason for boundless ranging. Since there are only 128 channels, but there may be 2000 RUs, the CU controls the situation as follows. RUs having data to transmit, send management and control messages to the CU saying they need bandwidth. The CU assigns one or more codes to the RUs according to whatever bandwidth allocation scheme is in use.
***Dynamic bandwidth allocation allows as many 64 kbps streams or channels as necessary to be allocated to a particular service so that high demand applications such as video teleconferencing or high speed internet access can be supported simultaneously with low demand applications like telephony over the same HFC link. Bandwidth allocation is managed at the CU through an activity status table in each RU and the CU that indicates the status of each timeslot and code assignments. The CU updates the RU tables by downstream messages. Bandwidth can be guaranteed upon request while other services with more bursty traffic may contend for the remainder of the total 10 Mbps payload.
The bandwidth assignments are sent downstream as code numbers to use during specific frame numbers. Referring to
Suppose RU 1624 in
There follows a detailed discussion of a ranging process which supports boundless ranging.
The Preferred Ranging Algorithm For Boundless Ranging
The ranging algorithm is the procedure by which an RU aligns its coarse transmission frame timing so that its frame start (i.e. the beginning of the frame gap) will be received by the CU exactly when the CU begins a new frame. This will cause all the RU transmitted frames to be received in the CU aligned to each other and to the CU.
The ranging process is performed after the RU is powered on and finishes its downstream initialization (clock and carrier recovery) functions and its received frames are aligned to its clock. This sequence of events allows the RU to be able to receive the data sent in the downstream by the CU.
Ranging is the first step of the upstream initial training process which includes the following steps:
1. Ranging
2. Time alignment.
3. Power alignment.
4. Time alignment.
5. Power alignment.
6. Equalization.
7. Repeat steps 4 through 7 N times.
The ranging process should:
- 1. Align the RU's frames to within +/−1 chip of the exact timing required to achieve frame synchronization
- 2. Align the RU's power level to achieve detectable power levels but not exceed the power level that increases the bit error rate of the operating RUs.
The objective of the ranging process are as follows.
1. To find the delay between the instant that the RU under consideration receives the frame start from the CU (in the downstream channel) until it has to begin transmitting the frame start in the upstream so that it will be received by the CU aligned with the CU frame start.
2. To find the power level with which the RU has to transmit its ranging signals, so that the CU receiver will be able to detect it, but at the same time will not introduce additional noise in the running RUs data channels (in the upstream). That is, it has to find the minimum power level that the RU should transmit the ranging signals, so that the CU receiver will detect them with high enough detection probability without unduly interfering with data being transmitted by other RUs should the RU ranging signals arrive at the CU mistimed and in the middle of another RU's payload data.
3. To find the frames offset that the RU under consideration has to have in its frame counter in order to be able to align its transmitted frames indices to the received frames in the CU. This offset is required to synchronize all the RUs in the frame level for control purposes (such as time-slot allocation, retraining initialization, etc.) and is needed because of the channel total-turn-around time (TTA) due to the required maximum distance between the CU and the farthest RU (100 miles). This element of the ranging is also known as the "boundless distance ranging".
The main idea in the ranging process is that the RU performs an efficient search of the [delay, power] plane in order to find the appropriate delay and power that will satisfy objectives 1 and 2 above. The RU has to transmit the ranging signal so that it will be received by the CU's receiver at a window located at the center of its gap as shown in
The ranging algorithm described here allows many RUs to perform their ranging at the same time and still achieve the separate frame synchronization of each of the RUs. Each RU sends a ranging signal that is a series of 17 pulses (one pulse per frame—gap). These 17 pulses include a starting pulse and 16 ranging ID pulses out of which 8 are "0" and 8 are "1". A "1" signal is represented by the presence of a Barker coded pulse with length of 13 chips. A "0" is represented by the absence of transmission of the Barker code. The ranging ID is randomly picked by each RU. It takes 17 frames for an RU to send its ranging signal. Because the maximum distance between the CU and the farthest RU is 100 miles, the TTA time is 16 frames. This means that the possible received ranging ID will be located within 32 (=17+16−1) frames in the CU.
The CU receiver looks for Barker signals in the six middle chips (i.e. the ranging listening window 1640) of the gaps of 32 successive frames whose positions (indices) were predefined by the CU in the ranging solicitation message it transmitted. The CU translates these 32, 6-chips-long vectors, into 6 32-digit vectors V1, V2 . . . V6 where V1 denotes all the received values (0/1) of chips number 1 in the successive frame listening windows 1640, and V2 contains the same for chip number 2 in the ranging listening windows, and so on. In the boundless ranging embodiment described below, the listening window in the middle of each gap is 8 chips wide, so vectors V1 through V8 are built by the CU CPU in the same fashion as described above.
In general, the CU receiver can have the following possible cases for each vector:
1. The CU does not detect any signal in these vectors—in this case it sends an "empty" message in the downstream.
2. The CU detects only valid. ID (up to six)—in this case it will send the "valid ID" message in the downstream with the detected IDs list.
3. The CU detects signals that do not have the structure of a valid ID—in this case the CU treats the received signal as being the result of contention of at least two RUs and sends a "collision" message.
Since the CU examines six vectors, it can also have the mixed cases: "valid+empty", or "valid+collision".
The scan of the delay-power plane is performed so that the RU scans all the relevant delays for a given power, and if this does not get the CU's response, the RU increases its power by a given step (ΔP), and re-scans all the delays. Assume A is the initial scanning point (minimum delay and minimum power). From A, the scan goes with constant power and increasing delays until the maximum delay is checked. If this is not sufficient, the power is increased and the delay is scanned from minimum to maximum values. Once the RU Barker code transmission has the correct delay and power level values to be received by the CU receiver in the gap, the CU will send an appropriate message that will stop the scan. If the message is "valid ID" with the RU's ranging ID, the RU completes its ranging while performing a last update of its delay according to the CU's adjustment data that is sent to the RU.
If the CU gets a non-valid ID with at least three "ones", it responds with a "collision" message that puts all the RUs that are ranging in this instant into a contention resolution mode. The contention resolution is performed in the ranging algorithm via a binary tree algorithm to "flip the coin" as previously described.
Summary of Steps of Preferred Boundless Ranging Process
- (1) CU solicits for ranging and scans a number of following frames equal to or greater than the TTA in frames to the farthest RU.
- (2) RU transmits Barkers continuously with 4 chip delay increase. After each iteration through all possible delays, the power increases by k dB (k is a constant which can be determined experimentally).
- (3) CU sends "ACTIVITY DETECTED IN FRAME#".
- (4) RU does not know which value of Td resulted in hitting the gap or even if it was the RU that hit the gap. There is a 16 frame ambiguity or 16 different possible delay values that could have caused the gap to be hit for the farthest RU in a system with a 16 frame TTA span. As a result, it assumes it was the RU that hit the gap and goes back to the delay value it used for the frame number 16 frames ago (for a system with a TTA span of 16 frames) and starts negotiation with the CU by sending one Barker code at a time and waiting for a reply. That is, a value for Td will be picked starting with the Td used 16 frames ago and a Barker code will be sent. The RU will then wait 16 frames for a reply message from the CU regarding whether activity was detected in the gap. If no such message is received, the next value for Td will be selected, and another Barker code will be sent with the RU again waiting 16 frames for a reply.
This is one Barker code at a time scheme is faster than some of the alternative ranging schemes previously described wherein the RU sends its entire ranging ID each time it picked a new delay value since in those alternative schemes a number of frames equal to the number of bits in the ID was consumed for each value of Td before the RU knew whether that was the correct Td value.
- (5) CU sends "ACTIVITY DETECTED IN FRAME#, START CONTENTION RESOLUTION".
- (6) all RUs that are ranging, transmit an ID sequence comprised of a start bit (always a logic 1) and 8 random Barkers out of 16 in the 16 consecutive frames following the start bit (in some embodiments, the ID code for each RU can be fixed and will be comprised of a start bit and an even number of consecutive bits exactly half of which are "1s"—in some embodiments, the ID code may have some other known number of 1's which are more or less than half with contentions being detected by detection in one vector of a number of 1s in excess of a predetermined threshold).
- (7) CU looks for valid ID (exactly 8 of the 16 gaps following the start bit contain barker codes), and, if a valid ID is found, broadcasts a message containing the VALID ID (the actual ID found), the FRAME# (frame # of the frame in which the start bit arrived of the valid ID), and a CHIP OFFSET (instructions on which way to adjust the value of Td to start the fine tuning process).
- (8) each RU with an ID found by the CU recognizes its ID in the CU broadcast message or messages, calculates its offset value and does fine tuning (one RU at a time under control of the CU) to center its barker code exactly in the middle of the gap.
- (9) For centering and confirmation, CU sends a downstream message containing PREVIOUS ID (the ID it just received) and a request, TX ANOTHER ID.
- (10) RU sends another random ID (randomly selected and usually different from the first ID it used in the ranging process). CU broadcasts new IDs found and RU knows it has completed ranging unless contentions found.
- (11) If there are contentions, CU sends "CONTENTIONS DETECTED"
- (12) each RUs that is ranging starts binary tree algorithm for contention resolution as previously described, and some stop ranging and some continue ranging.
More Details on Preferred Boundless Ranging Process
The preferred boundless ranging process is broken generally dowin into several phases: Activity detection; Contention detection resolution, and authentication, and Frame alignment and gap centering
In the preferred embodiment, only one pulse is transmitted per frame by each RU which is ranging for purposes of activity detection. For contention detection and resolution and frame alignment, gap centering nand authentication, the RUs send a 17 bit ID which comprises a sequence of one start Barker code followed by an ID comprised of 6 ON or OFF "bits" of which precisely 8 will be ON.
The details of the preferred boundless ranging process for use in an SCDMA embodiment where the codes are time shared are as follows.
(1) the CU solicits for ranging continuously and then analyzes each X frames following each Barker code transmission for activity. X is equal to the TTA to the farthest RU in number of frames, so if the farthest RU is 16 frames out, the 32 frames following each Barker code transmission from the CU will be analyzed for activity.
(2) each RU which is ranging transmits Barkers in consecutive frame gaps with a 4 chip delay increase in Td until CU reports activity detection. RU starts with a small power level so as to not cause excessive interference if it is not properly frame synchronized at the current value for Td. If it does not receive feedback from the CU at this power level after scanning all the possible delays, the RU increases the power by k3=3 dB and starts scanning again with 4 chip delay increases during each successive transmission. The power for ranging is limited to maximum power allowed less 4 dB. At this time the RU is only trying to hit the nearest gap and does not care what its offset is. That offset will be deduced later from a message from the CU.
In other words, in the beginning of the ranging process, the RU transmitter FRAME # has 0 offset relative to the receiver FRAME #, and the CU FRAME # is the same in the transmitter and the receiver.
(3) Activity detection: As CU detects activity in the gap, it notifies the RUs "activity detected in frame #m". The FRAME# is sent to the RUs to eliminate any software delay. No RU is allowed to start new ranging processing after the first activity detection. The RU optionally increases the power by k4=1 dB to ensure better Barker detection.
The TTA for the RU or RUs that hit the gap is unknown at this point. Therefore the RU assumes that it is the RU that hit the gap and needs to determine which of the values for Td it used for previous Barker code transmissions which caused it to hit the gap. The RU therefore backs up its delay value and starts a one Barker code at a time negotiation process with the CU to attempt to locate the value for Td which caused it to hit the nearest gap. The value of Td is always less than one frame or 125 microseconds. For RU located up to 100 miles from the CU the maximum latency or TTA delay is 13 frames obtained from:
For margin, assume a maximum TTA of 16 frames to the farthest RU. The RU uses the same chip delay used in frame #m identified in the CU's "activity detected" message in order to transmit the first Barker code. Only one Barker code is transmitted, and a reply from the CU is awaited for k5 frames (an experimentally determined number of frames). If the Barker is not detected, the RU continues to try all the 16 possible delays by increasing the delay by 4 chips for each trial. Each trial takes k5 frames until the RU receives feedback from the CU that activity has been detected in the gap. The value of k5 is set according to due to TTA and software delay. In alternative embodiments, a few pulses can be together.
Once the CU detects a pulse inside the gap, it sends to the RUs a message indicating "a pulse was detected in frame #n". This frame number can be used to set the offset value in some embodiments where there is known to be no contention (such as embodiments where only one RU is allowed to range at a time), but, in the preferred embodiment, since the RU is not yet sure it is the RU that hit the gap and there is only one RU in the gap, the offset number cannot yet be calculated.
(4) Contention detection and resolution: After the pulse was detected inside the gap, there are a few possibilities. First, there could be one pulse inside the gap was detected as well as other pulses which landed in the data portion. The source of the pulse in the gap can be from one RU, a contention of two or more RUs, or a noise hit. Second, there could be more than one pulse inside a gap or more than one pulse in several gaps. To resolve any contentions between multiple RUs, a contention resolution scheme similar to the scheme previously described herein can be used but with the addition of a start bit to accomodate the fact in boundless ranging that there is no longer any limitation that TTAmax be limited to one frame. Thus, the CU sends a downstream message, "ACTIVITY DETECTED IN FRAME #XX, START CONTENTION RESOLUTION".
(5) ***To determine if there are any contentions in the preferred boundless ranging embodiment, the RUs transmit a sequence of 17 bits comprises of a start bit (a Barker code) and 8 1's out of 16 successive gaps where a "1" is a gap with a Barker code transmission in it. Vectors V1 through V8 are created by the CU in the same manner described above for creation of vectors V1 through V6 for the 6 chip listening window embodiment. Contentions are detected by the CU when the number of 1's in any particular chip vector V1 through V8 is more than 3 and different from 8. If there are contentions, the CU goes into contention resolution phase. The CU looks for a valid ID sequence in each vector in the process of examining each vector for contentions. A valid ID is found when exactly 8 of 16 gaps following the start bit having a "1", i.e., a Barker code in them during an interval of 17+16=33 frames (17 frames for the start bit and 16 bit ID and 16 frames for TTA involving propagation of the command START CONTENTION RESOLUTION to the farthest RU in the system and propagation of the start bit back to the CU from the farthest RU. Each valid ID sequence starts with a start bit, and it is the frame number during which the start bit arrived at the CU which is the frame number in which the valid ID is deemed to have arrived if a valid ID is found. The use of the start bit in this manner give definiteness to the offset calculation by insuring that each valid ID starts with a 1 so that its time of arrival can be determined with no ambiguity. The receipt of the valid ID insures that there are no contention, because if more than one RU is in the gap, more than 8 "1s" will be received in the 16 gaps following the start bit. When the valid ID is received, it is broadcast downstream in a message "VALID ID, FRAME #, CHIP OFFSET", and the RU which transmitted the ID will know that it is the RU that hit the gap by virtue of seeing its ID in the downstream message.
An example of a table obtained at the CU in looking for valid IDs is given in
The table of
(6) After a valid ID is detected, the CU broadcasts messages for each correct ID. Each message is comprised of: valid ID (the actual ID received), FRAME number of the frame in which the start bit of the valid ID was received, and the number of chips the start bit was offset from the center of the listening window. The RUs which see their ID in the downstream messages know they have hit the gap and that there is no contention on the particular chip in the listening window in which they have landed. Each RU which recognizes its ID in the downstream message "VALID ID xxxxxxxxx, FRAME #, CHIP OFFSET" then calculates its offset number by using the CU frame number contained in this downstream message containing that RU's valid ID (this is the CU receive frame number during which the start bit of that RU's valid ID was received) and the CU transmit frame number count from the CU transmit frame counter in the frame detector of that RU's receiver (which matches the CU transmit frame count) contained in the message from the CU saying "ACTIVITY DETECTED IN FRAME #, START CONTENTION RESOLUTION (Send your ID)". The difference between these two frame numbers is the propagation time in whole frames of a signal from the CU to the RU and back to the CU which is equal to the TTA in whole frame for that RU. That offset number is set into the offset register 1632 in
In some embodiments, the Barker code listening window inside the gap is only 4 chips wide instead of 8, to prevent any of the Barker energy from being spilled into the data portion. Note that contentions are determined on a per chip basis in the listening window, so as long as only one RU has its Barker codes landing on that particular chip in every gap listening window, there is no contention on that chip. To avoid complication of the centering or fine tuning process, the CU in the preferred embodiment will fine tune only one RU at any particular time by messages in the downstream command and control channels telling the RUs which is to fine tune at any particular time.
(7) Fine Tuning: The RUs that see their valid ID in the downstream messages, then employ frame alignment and Barker centering by correcting the delay in terms of frames and chips to complete their ranging when so instructed by the CU.
(8) Contention resolution: When the RUs get a message of contention from the CU instead of the message "VALID ID xxxxxxxx, FRAME #, CHIP OFFSET", the RUs "flip the coin" using a binary tree algorithm, to decide whether they continue ranging. Each RU in contention resolution mode has a probability of ½ that it will retransmit the 17 bit ID sequence.
The CU should have the following commands for contention resolution:
- (A) "retransmit the sequence with probability of x, x=½ or 1"
- (B) "retransmit the sequence with probability of x, x=½ or 1 only if the RU transmitted the sequence one stage before"
- (C) "retransmit the sequence with probability of x, x=½ or 1 only if the RU transmitted the sequence one or two stages before"
(9) Centering and confirmation: The CU asks each successful RU which has centered its ID to transmit another random ID at the gap center in order to reduce the probability of errors.
After the CU finishes with sending IDs and contention resolution it should notify all the RUs that they are allowed to start ranging or restart ranging. The RUs that restart ranging should continue their ranging from the state it was stopped.
Although the teachings of the invention have been illustrated herein in terms of a few preferred and alternative embodiments, those skilled in the art will appreciate numerous modifications, improvement and substitutions that will serve the same functions without departing from the true spirit and scope of the appended claims. All such modifications, improvement and substitutions are intended to be included within the scope of the claims appended hereto.
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|Apparatus and method for SCDMA digital data transmission using orthogonal codes and a head end modem with no tracking loops
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|Apparatus and method for receiving upstream data transmissions from multiple remote transmitters
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|Apparatus and method for trellis encoding data for transmission in digital data transmission systems
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|Apparatus and method for trellis encoding data for transmission in digital data transmission systems
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|Apparatus and method for SCDMA digital data transmission using orthogonal codes and a head end modem with no tracking loops
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