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April. 2000
Digital Switching
EETS8304/TM-708N SMU/NTU Lectures April 17 & 24, 2001 Cellular & PCS
(print in PowerPoint notes pages format to see additional notes below each slide) Two suggested reference books by L.Harte and R. Levine : Cellular and PCS: The Big Picture (1997) GSM SuperPhones (1999)
both published by McGraw-Hill
Revised 2001
©1996-2001, R.C.Levine
Print in Power Point Notes Pages format. Many pages have notes in this lecture. This set of lecture notes extends over two class sessions.
©1996-2000, R.C.Levine
Page 1
Cellular & PCS
April. 2000
Cellular and PCS
General Background of Cellular & PCS •Different Access Technologies •System Structure
– Physical Description Radio Um Interface – Signal Description
•Call Processing
– Initialization – Call Origination
• Mobile origin, mobile destination
– Handover – Release/Disconnect
– – – – Voice Data & Fax Short Message Service (SMS) Packet Data (particularly EDGE and GPRS)
Revised 2001
©1996-2001, R.C.Levine
This presentation is a condensed version of lectures used for people actually working in the cellular and PCS industry. It is intended for students having no particular prior exposure to radio or cellular/PCS systems. The author will appreciate notification of any errors, regardless ho w minor. Please send a marked copy of the relevant page(s) to the address below. Copyright notice: This material is copyright © 1996 by Richard Levine and Beta Scientific Laboratory, Inc. It may not be copied without written permission of the copyright holder. To apply for permission to reproduce, contact Beta Scientific Laboratory, PO Box 836224, Richardson, Texas, 75083-6224, Telephone +1 (972) 233 4552
©1996-2000, R.C.Levine
Page 2
Cellular & PCS
April. 2000
History and Jargon (North America)
• Analog cellular on the 800 MHz band
– Since 1979 (experimental), 1981 commercial; analog now being phased out • Digital Cellular/PCS on 800 and 1900 MHz bands – Since ~1991 with immense growth rate
• Systems on the 1.9 GHz (1900 MHz) band
– Usually called Personal Communications Systems
• even when technologically identical to 800 MHz systems (such as IS-136)
• 900 MHz and 1.8 GHz bands used in Europe
Revised 2001
©1996-2001, R.C.Levine
There is some confusion in the industry about the similarity or distinction between the terms cellular and PCS. In some cases there is no distinction. In other cases the distinction is not technological, but is based on the frequency band of operation or on who owns the license to operate the system.
©1996-2000, R.C.Levine
Page 3
Cellular & PCS
April. 2000
• Technically, a cellular system has 2 properties: – Cellular frequency re-use – Handover (also called handoff) • So do most PCSs (personal communications systems)
– only exception is CT-2 public cordless (current implementations) without handover
• Today the North American business distinction is mainly based on frequency band...
– 800/900 MHz is described as cellular • including digital cellular such as GSM, IS-54, IS-136 – 1.9 GHz is described as PCS Warning: jargon subject to change without notice! Beware of total confusion...
Revised 2001
©1996-2001, R.C.Levine
The jargon of the cellular and PCS industry is unfortunately not fully stable. Since not everybody agrees on the distinction between a cellular and PCS system, AT&T was criticized for calling their IS-136 roll out by the marketing name digital PCS system, because it operates on the 800 MHz band. From the purely technological point of view, there is no fundame ntal distinction between systems which operate on the two bands which justifies using a different name for each band. This course will follow the description on the slide above just to be unambiguous and agree with what the majority of people in the industry are currently saying. A cellular operator is then a company which owns an 800 MHz North American cellular band license. A PCS operator has a 1.9 GHz band license. Terminology has changed in the last 2 years, and may change again. To avoid pointless arguments, verify definitions before proceeding to argue!
©1996-2000, R.C.Levine
Page 4
Cellular & PCS
April. 2000
Brief History of Cellular/PCS
• Manual operator-handled mobile radio (1945…)
• Automatic Mobile Radio, e.g. Secode, IMTS (1960…)
• Trunked radio (1960…) – cellular-like frequency re-use – but no handover! • Cellular radio (1978…) required new technology: – control of mobile radio operation via messages from base
• Mobile transmit (Tx) frequency and power • Can be changed during a conversation to select best base station or compensate for distance • Handover continues conversation as mobile station moves from cell to cell
Revised 2001
©1996-2001, R.C.Levine
Cellular radio did not exist until the relatively simple microprocessors of the 1970s were available to provide remote control and sufficient sophistication to act on commands from the base station.
©1996-2000, R.C.Levine
Page 5
Cellular & PCS
April. 2000
Cellular Frequency Re-use
• Certain types of radio modulation exhibit "capture effect" • When ratio of desired signal power to undesired (interference & noise) power is greater than the "capture ratio," only the stronger signal is apparent • Capture ratio depends on
– Type of modulation: FM, Phase Modulation -but NOT AM – Bandwidth of signal compared to data rate:
• Analog cellular 30 kHz: capture ratio 63/1 or 18 dB • Narrow band NAMPS 10 kHz: c.r. is 200/1 or 23 dB
Revised 2001
©1996-2001, R.C.Levine
Every few years someone re-proposes some type of amplitude modulation (AM) single sideband (SSB) cellular reuse system. These proposals often include elaborate audio compression-expansion ("companding") and audio noise reduction methods. The objective is to exploit the much narrower bandwidth of AM (only 4 kHz for a 4 kHz audio signal). These proposals ignore the fact that the C/I ratio measured at the antenna is the same audio S/N or S/I ratio that will be heard at the earphone. Therefore, the objective of a 30 dB S/I audio ratio would require something like an n=28 frequency plan. In other words, one could not reuse the same carrier frequency in the same city in many systems! In contrast, FM and phase modulation both exhibit a fairly distinct threshold in C/(I+n). When the desired signal power C is greater than the sum of interference and noise power (I+n) by this ratio, the audio output (or digital bit accuracy) is almost perfect. The audio exhibits a lack of no ise corresponding to 30 dB (1000/1 ratio) of S/N, although the FM C/I in an 30 kHz analog cellular system is only 18 dB (63/1). Once the C/I ratio falls below that threshold, clicks and pops are heard, and at even lower C/I, only a random noise hiss is heard. Due to widespread use of FM during World War II (the inventor, Col. Edwin Armstrong, donated his patents free of charge to the armed forces) many people got the idea of cellular reuse about the same time, but only in the 1970s was handover and remotely computer controlled radio carrier frequency selection and transmit power added.
©1996-2000, R.C.Levine
Page 6
Base Rx • Two frequencies are used simultaneously for Frequency Division Duplex(ing) -FDD. there would not be enough spectrum for a major fraction of the population to use cellular and PCS radio systems.C." The objective is to make a system which is no more complicated to use than the ordinary landline telephone. usually at the cell center – The radio coverage in the cell may be optionally: • Omnidirectional (azimuthally) with one antenna group • Sectored (typically with 3 antenna groups. To quote Captain Queeg in the Herman Wouk novel The Caine Mutiny.C." Some special cellular phones such as the GTE "TeleGo " handset have even been designed so that the user hears dial tone first so it is perceived exactly like a regular wired telephone! ©1996-2000.Cellular & PCS April. Without handover. Mobile Rx – uplink (reverse): Mobile Tx. (So-called trunked radio systems do not have handover. but do have frequency reuse. it would not be practical to continually select the proper frequency for a conversation or a handover. The only significant operational difference is that the user dials the desired destination directory number first. calls would need to be limited to the time one dwells in a single cell. All these complicated continual adjustments are done without the need for the user to be a technical whiz and constantly adjust dials and buttons. before engaging the central switching equipment and hearing a "dial tone.Levine Page 7 .) Without computer remote control of the mobile station. R. the system was "designed by geniuses to be used by idiots. Some technologies (DECT. some modes of 3G) use time division duplex (TDD) with alternate time interval transmit-receive on the same radio frequency Revised 2001 ©1996-2001. 2000 Radio Cells • A cellular service area is covered by numerous smaller cells – Each cell has one base station (base antenna location). and control the mobile set transmit power accurately as the MS moves close to and away from the base station. R.Levine 7 Without cellular frequency reuse. and that is their limitation. 120º each) Warning: Some documents use the words cell/sector differently than we do – Each sector has at least one RF carrier frequency • A carrier frequency identification number describes two different (paired) frequencies: – downlink (forward): Base Tx.
and other objects on that surface which affect radio propagation. R. R. The input information comprises the following factors: Expected geographic density of call traffic in each area of the city over the planned service life of the system. the threshold values appropriate to each cell or sector must be set in each cell.) should be identified and properly handled. electric machinery. the radio path losses from overhanging foliage reduces the street level radio signal strength during the spring and summer. This includes populations within buildings and underground in tunnels and parking garages. among other things. The objective of 90% of the time recognizes that. the choice (omni. high path loss requires small cell cluster (3 or 4) • High capture ratio. etc. External sources of radio interference (other radio systems. Topography of the ground surface and the buildings. building installation of equipment and mounting of antennas.C. ©1996-2000. electrical radiation from signs.Levine Page 8 .C.Cellular & PCS April. Relative costs of real estate for towers. For proper control of handover. etc. The traditional objective is to produce a plan for radio coverage of 90% of the service area which works 90% of the time.or sectored directionality) and placement of the antennas. compared to the fall and winter seasons. not based on theory • Exponent in range 2 (open space) to 4 (cluttered urban environment) • A frequency plan is characterized by a cell cluster count in which each frequency is used in one cell • Low capture ratio. low path loss requires large cluster (7 or 12 or more) Revised 2001 ©1996-2001. 2000 Cellular Frequency Plan • Frequency plan depends on – capture ratio resulting from RF technology – Radio signal strength path loss or distancerelated attenuation • Approximately: received power =1/distance 4 in city – Empirical approximation. The mutual interference between cells having the same reuse radio carrier frequencies (cocarrier interference) should be below the capture ratio value. and the assignment of the proper carrier frequencies to each cell or sector. trees.Levine 8 The major task of engineers who design and install a cellular system is the placement of the cell base stations.
(In the GSM/PCS-1900 system.5 kHz bandpass telephone audio channel) • Unfortunately.Cellular & PCS April.C.Levine Page 9 . since the customer is either experiencing "garbage" audio. R.Levine 9 In most systems. the TIA-553 North American standard only provides 3 SAT choices (5970. Many choices (2 42). if a problem of reception of the wrong radio signal (or a radio signal not containing the correct identification code or signal) persists for 5 continuous seconds. or the customer is in communication with the wrong person. In some cases. The long term solution is to identify those areas where such problems exist. due to radio interference which is so strong that it interferes with communication.SAT (above the 3. but only 62 downlink code choices in each cell. but this is very costly so it is saved as the last measure. ©1996-2000. The correction of the radio coverage may require altering the base transmitter power. practical systems would not have enough traffic capacity (insufficient total radio spectrum) However. changing the height and/or the mechanical or electrical downtilt of the base radio antenna. Revised 2001 ©1996-2001. 6000. or use of a radio repeater. the immediate way the system design deals with it is to release (disconnect) the radio link. 2000 Special Frequency Reuse Problems • • • Without multi-cell frequency reuse. normally on another radio channel which hopefully is not experiencing such bad interference. the possibility of co-channel (or co-carrier) interference is always of concern Cellular/PCS systems have various methods to prevent miscommunication with a co-channel signal from another cell: – Analog systems include a different Supervisory Audio Tone . there is an automatic reconnection following such a release.C. R. 6030 Hz) – TDMA digital systems include a repeating digital identifier code in each transmission burst and associated with each control message • 8 synch/training code choices in GSM/PCS-1900 • 255 CDVCC code choices in IS-136 – CDMA systems plan to use the same radio frequency in all cells w ith many different uplink CDMA spreading codes in different cells. a relocation of the base antenna may be needed if other methods are not sufficient. and to correct the radio coverage in these areas.) There is not much else to do in the short term.
5 cell diameters away from the central 1 cell.5 diameters away and two are 2 diameters away. R. etc. Thus this system does not have as high a capacity as the n=3 frequency engineering plan. there are 4 nearest co-carrier cells (also labeled 1) but 2 of them are 1. shown on another page. The next rank of co-carrier cells are about 3 diameters away.C. This BSIC code is broadcast periodically by the base station. and the other 3 bits indicate one of 8 permitted values of the training/synchronizing sequence which is used in full TDMA bursts (explained on another page).10. if at all. Observe the many near (but not adjacent) cells also labeled with 1. Because of using (horizontally) omnidirectional antennas at all sites. ©1996-2000. the co-carrier interference frequencies are very close to that cell. 22. at a distance of about 2.13. 15. The BSIC is used in several places in the coding to prevent a receiver from using a co-carrier interfering signal from another base station operating on the same carrier frequency. Also. One example relates to the random access burst.Levine Page 10 .Levine 5 3 3 The number in each hexagonal cell represents the first (lowest usually) carrier frequency number assigned to that cell. In general there are 6 nearest co-carrier neighbor cells. The same BSIC should only be used in very distant co-carrier cells.5 diameters. R. All the full burst transmissions in this cell also use the specific training bit sequence code specified by the BSIC.C. etc.4. 7. cell 1 can also be used for carrier frequencies number 4. In the n=4 cluster. but they are not shown on the diagram and may not be significant because they are beyond the horizon. so there are 1/3 of all available frequencies used in each cell. There is also a second and third rank (and even more distant) of co-carrier cells. Around each cell labeled 1 there are 6 nearest co-carrier cells (only 4 are shown in the diagram). so the MS receives it. However. the cells labeled 1 can also be used for carrier 5. In the n=7 cluster. A distinctive base station identity code (BSIC) can be assigned by the operator to each cluster. and their centers are only 1. Three of the 6 bits are arbitrarily chosen by the operator. each cell is subject to co-carrier interference from all co-carrier cells in all compass directions.Cellular & PCS April.7. etc. In the n=3 clusters. 2000 Frequency Clusters Ideal hexagon pictures of n=3. omni-directional clusters 2 2 1 4 2 4 2 1 2 1 3 2 4 1 2 Revised 2001 2 3 2 1 3 1 3 2 1 1 3 5 3 7 4 3 5 3 6 1 1 3 2 4 3 2 3 7 1 6 5 3 5 3 6 2 4 7 1 6 10 1 3 2 1 7 1 2 4 7 1 6 2 4 2 4 3 4 1 3 2 4 1 ©1996-2001.9. cells labeled 1 may also be used for carriers 8.
and the amount of such back and side "lobe" power is greater for narrow sectors than for wide angle sectors. This is a result of segregating the overall set of carrier frequencies into 3 (or 6) subsets in a sectored cell. The higher traffic capacity available when the same number of MSs in the cell can use any or all of the various carriers in the omnidirectional case is called "trunking efficiency. This improves the signal quality. The overall blocking probability of a number of channels is increased (and thus the usable traffic capacity is reduced) when they are subdivided into a number of exclusive subsets. Sectored cells have two advantages over omnidirectional cells." ©1996-2000.C.Cellular & PCS April. and the MSs in each sector can only chose from those carriers available in that sector. the traffic capacity actually goes down.C.Levine Page 11 . but there is no change in the frequency plan. When sectored cells are used in place of originally omnidirectional cells. or from n=7 to an n=4 plan. Because the total interference from other cells is reduced. 2000 Sectored Cells • Ideal hexagon representations. R. the cluster can be redesigned from a n=9 to n=7. One important difference: There is non-negligible power radiated in the back and side regions. each with shielding on the "back. Revised 2001 ©1996-2001. which is manifested as a lower BER in a digital system.Levine 11 Sectored cells are created by installing multiple antenna sets at the base location. in some cases." Each set of antennas is directional rather than omnidirectional. by limiting the radio reception/transmission to the "front" of the angular sector and not transmitting or receiving a signal from the "back" they reduce the level of interference by a ratio of 3/1 or 6/1 for 3 and 6 sectors.ideal in several ways. respectively. ideally no "back" antenna signal transmission/reception Back and side lobes Back of blue sector 60º 120º "Real" Sector 3 sectors Front of blue sector 6 sectors Real sectored cells are non. thus increasing the capacity per cell. R. First.
This capability to move channels to the sector with the most traffic overcomes the trunking limitation imposed by provisioning a fixed number of channels in each sector without regard to the changing traffic load demand in each sector.Cellular & PCS April. 2000 Sectored Cells • Narrow sectors reduce Co-channel interference – Permits geographically closer frequency re-use – Thus more carriers/cell. more capacity • qualification: smaller trunk groups reduce "trunking efficiency" • But… back and side lobes are problems – Permit "spot" co-channel interference • "sneak path" interference which only occurs intermittently. this requires a more sophisticated base transceiver (BTS) than the normal design.Levine Page 12 . A limited approach to this idea using passive "dumb" antennas is to use an omnidirectional pattern for the beacon carrier (which is used to start the setup of calls). The omnidirectional coverage can be achieved by either a separate antenna for that one carrier frequency. This requires a high degree of interaction of information between the base station signaling hardware and software and the antenna beam forming sys tem. The special BTS must have multiple receiver inputs so it can determine which sector the MS is located in. and then transfer the MS to a carrier which is used in only one sector to continue the call. In general. or by connecting the beacon carrier to/from all the sector antenna sets. In some systems.C. difficult to identify and debug – "smart antennas" (adaptive phased arrays) address this problem better (but at high cost) Revised 2001 ©1996-2001. ©1996-2000.C. "smart" antennas have been proposed which permit dynamically forming the radio directionality beam for each TDMA time slot separately so that the beam points to the MS it communicates with. R.Levine 12 Some systems separate the carrier frequencies into subsets which operate separately in each sector. R. In some vendor's systems the cells are sectored but individual carriers can be "switched" from sector to sector.
system operators first design for all but the lowest power class. 2000 Uplink-Downlink Balance • Need equally good signal quality both directions– two-way communications is the objective – areas covered only by downlink are not useful. There is also. to eventually handle all power classes. and by default they will also support any higher MS power class as well.C. The immediate need for many operators is to meet a legally mandated target of overall population area coverage as soon as possible. the operator knows that their design will not provide 90% area and time coverage to the very smallest power class MS units.g.Levine Revised 2001 13 The downlink limits for different samples of the same mobile station production run will differ very slightly because of minor variations in the internal noise of the MS receiver. R. 5 to 10W/carrier) than MS (max 2W for PCS-1900. R. ©1996-2000. In general. than in the case of a GSM or North American cellular system. and then adjust the system coverage as described on another page. Similarly the uplink performance will vary slightly due to minor differences in actual compared to nominal transmit power. where the power difference between the largest and smallest power class is very significant. In some cases. 3W for IS-136) • Compensate for this via: – Base Rx diversity (equivalent gain of 2-5 dB) – Base Rx antenna gain (typ.Cellular & PCS April. These adjustments may take 1 or 2 years.Levine Page 13 . where all the power classes are slightly different low power levels below 1 watt. In general this difference is much less significant in a PCS-1900 system. a greater uplink operating range and consequently a larger useful cell size for a mobile station of a higher power class (a higher rated maximum transmit power level). clearly. Typically. 5-7 dB or more) – Low-noise amplifier (LNA) in base receive multi-coupler ©1996-2001. may cause excessive co-channel interference to other cells • Base Tx is more powerful (e. the system operator makes a decision to support a certain power class.C.
As the system traffic load increases.Levine 14 These steps are common to all cellular system designs. regardless of the specific RF technology. 2000 System Design and Installation • System designer estimates geographical traffic density – Market. to provide more carriers and thus more traffic channels. charts or computer programs based on Erlang B or C probability distribution (or other statistical traffic models) are used to estimate the relationship between number of cells and expected total number of hours of simultaneous conversations per clock hour for a given probability of blocking (or grade of service GOS). The relation between number of installed traffic channels and the traffic load which they can carry is a well understood process.Levine Page 14 ." then: • Chose some cell sites for longest term usefulness – so no cells need be abandoned at later date • First increase capacity by adding channels at a site • Then "split" cells into smaller cells – new antenna sites installed Revised 2001 ©1996-2001. often expressed by the symbolic expression P02. and specific geographic features such as high-traffic roads. Tables. areas of pedestrian congregation. R. R. When the full allotment of carriers has been installed under the frequency plan that is in place. usually by subdividing a cell into 3 or 7 smaller cells covering the original cell area (so-called cell splitting). mid and distant future dates – Conduct design "backward in time. Although a number of different statistical models are used in the industry.C. demographic. ©1996-2000. For cellular and PCS systems the legally accepted GOS is a 2% probability of blocking.Cellular & PCS April. etc. the first stage of upgrade uses additional base transceivers installed at each cell having increased traffic demand.C. – Desirable to do this for near. the ultimate refinement or fine tuning of the overall traffic handling design is best when based on actual in-service traffic measurements. additional cell (antenna) sites can be constructed.
2000 Typical Downlink Cell Map Coverage Diagram • Omnidirectional (intent) cell shown.Cellular & PCS April. R. We can use a radio repeater to increase the radio illumination in the weak signal area. in general.Levine Page 15 . One of the simplest is to increase the base transmitter power. or use a larger number of small cells.C. ©1996-2000. If it is in a highly populated area (a shopping center or major business district) then we need to increase the signal strength there. We would need to see terrain data to be sure. if it is in the middle of a garbage dump. This may be done by any of a number of methods. or a lake not used by people) then it is of little concern. These last methods are the most expensive. In an extreme case. tall building. If this weak signal area does not coincide with an area of population (for example. due to locally strong absorption or shadowing of the radio signal.Levine 15 The indentation in the north-by-northeast portion of this cell is probably explainable due to a hill.C. or other obstacle. but this also causes some increase in multipath at the edges of the weak area where both the repeater and the direct signal appear. and it will increase overall cell size and cause more interference to other co-carrier cells in the system. but this may not be sufficient. usable power contour Other Isopower Contour Latitude (North) Handover power threshold contour Revised 2001 Longitude (East) ©1996-2001. only 3 contours shown Base Antenna Min. or heavier overhead foliage (particularly in and near summer season) in that portion of the cell. This picture does not illustrate the appearance of small (blue) areas of weak signal strength which will often appear in the magenta (central) area of the diagram. The the shorter range to the east compared to the greater range to the west and south is probably due to generally greater path loss in the eastern propagation direction. we may chose another base antenna location during the design phase to get more complete illumination. This in turn may be explainable by more convoluted terrain in the eastern part of the cell. R.
Experimental (Measured at actual site) – Vehicle equipped with calibrated Rx and Global Position System (GPS) Revised 2001 ©1996-2001. R.Levine Page 16 . and utilize the actual directionality data for the antenna used at the base station. trees and other objects on the radio propagation in a very approximate and somewhat subjective way.Levine 16 Although an antenna directionality graph (dB on radial scale and angle on the angular scale of a polar graph) shown in a manufacturer's catalog is a good guide to the ground cover from that antenna. the final overlay showing lines of constant power (radio signal strength indication .) as well • Each color boundary is a specified iso-power curve (only a few important contours shown) – Like isotherms on a weather map – Iso-BER can also be plotted for digital systems • • Theoretical – Output from site-specific software: LCC. etc. Experimental data is even more accurate and should be gathered at each cell site before starting service. 2000 Contour Map Notes • Transparent overlay for US Coast and Geodetic Survey (USCGS) map is typically used to display coverage – (other similar government maps on other countries) – Positions on paper match Lambert conical projection • Not an antenna directionality graph – Isopower curves are a result of antenna directionality and local site effects (terrain. R. etc.Cellular & PCS April. One can make a reasonable estimate of ground radio coverage using any one of a number of computer software packages which estimate path loss. Measured data from the field indicating RSSI can be plotted as an overlay in the form of contours of constant RSSI and/or BER as well.C. MSI Planet.RSSI) and constant BER value on a map is the true indication of cell coverage.C. Real measured data does not involve assumptions to the same extent that these radio coverage software packages do! ©1996-2000. A major reason for the greater accuracy of experimental data is that most computer programs for computing radio wave propagation treat the effects of buildings. as well as data from the US Coast and Geodetic Survey (USCGS) which indicates the height of the ground above sea level at each 15 minutes of latitude and longitude. From this we get a theoretical graph of signal strength and/or BER contour s.
and then decreased slightly in the winter. 2000 Control of Cell Size • Antenna height should be sufficient to cover largest expected cell via line of sight – Inflexible situation to be limited by height • Downlink range mainly controlled by – Base Tx power level adjustment – Antenna gain. etc. Usually no physical change is made in the base receiver usable range. Changes in the nominal uplink (base receiver) cell size are actually mainly sofware changes in the handoff thresholds for RSSI or BER. except by mechanically tilting the antenna to point its main lobe at the most distant service area.Cellular & PCS April. which are discussed later in these notes. R. R. Base transmitter power must be increased slightly in the summer.C. ©1996-2000.) on trees over the street and sidewalk areas where the mobile users are located. The uplink is designed with adequate coverage so it is as large as the largest expected downlink coverage area.Levine 17 Cell coverage must frequently be adjusted seasonally due to the different amounts of absorption from foliage (leaves. directivity – Use of electrical and/or mechanical antenna downtilt • Electrical downtilt is preferable for omnidirectional antennas • Careful about back lobe effects with mechanical downtilt in sectored cells Revised 2001 ©1996-2001.C.Levine Page 17 .
Part of this dynamic range is the result of automatic gain control (AGC). 2000 Minimum Signal • Minimum usable signal strength must exceed total noise and interference by the appropriate C/(I+n) ratio (typically 17 to 18 dB for analog 30 kHz) • Noise level of receiver – Fundamental physical property: thermal motion of discrete electrons – Calculate from temperature. the minimum usable signal strength at the periphery of an interior (interference limited) cell is typically about -95 dB. Stronger signals are automatically amplified at a lower amplification setting. We thus say that the receiver has a 50 or 60 dB dynamic range. Only the the outermost boundary of the outermost cells of a system are noise limited. which internally adjusts the receiver RF amp lification to suit the incoming signal strength.Cellular & PCS April. Weaker signals are amplified to the fullest extent possible. We design the system to use the lowest feasible transmitter power all around (bases and mobiles) so we get maximum talk time from battery powered mobile sets and minimize wasted excessive power. ©1996-2000. at about the same voltage level regardless of their radio signal strength at the Rx antenna.C. R. Cellular and PCS systems are designed to be interference limited.Levine Page 18 . where they are demodulated. Thus. level set by design as low as possible – Greater Tx level in all cells works. In all other parts of the system. The result is that signals appear at the internal detector or discriminator stage of the receiver. bandwidth: n=kT•∆f.8 fW or -121 dBm for 200 kHz signal bandwidth – Noise Figure of receiver (added noise from internal amplifier) adds typ. this is 0. but wastes power Revised 2001 ©1996-2001. 3 to 7 dB more noise (result: -119 to -116 dBm) • Interference: primarily co-carrier signals. the maximum power that a radio receiver can use without distortion or intermodulation is typically about 50 to 60 dB greater than the minimum power due to internal noise.Levine 18 A noise limited system would require a signal strength at the outer periphery of a cell to be at least -101 dBm for a receiver with a -119 intrinsic noise and using 18 dB as the desired S/n ratio for good reception. R.C. Incidentally. the interference (primarily co-channel interference) from other cells in the system is the primary factor.
Cellular & PCS April. R. 2000 Multipath. earth surface) • At the Rx antenna the total signal is the sum of – direct rays – rays delayed due to several reflections and a zig-zag path • Multipath can cause both fast fading and inter-symbol interference (ISI) Revised 2001 ©1996-2001.Levine 19 Radio multipath occurs in all frequency bands. but the way in which it affects UHF radio for PCS systems is primarily due to the fact that the wavelength is smaller than human size. so we can move (on foot or in a vehicle) at a speed of several wavelengths per second. Fading and ISI • RF transmission is degraded by "multipath" • Multipath propagation occurs when there are radio reflective surfaces in the environment (cliffs.Levine Page 19 . buildings.C.C. R. ©1996-2000.
etc .5 .7 . R. adds to this to produce the "floor" for interference limited operation. Due to imperfections in the transistors and diodes used in the radio. and used to initiate a handover.2 .Levine Irregular but approximately periodic fades are a characteristic of multipath radio propagation. When the signal strength fades. R. and as we approach the outer edge of the cell.6 . the interference and noise in the receiver dominates for a short time and the output depends on chance rather than the transmitted signal. 5 or even (briefly) 8%.1 . This can be detected due to the use of error detection codes in the digital signal transmissions. 2000 Multipath Fading -30 Received signal level (dBm) Due to unpredictable approximately periodic fading. producing a total equivalent noise n. The radio receiver internally produces a noise level which is the result of thermal agitation of the electrons which make up the small electric current. here. ©1996-2000. bursts of bit errors occur at a speed-related rate here. there is a further increase in noise level described by a so-called noise figure.9 movement distance (meters) Revised 2001 thermal noise level 20 ©1996-2001. The major fades occur approximately a half wavelength apart when the various delayed radio rays are almost parallel.Cellular & PCS April.4 .C. primarily from other cocarrier sources in other cells. by designing the system so the average signal strength is stronger than the combination of interference (I) and noise (n) by the capture ratio (typically approximately 17 dB) even at the outer edges of the cell.Levine Page 20 . The interference. -50 -70 C/(I+n) -90 Average signal power I+n -110 n (w/ noise figure) -130 0 . The power level of thermal variation in current is proportional to the absolute (Kelvin) temperature and the bandwidth of the receiver.3 . This goal is not always achieved fully. here.C.8 . The objective of a good design is to keep the intrinsic bit error rate (BER) below about 1%. here. the BER increases to 3.
C. • In the special case of two waves of equal amplitude. 2000 Fading Happens Because. although the major fades are approximately periodic.Levine Page 21 .. ©1996-2000.C. R. The strongest peak signal levels are about 6 dB above average level.9 GHz Revised 2001 ©1996-2001. • Direct and delayed rays are out of phase at some locations – During part of an oscillation cycle. exact cancellation occurs at some locations – Partial cancellation from rays of unequal power is more likely • Due to short wavelength. R. The deepest fades are about 20 dB below average power level. one electromagnetic wave ray is pushing the electrons in the antenna in the opposite direction from the other ray.Levine 21 We almost never get two equally strong rays producing total cancellation. a very tiny delay time spread can produce significant fading – ~300 mm (12 in) wavelength for 800 MHz – ~150 mm (6 in) wavelength for 1.Cellular & PCS April. In most situations there are a large number of delayed rays having a variety of signal strengths. but most peaks are lower than that. The depth of the fades in most cases is limited. The result is a random fading pattern rather than a strictly periodic fading.. and that does not occur very often.
Levine 22 We never get perfect error. 2000 To Combat Fading. particularly when they are an odd number of quarter wavelengths apart • Time and/or frequency diversity of the signal – Fading seldom occurs simultaneously at two different frequencies in the same place. • • Fading allowance (margin) in RF coverage design • Design to obtain strong enough average received RF power level so that fades are very brief (when antenna is moving)..free digital radio transmission on a moving UHF transceiver. or a wideband signal is used made up of many different frequency components • Interleaving. ©1996-2000.C. frequency • Receive antenna diversity (at the base station) – Fading seldom occurs simultaneously at two places. time. but we can achieve almost perfect final processed data rate if we design the system with adequate compensation for raw bit errors by means of error protection coding and proper use of antennas and equalizers. then reassembled in original order • Error Protection Coding (using additional digital bits): • Error detection codes together with re-transmission algorithms replace badly received data (good for command messages) • Error correction codes allow identification and reversal of identified wrong bits (good for digitally coded speech) Revised 2001 ©1996-2001. R.C.Levine Page 22 .. R. a form of time diversity • Initially consecutive bits of a digital signal are separated and some are sent at a later time than others.Cellular & PCS April. and always operate in regions of adequate signal strength. so signal could be transmitted again later in time. Diversity: place. or RF frequency can be changed intermittently.
there was a great deal of concern that large delay spreads would make a high bit rate system like GSM impractical. it is desirable to design the placement of base antennas so that strong delayed reflected radio signals (such as from the side of a cliff or building) are minimized by placing the base antennas so that illumination of such reflective vertic al wall. R. Typical delay spreads in crowded city areas are 4 to 8 µs.Levine Page 23 . receivers are equipped with an adaptive equalizer – Adaptive equalizer (and also the similar "RAKE receiver" used for CDMA) produces delayed copy/ies of the received signal waveform and use(s) these copy/ies to cancel the physically delayed radio signals – This equalizer examines the effect of multipath delay on the known training sequence.6 µsec).C.Levine 23 Before the actual field tests. Again.like surfaces is avoided.. these concerns appear to be unwarranted based on actual system performance. However. and then uses this information to undo that effect on the other bits in the cell using the internally delayed replicas of the signal • Second.Cellular & PCS April. Measured delay spreads in the foothills of mountains are as large as 16 µseconds. Nevertheless. although the lower bit rate there makes the symbol duration more than twice the worst measured delay spread. ISI can be a problem.. R. ©1996-2000. Similar concerns were expressed before the testing phases of the North American TDMA system. • First.C. the error protection codes help detect/correct errors regardless of whether they are due to fading or ISI • ISI cannot be combated by just using a stronger signal. Revised 2001 ©1996-2001. This is a larger time interval than the bit duration of the GSM/PCS-1900 bit (only 3. the adaptive equalizer used in GSM and PCS-1900 does a more than adequate job correcting this ISI. 2000 InterSymbol Interference (ISI) • When the multipath delay spread is greater than about 20% of the digital symbol duration. To combat ISI.
R. 2000 Handover Requires Coverage Overlap Idealized circular omnidirectional cells don't appear circular in display.Levine Page 24 .C. then the handover process may start before the MS enters the valid service region of the adjacent cell. can receive adequate service from any one of 3 cells. ©1996-2000. If the handover boundary is set too far out. but the recurrence of such problems is a clue that the handover threshold should be fixed. Note when this happens on a high-speed expressway which crosses the mutual cell boundary lines.Cellular & PCS April. there are two possible handover target cells. like region z. When a MS enters that area from one cell. In this diagram. equal BER in a digital system). but also involving RSSI . a circle is used to illustrate two important contours of equal power (or more aptly. It is desirable that the handover threshold (usually based on BER. The MS can operate adequately all the way out to the outer circle. Some areas. The GSM system does have a recovery algorithm to reconnect a call which is dropped because of such a problem. Minimum performance contour z A x y B Handover threshold contour Note desired match of handover areas • • curvilinear triangle z has choice of 3 cells Distance x-y should be sufficient for fastest vehicle to stay in "green" band during the slowest handover Revised 2001 ©1996-2001.C. If the handover boundary is too close in. R. data based on historical patterns of handover and geography of roads in area z will help to chose the best target most of the time. in both the yellow and darker areas.radio signal strength indication in many systems) should be aligned with the outer boundary of the adjacent cell or sector.Levine 24 Cells are often represented by simplified abstract shapes such as a hexagon or a circle. If the relative signal quality or the ava ilable number of traffic channels in the two cells does not immediately settle the issue. then the system may not have time to perform a handover for a very fast vehicle moving from one cell to another.
it is desir able that the MS can actually come in a little closer than the starting radius before canceling the handover process. it is likely that there is an unsuspected source of RF interference which is increasing the RSSI but corrupting the data. R.. R. if you find that BER and RSSI contours which should match geographically are very separate. Then the thresholds must be set as described in the previous page so the cell boundaries line up with adjacent cell thresholds. and also to not start a handover (or cancel it) when it is not needed (since it consumes internal processing and data communication resources within the infrastructure).C.Levine Page 25 . (uplink) radio signal strength indication (RSSI) is the major measurable parameter – Sometimes RSSI is misleading. and you should search for and remove it. More on handover later in the course. is the adjustment of handover threshold values. The preferred parameter to control handovers is the bit error rate (BER). The prerequisite to meet these objectives is proper RF coverage in all adjacent cells. particularly when significant interference is present • For digital systems.Cellular & PCS April. 2000 Handoff Control Parameters • For analog systems. However.Levine 25 Revised 2001 One of the few things which the system operator can do to "tweak" or "fine tune" the system after all the antennas are fixed in place. BER theoretically tells all. ©1996-2000. – Incorporates effects of weak RSSI and/or bad interference – BER reported for traffic channel in mobile-assisted handoff (MAHO) • Some operators like to trigger handoff based on occurrence of either one or the other: – RSSI below sector-optimal threshold – BER above sector-optimal threshold • Handoff process cancellation levels (of BER. The "cancel" threshold is normally set at a better signal quality than the start handover threshold to avoid "ping-pong" starting and stopping of the handover process for a MS which is moving along the threshold boundary.C. and then turns around and moves back toward the center of the cell.. The objective is to hand over all calls without dropping any. RSSI) are also important – usually set at better signal levels than the start threshold. If an MS enters the darker handover zone on the previous page. for intentional hysteresis ©1996-2001. Everyone treats this as a "magic number" and there is as much superstition as fact surrounding the methods used by various operators to set optimal threshold levels.
In a TDMA system like GSM or IS-136.Cellular & PCS April. Of course. and 10 carriers in the remaining 30%. where n is the cluster count) • Sectorize (if originally omni) and reduce cluster group n from e. from 7 to 4.g. you can change out the antennas and go to 3 sectors per cell. The half. You can then increase the number of carriers in each cell from 10 to 18 (75/4)..C.rate codec and improved smart antennas have great promise for the future. or C band PCS license). • Install more RF carriers in cell (up to limit of TotalCarriers/n. most PCS systems are initially designed with sectored cells to begin with. that first beacon carrier can support the shared channel used for call setup and related operations..) If your initial design was not sectored. To add more capacity.CDMA is not amenable to overlay carrier use. If your license only permits 75 PCS-1900 carrier frequencies (A. Overlay of some additional carriers which do not fit into the normal frequency plan is helpful only if you have a heavy concentration of traffic near the center of the cell. and install TC/n carriers in new small cells • Use half-rate speech coder (if acceptable quality) • Use "smart" directional adaptive antennas (when available and economical) Revised 2001 ©1996-2001. R.714… you can install 11 carriers in about 70% of the cells. you can only install 75/7 or 10 or 11 carriers per cell. Each additional carrier can each support up to 8 (or 3) conversations. You need to justify the capital cost by an almost immediate increase in traffic density. 2000 To Add Capacity.Levine 26 An operator will begin by installing at least one carrier supported by one base transceiver (BTS) in each cell (or each sector). R. Cell splitting is workable but expensive. just add more carriers. immediately following up with a new frequency engineering plan with n=4. then install more carriers • Overlay center part of cell with lower power carrier(s) [also called tiered cell coverage] – Only adds capacity in central cell portion – not applicable to single frequency CDMA • Split cell. But there is a limit due to the number of carriers in your licensed band. and your frequency plan is n=7.B. ©1996-2000. The remaining 7 (or 2 in IS-136) physical channels (time slots) can support conversations.C.Levine Page 26 . (Since the exact ratio is 10.
This limits the cell size to not less than about 50 to 100 meters.Levine 27 Academic sources inaccurately describe cell splitting as an essential feature of cellular frequency reuse systems. If the growth is too slow. This is incorrect for two reasons.C. R. the cellular operator may lose money for months or years until the traffic and revenue increase by a factor of (typ ically) 7 in the split cell area.C. Second. First. R. minimum 5 mW handset Tx power) so you can't split again and again without limitation • High real estate cost of new antennas/cells is a deterrent • Cell splitting is the last choice economically. which allows the capacity to be increased without limit. the lower transmit power which can be controlled in a mobile set cannot go below about 5 milliwatts. after using methods which add capacity to an existing cell site Revised 2001 ©1996-2001. One method is to put only the base transceiver (BTS) at the antenna site. 2000 Before: Cell Splitting After: • Increase of capacity by 7 in center cell (for n=7 plan) • But there is a lower limit on cell size (due to approx .Cellular & PCS April. ©1996-2000. even without an external antenna. and must be compensated by an almost immediate increase in traffic density and revenue. Another method is to feed the antenna remotely using either CO-axial cables or fiber optics to carry the RF signals to/from a central base equipment installation. and then use one base controller (BSC) to serve several BTS locations.Levine Page 27 . Attempting to use a lower design cell size will produce unacceptable co-carrier interference to other small cells. There are several methods for adding more antenna sites without the full cost of a complete base installation. A set of small RF amplifiers and electrooptic convertors is needed at the antenna location for both outgoing and incoming RF signals. This is due to leakage of RF from the internal electronic circuits. the cost of additional cell sites is very high.
Design documents issued 1989-91.C. but later became a generic name for the North Americ an analog 800 MHz technology. Total Access Communication System (TACS) was originally a Motorola trade name. particularly when used by non-British speakers and writers. CEPT is the acronym of the Conférence Européenne (des Administrations) des Postes et des Télécommunications. 2000 Early Cellular Systems • First cellular experimental analog systems (~1979): – AT&T AMPS system (Chicago. R. 9 incompatible cellular radio systems were in service in Europe – 7 different incompatible analog technologies – 2 nations' technology compatible to 2 others. but became a generic name for the British 800-900 MHz analog cellular system (which is also known as E-TACS. for European-TACS).Levine Page 28 . ©1996-2001. R. particularly when used by non-North American speakers or writers.Levine Revised 2001 28 The trade names of the first two systems later came to have slightly different meanings. France). ©1996-2000. DC) • • First commercially operating systems were NMT-450 (Scandinavia) and NTT-MCS (Japan) ~1980 In early 1980s. an international standards body which still exists but today is more devoted to legal and tariff issues. France (a suburb of Nice. such as the MicroTAC™ handset. but no roaming service agreements! • Clearly incompatible with the technology unification plan for the European Union. – CEPT (later ETSI) convened Groupe Spécial Mobile (GSM) meetings (1982) to develop Pan-European second generation (2G) cellular technology. IL) – Motorola TACS system (Baltimore.C.Cellular & PCS April. Advanced Mobile Phone Service (AMPS) was originally an AT&T trade name. Washington. The TAC acronym survives in other Motorola products. Most of the technological standards activities of CEPT (particularly for cellular and PCS systems) have now been taken over by the European Telecommunications Standards Institute (ETSI) with headquarters in Sophia Antipolis. MD.
R. ©1996-2000.5 committee and IS-95 in '92 – TR45. R. More comments on this at the ve ry end of the lecture.C.C.3 decision on TDMA in 1989-90 led to IS54 "dual mode" digital cellular in 1990 (later IS136) – Interest in Qualcomm CDMA proposal in '89 led to TR45.) Revised 2001 ©1996-2001.3 also designed IS-136 "all digital" TDMA in '94 (All these are also called 2G.Levine 29 The FCC and the industry in general has followed a policy of free competition. since nobody really wants to continue indefinitely with so many different incompatible radio technologies. 2000 More North American History • Concern about traffic saturation led to experiments in digital cellular: – Lucent (then AT&T) Chicago FDM demo (1988) • TIA TR45 Standards sub-committees formed to design digital cellular – TR45.Cellular & PCS April.Levine Page 29 . This will eventually lead to an expected "shake-out" in a few years.
No. all three of these access technologies have about the same inherent capacity.actually used in GSM/PCS-1900 and IS-54). Baier. particularly FDMA (frequency division multiple access or use of narrower bandwidth radio channels for each conversation). as well as a few papers which conclude just the opposite.C. In North America." IEEE Transactions on Vehicular Technology.Levine Revised 2001 30 People frequently ask. P. pp. Paul Newson. V. 2. there are also numerous publications which conclude that CDMA is inherently capable of greater capacity than other technologies. with each side claiming that their proposed technology had inherently higher capacity than the others. The most important system differences relate to secondary factors like the speech coder. Jung. Heath. P. a significant amount of time (almost 2 years) was wasted arguing about the access technology. August 1993. ©1996-2000. The following two references both conclude that the theoretical capacity (conversations/kHz/km2 ) of CDMA. 1. Steil. R. provided that all systems compared either all do (or all do not) use dynamic channel assignment (DSI) via voice activity control to fully utilize available channels during pauses in speech. the European public switched telephone network (PSTN). – First objective: higher capacity – Second objective: backward compatibility – These North American objectives somewhat complicated the design of an otherwise simpler system ©1996-2001. Of course. with the attendant technological and regulatory differences. or the economics of sharing common base equipment in TDMA. Mark R. 42. "Why are many aspects of the designs of North American TDMA and GSM/PCS-1900 different?" Some of the reasons relate to the intended connection to the North American vs." IEEE Journal on Selected Areas in Communications.C. "Advantages of CDMA and Spread Spectrum Techniques over FDMA and TDMA in Cellular Mobile Radio Applications. No. 357-364.Levine Page 30 . A. Much of the fundamental difference. "The Capacity of a Spread Spectrum CDMA System for Cellular Mobile Radio with Consideration of System Imperfections. however. and later CDMA (code division multiple access). 2000 Design Objective Contrasts • Original first objective of GSM design was PanEuropean technological standardization – Secondary objective was high-technology to stimulate European production capabilities – Traffic capacity target nominally equivalent to pre-existing 25 kHz European analog cellular bandwidth (200kHz/8) • Backward compatibility was not an objective – No "dual mode" handsets • Contrast this with North American TDMA (IS-54). 12. R.Cellular & PCS April.673-684. May 1994.4. 3. In fact. pp.W. TDMA and FDMA are all equal. V. TDMA (time division multiple access -. is based on the difference in design objectives. DSI.
it is easier for me to work with a person who understands the technology than with someone who resists learning the technology and just wants a "go/no go" technical opinion from an expert. Even though I make much of my income advising executives about technology. I feel that the underlying technology is not mysterious. upgraded • Note recent enhanced full-rate (EFR) coders (~1998) – Digital Speech Interpolation (DSI) can be added.C. "It's too complicated to explain. Feynman. The late physicist. R. or when the y do it is not heavily flavored with the specific technology topics which are most significant for PCS system evaluation. upgraded – Modulation can be changed in design stage – Features and services can be added – Are all other factors held constant? – Is inter-cell reuse interference accurately taken into account? • Over-optimistic CDMA capacity estimates arose partly from mischaracterization of adjacent cell interference as RF white noise Revised 2001 ©1996-2001.C. and the problem is aggravated by the fact that many of the people who need to make executive decisions about which technolo gy to buy do not have a technological education or background.Cellular & PCS April. You can learn what is required. said. Richard P.Levine Page 31 . R." ©1996-2000.Levine 31 Unfortunately the objective of some participants in the public debate about PCS and cellular technology is not always to present all the facts and evaluate them dispassionately. There is more fluff and puffery already thrown out on this subject. 2000 Access Technology Arguments • Many arguments ostensibly raised about access system technology comparisons actually relate to other. You can learn the jargon and read the documents and ask questions. alterable aspects: – Speech coder can be changed. And don't take "expert opinion" as the only answer. "I finally recognized that the reason I could not explain the Pauli exclusion principle [a rule in atomic physics that certain different elementary particles never have the same energy] to my students in simple terms was because I do not really understand it!" Keep that in mind when people tell you. and anyone with an interest and a reasonable background can learn enough to make valid decisions based on their own understanding of the issues.
Levine Page 32 . Second VLR is optional AuC EIR F D VLR G BSC BTS BTS HLR VLR B C BSC A-bis BTS A MSC BSC BTS E OMC to PSTN to other MSCs Um MS BSS Many practical items not shown: power supply. BSC Base Station Controller BSS Base Station Sub-system (collective name for BSC + BTS) BTS Base Transceiver Station EIR Equipment Identity Register (data base). one OMC serves multiple MSCs and other equipment. Can be located with the MSC. In some implementations.includes portable handsets MSC Mobile-service Switching Center. multiple MSCs share the same HLR. HLR Home Location Register (data base). it is only connected to other MSCs. etc. B. PSTN Public Switched Telephone Network VLR Visited Location Register (data base) . In some implementations. Usually built into the MSC. they are after-the-fact. ©1996-2000.C.) were arbitrarily assigned in alphabetical order. --------------------------Interface names (A. C. 2000 General PCS System Structure "Official" block diagram (from GSM) showing major defined interfaces…. HLR and VLR are the same physical data base. MS Mobile Station (or Set). with records active in the VLR specially/temporarily marked as required. Abis.C.Levine 32 This diagram and the names used with it are due to the GSM standards. R.includes both visiting and active home subscriber data.Cellular & PCS April. antenna couplers. Associated with HLR. OMC Operations and Maintenance Center. In other cases. The almost synonymous term Mobile Telephone Switching Office (MTSO) is frequently used for this switch in older cellular systems. A very similar terminology has been adopted for North American standards by the TIA. Associated with HLR. AuC Authentication Center (data base). In some cases an MSC can also serve as a gateway MSC (GMSC) to the public network. Although mnemonics have been proposed for these letters. Revised 2001 ©1996-2001. R. or may be distant. The Um label is taken from the customer-network U interface label used in ISDN. etc. In some implementations. air conditioning.
etc. including the generation of the detail billing record for billing purposes. other physical attributes of MS – List of special services available to this subscriber [e.C. Revised 2001 ©1996-2001.Levine Page 33 .Levine 33 Data from the VLR is used to set up a call and maintain data about the call. R.C. circuit-switched FAX. R. ©1996-2000."Visited" Location Register • Data needed to communicate with a MS – Equipment identity and authentication-related data – Last known Location Area (LA) [group of cells] – Power Class. All the physical.Cellular & PCS April. 2000 VLR Data Base • Misleading name.] • More data entered while engaged in a Call – Current cell – Encryption keys – etc. radio and electronic information needed for setting up a call is available in the VLR.g.
the HLR contains the system identification number and the most recent LA in which the MS was located. also called MAP.C. – Current city and LA • particularly when not in home system (when "roaming") – Authentication related information • In some implementations HLR and VLR are the same physical data base – VLR records distinguished logically via "active in VLR" bits Revised 2001 ©1996-2001. The visited MSC notifies the HLR by means of messages through the signaling network which connects all the MSCs and their associated VLRs and HLRs. the signaling messages used for this purpose form a part of a vocabulary or set of messages described as MAP (mobile application part). 2000 HLR Data Base • Home Location Register • Need not be part of the MSC – One HLR can be shared by several MSCs • Some operators plan a single regional HLR for shared use by several MSCs • Contains "everything" permanent about the customer – IMSI.25 packet data networks.Cellular & PCS April. In the GSM or the IS-136 systems.C.Levine 34 The HLR contains all the "background" information about each subscriber needed to "reload" the VLR when that particular subscriber appears in the service area of the appropriate VLR. These messages can be transmitted via Common Channel 7 signaling associated with a telephone network.Levine Page 34 . In North America. The MAP was developed just for GSM. IMEI. In the European GSM network. This occurs automatically when the MS enters a new LA or new system area. 7 signaling. R. R. Each operator and his vendors make a choice about the specifics among these implementation choices. etc. when the customer is roaming to another city or system. are described in TIA standard IS-41. A series of transactions take place in the cell and on the carrier frequency which the MS identifies as new (different from the System ID and LA of the previous carrier frequency just used before that). ©1996-2000. or they can also be transmitted via other types of data communication networks such as TCP/IP or X. Directory Number. which is a special subset of Common Channel No. classes of service. a similar (but not identical) set of messages.
is a major cost saving compared to the need to fully equip each base station with its own control equipment installation in analog technology. R. The multiplexing and the low RF power level used in PCS systems also makes the equipment smaller and less costly to install. 2000 Base Station Assembly • Antennas – Transmit Combiner Processing – Receive Multi-coupler/Low Noise Distribution Amplifier • Base Transceiver – Transmitter Section – Receiver Section • Antenna Diversity Processing in Receiver • Base Station Controller • Support equipment: power.Levine Page 35 . air conditioning. Revised 2001 ©1996-2001. The ability to operate several BTS units off of a common BSC.Levine 35 Many aspects of the base station design are specifically intended to make the cost lower than analog systems.C. reducing the cost and space for equipment.C. Eight channels on one carrier in one transceiver implies that less transmit combiners (or none for a one carrier base installation) are used. R. whether yo u rent or buy. The cost of smaller building space is lower.Cellular & PCS April. the use of only one base transceiver is feasible in a low traffic cell (compared to at least two transceivers in an analog system). even when they are geographically separated. and the power wasted. For example. ©1996-2000.
Tx Combiner Rx multicoupler Rx multicoupler second Rx ant. etc. first Rx ant. are on this carrier and transceiver.C.. In a GSM system.) (not on this slide) Other abbreviations: BCF Base Control Function BT Base Transceiver (with one carrier. etc. Tx ant. dedicated. power equipment. at least 6 of the time slots on this carrier zero are used for customer traffic.. test transceiver. Some examples: Tx = transmitter Rx = receiver Xtal or Cx = crystal (used in some oscillators. reverse access.Levine Page 36 . and the common shared channels such as broadcast. R. Standard industry jargon replaces part of many words with the letter x.Levine 36 Readers who are familiar with analog cellular equipment will note that there is no locating receiver here. The other transceivers devote all 8 (3 for IS-54 and IS-136) of their time slots to customer traffic. A A-bis BSS Two standardized interfaces (A and Abis in GSM) permit competitive suppliers for base equipment. Revised 2001 ©1996-2001. alarm equipment.C. R. 8 time slots in the case of GSM) ©1996-2000. BSC BCF BT 0 BT 1 BTS BT n . air -conditioning. etc. Carrier number zero is associated with Base Transceiver zero. 2000 Base Station Equipment Not shown: band pass or band reject filters in antenna lines.Cellular & PCS April..
then two stages of combiners are used.Cellular & PCS April. and it can add a little bit of gain to the signal. Transmit combiners normally discard half the power from the transmitter in the form of heat.Levine 37 The receive multicoupler can compensate for splitting the receiver antenna signal between several receivers. Most combiners contain frequency filters which must be manually retuned when the carrier frequency of the associated input is changed. but like all amplifiers it also introduces more noise of its own. R. but it is still a significant consideration in the overall power budget. ©1996-2000. R. The important part of its operation is that (almost) none of the power gets into the output of other RF transmitters which share the same antenna. to cause overheating and damage there! A transmit combiner typically has 4 (or for some units.C. 8) inputs and one output.C. because they use a directional coupler which splits the power so half of it goes on to the antenna and half to a suitable power-absorbing resistor with cooling fins. thus facilitating changes in the carrier frequencies at a cell site without dispatching a technician to the site. not to other transmitters • Receive multi-coupler is RF low-noise preamplifier – similar to TV community antenna distribution system – distributes Rx signal to all receivers at same level they would get from an unshared Rx antenna Revised 2001 ©1996-2001. This situation is better with a digital system having less stages of combiners than for an analog system. If more than 4 transmitters must be combined onto a single antenna. and 75% of the power is turned into heat. 2000 Inside the Boxes • Transmit Combiner contains – Tunable resonant cavity filters – Directional couplers • Its purpose: feed most Tx power to Tx antenna.Levine Page 37 . Many vendors make combiner filters which can be remotely tuned by means of a precision remotely-controlled stepper motor which rotates the tuning axle.
or a properly constructed adaptive phased array.C.C. not standardized Revised 2001 ©1996-2001.Levine 38 Early (ca. can improve the signal/noise even more. Diversity usually gives at least 2 dB improvement or more in C/(I+n) even for selection diversity. From time to time. ©1996-2000. preferably >8•λ apart • Several methods for diversity combining: – Switching/selection – Equal gain – Maximal ratio vendor design choice.Cellular & PCS April. 2000 Why 2 Base Rx Antennas? • Dual antennas diversity improves base reception sensitivity by as much as 2 to 5 dB vis-à-vis a single antenna • Spacing of antennas should be odd multiple of λ/4. Most customers were unwilling to pay the extra installation costs for a two-antenna system. vertical separation diversity is not as effective as ho rizontal separation diversity. The amount of improvement depends on the placement and separation of the antennas and the technology used to combine the two diverse signals.Levine Page 38 . Use of more than 2 antennas. 1978) analog cellular mobile stations used two antennas for mobile diversity reception as well. various manufacturers have shown special mobile antenna units with two individual antennas mounted one above the other in a slender tube which is no more difficult to install than a single antenna. and single antenna mobile sets have since dominated the market. but is rarely done in cellular and PCS systems due to increased cost. System designers must design with a single antenna mobile set as their objective. Use of a more sophisticated system such as equal gain or maximal ratio combining can improve that by as much as to 2 dB more. R. Receiver diversity using two antennas improves receive signal/noise ratio by 2 to 5 dB. R. which is universally used at base stations. However. which implies at least 3 dB more signal strength must be delivered on the street compared to the "old" mobile diversity sets.
The switching instants must be restricted to the beginning or end of a TDMA time slot when phase modulation is used. but typically 2dB better than switching diversity Revised 2001 ©1996-2001.Cellular & PCS April.C. IS-95.C.5 to 4 dB C/(I+n) increase in signal/noise • • Equal gain – Adaptive phase shift hardware used to phase shift one channel to match carrier phase of other. where switch times between antennas is restricted to the boundaries of data bit fields – Simplest. and thus the effectiveness of this form of diversity is further reduced if the fading rate corresponds to time intervals generally shorter than a time slot. ©1996-2000. R. but weaker signal is dynamically amplified to same average level as stronger signal as well – Most complex. about 1. 2000 Diversity Combining • Switching/selection – Stronger of two signals instantaneously selected • ~1 dB hysteresis in selection – Causes random phase shifts • a problem for phase modulation like IS-136.Levine 39 The random phase shifts which occur as a result of switching/selection diversity restrict its use with phase modulated (as opposed to frequency modulated) signals. then added coherently – about 1.Levine Page 39 . R.5 dB better than switching diversity Maximal ratio – Like equal gain.
Levine Page 40 . Don't omit it if you are designing or provisioning an installation! ©1996-2000.C.Levine 40 Revised 2001 A test transceiver at each cell is an invaluable piece of equipment.Cellular & PCS April. R. R. but available from most vendors • A remotely controllable transceiver which mimics a mobile set – Controlled from operation-maintenance position (OMP) – Temporarily uses a voice channel in the A interface • Permits many useful tests without sending a technician to the site: – – – – Place or receive a call Talk over the radio link Check RSSI independent of BSS equipment etc.C. ©1996-2001. 2000 Test Transceiver • Not required in standards.
01-0.30 MHz 30-300 MHz 300-3000 MHz (0. This chart shows the terms and shows a typical use of each band: MF.3. absolutely no available bands! – Former point-point microwave and military bands were made available around 2 GHz band • Still some incumbent microwave systems – Government auctioned bands to highest bidder in 1990s • Strong financial motive to move quickly • Technological reasons: – UHF follows "line of sight" propagation – Little/no over-horizon or "skip" radio propagation • MF.C. high.3 MHz) 3. 2-13 North America) UHF. have very specific meanings. 1.Ultra High Frequency (800 MHz cellular. R. R. – VHF and below. 900 MHz GSM. 2000 Why UHF Bands? • "Because they are available" is a legal/historical reason only. and some bands are used for extensive installed microwave or have strong atmospheric attenuation Revised 2001 ©1996-2001..3 GHz) 0.9 GHz PCS-1900) SHF.Very High Frequency (TV Ch. HF short-wave bands would be impractical for cellular – SHF bands require much more costly components. etc.Levine Page 41 .C.1 meters wavelength 3-30 GHz ©1996-2000..1-1 km) wavelength 10-100 meters wavelength 1-10 meters wavelength 0.High Frequency (Short Wave Bands) VHF . terms like medium.Medium Frequency (AM Broadcast band) HF.Cellular & PCS April. although very significant.Super High Frequency 100-1000 meters (0.1-1 meters wavelength 30-300 kHz (0.Levine 41 In radio frequency band jargon. ultra high.3.
with proportionately higher numbers of carriers per cell. and most "B" licenses are owned by former Bell System operating companies. Ordinarily used for voice – IS-136 allows any frequency to be used for TDMA setup carrier – IS-95 uses 10 "chunks" each 1.Levine The North American 800 MHz cellular spectrum consists (at present) of 832 RF carrier frequency pairs.Cellular & PCS April. Each pair is called a channel.C. The 832 carriers are divided legally into two subgroups of 416. A landline telephone operating company can own a financial interest in an "A" license. the total number of carriers in use are divided into 7 subgroups. Analog cellular systems can perform adequately with a 63/1 (18dB) carrier to interference ratio. There are also 21 secondary control channels (used only by IS-54 TDMA dual-mode radios) which may be used for voice instead. with each subgroup (of about 60 carriers) operating in a cell. 2000 North American 800 MHz Band Cellular Spectrum Setup-control carriers (21 each operator) Paired Bands SMR band A' B' 667716 717799 Uplink-Reverse sub-band A'' 9911023 Downlink Forward Sub-band A'' 9911023 A 1-333 B 334-666 Specialized Mobile Radio use A 1-333 B 334-666 A' 667716 B' 717799 824 MHz 825 MHz 835 MHz 845 846. and the geometric pattern of this cluster is repeated throughout the service area (typically a city and its suburbs). In systems using such a modulation and coding technology that the radios can perform adequately with a lower C/I ratio. each subgroup allocated to one of two competitive operating companies (also called "common carriers" in the legal sense arising historically from railroad terminology). As a memory aid. Southwestern Bell owns a portion of the "A" license in New York. In most systems.C. although this term is also used in different ways when a carrier can carry multiple TDMA channels. many "A" licenses are owned by AT&T wireless (formerly McCaw).5 894 MHz MHz MHz • • • • Original 30 kHz carriers 1-666 assigned 1981 Additional carriers assigned 1987 No more carriers likely until after year 2000 Operator optional additional IS-136 setup carriers in middle of A' and B' sub-bands. by means of three sets of directional base station antennas. but is prohibited from owning even part of an "A" license in Dallas. R. and are prohibited from use for voice.25 MHz bandwidth Revised 2001 ©1996-2001.Levine Page 42 8 . so long as it is not in a city where they also own part or all of the landline operation. the cells (particularly in the hightraffic areas of downtown) are further subdivided into three sectors. For example. of which only one is a control carrier (channel) in analog cellular systems. 21 are legally designated as primary control channels. may be used. R.5 849 MHz MHz MHz 869 870 MHz MHz 880 MHz 890 891. ©1996-2000. The "A" operating company is legally restricted to not have a financial interest in the local telephone operating company. where they already own the "B" license. The 7 subgroups are arranged in a cluster consisting of 7 cells. Each sector then uses about 20 carriers. This produces greater capacity in conversations per square km. although many are held by others. four cell or three cell clusters. The "B" operator in general also operates the local "landline" telephone service in the same city. each covering about a 120 degree wedge of the circular cell. although a few are not. In a typical analog cellular system frequency allocation plan. Within the 416 carriers. using the same cell size as comparable systems. at the option of the system operator.
40 MHz block combinations are permitted i Cellular operators are eligible for only one 10 MHz block in their existing services areas Revised 2001 ©1996-2001. Other sections of the spectrum are reserved for later auction. ©1996-2000.C.Levine 43 This shows only a portion of the 1. MTA. R.C.Levine Page 43 . R. Certain bands are reserved for women and minority owned businesses. 2000 USA 1900 MHz Band PCS Spectrum FCC PCS Spectrum Allocation . E & F for use in Basic Trading Areas (BTAs) [suburban or rural] i In any service area.Cellular & PCS April.2 GHz spectrum which has partly been auctioned for voice and data PCS.Basic (rural.8-2. D. to be politically correct in allocation of the spectrum resources.June 9.Metropolitan Trading Area BTA. 1994 Licensed Uplink Paired Bands Unlicensed B T A B T A Licensed Downlink MTA A 1850 MHz 1865 MHz B T A MTA B 1870 MHz BTA C 1910 MHz Data Voice MTA A B T A MTA B 1950 MHz B T A B T A BTA C 1975 MHz 1990 MHz D E F 1885 1890 1895 MHz MHz MHz D 1945 MHz E F 1965 1970 MHz MHz 1920 MHz 1930 MHz i Blocks A & B are for use in Metropolitan Trading Areas (MTAs) i Blocks C. suburban) Trading Area (these names come from Rand-McNally commercial atlas maps of business districts in the USA).
an enhanced full rate speech coder. Amplitude Modulation Advanced Mobile 'Phone System.6 Binary and or 13 kb/s. originally Groupe Spécial Mobile NAMPS Narrow-band AMPS QCELP Qualcomm Code Excited Linear Predictive speech coder. RELP Regular Pulse Excited Linear Predictive speech coder. DQPSK Differential Quadrature Phase Shift Keying. ACELP * 3[6] refers to 3 conversations at present. VSELP Vector Sum Excited Linear Predictive speech coder. Time Division 30 kHz IS-136 Multiple Access CDMA IS-95 Code Division 1280 kHz (Qualcomm) Multiple Access GSM and PCS-1900 TDMA 200 kHz 3 [6]* 62 planned [typically 12 to 18 achieved] 8[16] analog FM (subcarrier AM for control signals) VSELP 8 kb/s Differential + 5 kb/s FEC. FM Frequency Modulation GMSK Gaussian Minimum Shift Keying. R. 2000 Access Technologies Name Technology Analog (AMPS) TIA-553.C. IS91. R.Cellular & PCS April. Revised 2001 ©1996-2001. PCS-1900 North American version of GSM on 1.Levine Page 44 .4 kb/s GMSK FEC. 94 N-AMPS IS-88 (Motorola) Analog FM Modulated Carrier Bandwidth 30 kHz conversations/ speech carrier coding 1 analog FM (telephone 3. FEC Forward Error Correction code. planned 6 in the future with half rate speech coder.5 kHz audio) analog FM Modulation analog FM (FSK for control signals) Narrow Band Analog FM 10 kHz 1 TDMA IS-54. Phase ACELP Shift Keying RELP 13 kb/s Digital FM +9. ©1996-2000.9 GHz band. a special type of digital FM with controlled gradual transitions between the two frequency extremes for the purpose of producing an optimal combination of narrow bandwidth and low susceptibility to interference. Quad. originally an AT&T trade name. π/4 offset ACELP DQPSK QCELP 9.C. GSM Global System for Mobile communication.Levine 44 ACELP AM AMPS Algebraic CELP.
frequency hopping. 9 dB C/I for GSM). In that case what is actually needed to make a fair comparison is the geographic spectral efficiency. in Hz (cycles/s) • At the same time. a higher C/I ratio is required).C. etc. and adaptive equalizers.Levine Page 45 . In a system used in an office or multilevel building. increase from a 2. Many types of modulation have high spectral efficiency.level FM signal to a 4 or 8 level FM signal (this is actually done in some radio paging systems). conversations/kHz /km2 (or other appropriate measure of area). to bandwidth. One can demonstrate this by merely increasing the number of discrete levels used in many existing types of modulation. in bits/sec. ultimately permit the operation of the system at a truly lower C/(I+n) ratio (for example. use of improved antennas. and thus allow more carrier frequencies and more capacity in each cell. The term is sometimes loosely used to describe overall capacity of a PCS system when comparing two technologies. R. Improvements in the fading performance by use of frequency hopping. Our objective is an optimum combination of the two properties." the ratio of data rate. R. we want high resistance to interference – Usability at a low C/(I+n) [capture] ratio is desired – Initial C/I used for GSM was 17 to 18 dB (63/1) • Now operating at about 14 dB (25/1) even 9 dB(8/1) • permits n=4 clusters with 60º sectors – Theoretically we can approach 9 dB (8/1) • theoretically permits n=3 clusters with 60º sectors • requires optimum performance from antennas.Cellular & PCS April.Levine 45 The term "spectral efficiency" is only part of the story. but (with the same signal level) the effect of interference is much worse (that is. but rather poor ability to operate error. the space aspect of this comparison should rightly be based on cubic meters rather than land surface area.free in the presence of interference. Revised 2001 ©1996-2001. For example. 2000 Radio Design Objectives • We want a signal which has small bandwidth in proportion to the information bandwidth transmitted – Relatively high "spectral efficiency. The bit rate increases (4 bits per symbol can be encoded using a 4level FM signal). ©1996-2000.C.
Cellular & PCS April. R. The desired transmit frequency signal can be produced by multiplying the modulated signal with the adjustable LO 2 which is set to 70 MHz below the desired Tx frequency (for example.band emissions. This multiplication of the two waveforms produces two new frequency components. A particular manufacturer's design may differ in many details. Complicated signal processing such as modulation in the Txand amplification and Rx filtering are performed at a relatively low frequency using less precise and less expensive components. The base Tx is similar.C. In the Rx (next page) LO1 is adjusted to 70 MHz below the desired carrier frequency. In the Tx.C. Armstrong. Greater thickness indicates higher power level Revised 2001 ©1996-2001. which are optimized for use at only one frequency.H. Both the Txand the Rx use the "super-heterodyne" technique devised by pioneer radio inventor Col. ©1996-2000. 2000 Mobile Station Structure: GSM Transmitter (Tx) 7654321 Control Microprocessor & memory Microphone speech coder Digital Processes 123 456 789 *0# Band Filter to T/R swtich Attenuates harmonic frequency or spurious out-of. such as performing more or less of the operations in digital form by means of a digital signal processor (DSP) computer chip.2 MHz). E. LO2 is set at 1780. The RF power amplifier (PA) has adjustable power output level.Levine 46 This is a "generic" block diagram of a PCS-1900 Tx section in a handset transceiver. GMSK to Mixer) RF band radio waveforms (red). the modulator is design to work at a so-called intermediate frequency (IF) from LO3 which is typically 70 MHz (although some designers use 10. •Bit interleaving •Encryption •Append frame bits LO 3 Baseband analog waveforms (black.2 MHz.Levine Page 46 . SACCH. to transmit at 1850. and multiplexes 8 different channels onto one carrier. including the initial scanning of the RF spectrum to find and camp onto a broadcast channel so the handset can be initialized to work with the local base system. aside from ramping the power up and down at the beginning and end of each Txburst.7 or 26 MHz). coder to GMSK) Intermediate Frequency (IF) radio waveforms (blue. The "image" signal also produced at 1710. etc. one at a frequency equal to the sum and the other at a frequency equal to the difference of each signal's frequencies. send end etc… Display Keypad Tx carrier selection (tuning) LO 2 analog | digital GMSK Modulator "Mixer" (upconvert) Tx Power Control RF Power Amplifier (PA) analog | digital •Error protect coding •FACCH. All the major functions of the handset are controlled by the microprocessor. microphone) Baseband digital waveforms (azure. but produces higher RF output power levels. R. A replica of the desired signal can be produced at a lower (for the Rx) or a higher (for the Tx) frequency by effectively multiplying it with a local oscillator signal. respectively.2 MHz is removed by filters and/or a dual mixer which subtracts the image signal.
Levine 47 This is a "generic" block diagram of a PCS-1900 Rx section in a handset transceiver. The T/R switch is uses a positive-intrinsic-negative (PIN) semiconductor diode to prevent high transmit power from getting into the Rx.interleaving •FACCH. but has duplicated modules from the band filter up to the FM demodulator for implementing Rx diversity.C. named for Andrew Viterbi. A PIN diode switches from the ON to the OFF condition very quickly. and then evaluating the overall quality of a sequence of three or more received data bits.. Detector to decoder) Intermediate Frequency radio waveforms (blue. Revised 2001 ©1996-2001. it uses the last 3 or 4 bits. R. The received signal strength indication (RSSI) is used in the MAHO reports send to the BS. Most GSM and PCS-1900 Rxs use a Viterbi adaptive equalizer. for use in Viterbi adaptive equalizer. etc. IF Amplifiers FM Detector "discrim inator" Automatic Gain Control (AGC) feedback From Tx RF pre -amp gain is electronically adjustable digital | analog analog | digital Data bits and "data quality" value. The base Rx is similar. a competitive technology.Levine Page 47 . earphone) Baseband digital waveforms (azure. The Viterbi equalizer corrects for ISI by encoding an XOR of two time-adjacent bits in the GMSK modulator (see other page). Other digital technologies such as IS-54 and IS-136 or the RAKE equalizer in CDMA IS-95 mostly use a multiple delay line adaptive equalizer.. Typically.9 GHz band so that the Rx will not have "image" signal reception (fro m antenna signals which are below the LO 1 frequency by the IF value).Cellular & PCS April. and thus has a corresponding 3 or 4 bit delay. the base transceiver uses separate Txand Rx antennas (no T/R switch) since it transmits and receives continuously on all 8 time slots. RSSI T/R* switch Band Filter PreAmp "Mixer" (down convert) Earphone Adaptive Equalizer Digital Processes decoder Tx/Rx control . SACCH. Mixer to Detector) RF band radio waveforms (red). who incidentally is one of the developers of CDMA. The band filter attenuates radio signals from out of the 1. IS-136 allows an electronic Transmit/Receive switch using a PIN diode. The Viterbi algorithm saves bit and accuracy data and checks all combinations of previous bits to find the sequence which has the smallest total error. The amplification of the RF pre -amplifier is adjusted continually by means of the AGC feedback signal to provide extra amplification for very weak signals. R. up to the noise limited minimum receivable signal level. Continuous transmit -receive in analog and IS-95 CDMA requires a radio frequency filter duplexer [diplexer]. with regard to a numerical measure of the signal accuracy.C. a different method altogether. 2000 Mobile Station Structure: GSM Receiver (Rx) Antenna Rx carrier selection (tuning) LO 1 Intermediate Frequency (IF) amplifiers also incorporate filters for 200 kHz bandwidth. ©1996-2000. Also. If the frequency of the received signal is not exactly on the expected value for one bit symbol interval. Greater thickness indicates higher power level * TDMA operation in GSM. a measurement of the amount of frequency error is passed to the equalizer along with the binary bit value (0 or 1). •Slot separation •Remove frame bits •Decryption •Bit de. •Error protect decoding Baseband analog waveforms (black.
When it is close in to the base.1 or 0.C.Levine Revised 2001 48 High power class MSs. ©1996-2000. The PCS-1900 system is designed overall to work with lower power at both the base and mobile transmitter. and to use smaller cells than GSM. The high power MS can operate much further from the base.6 W max • Handsets are by far most popular for "use anywhere" convenience • Some "low tier" PCS systems use 0. most likely in urban areas rather than in the open country. to use only the amount of power needed to communicate with the particular MS now connected. and save some electric power cost.2 W tiny handsets with limited range to base station • Many vendors make a vehicle adapter for handset which gives RF power amplification to ~3W level. R.Cellular & PCS April. reduce co-carrier interference in other cells. In the PCS-1900 system. must have a final PA which can be adjusted for the full range of 16 power steps. like the 20 watt vehicle mounted mobile unit in GSM. ©1996-2001. Since GSM operates on a different radio band entirely. there is no possibility of someone bringing in a MS with power capabilities in excess of the PCS-1900 design limits and causing technical problems in the system.C.8 W max – Handsets typically 0.Levine Page 48 . They use the lowest power step when they are very close to a base station. or when they are in a small cell. turn down the base transmitter power. R. The system operator has the option in PCS-1900 to adjust the base transmit power on a moment by moment basis. 2000 Mobile Station Power Classes & Control • Mobile sets are manufactured in various power classes – Large high-power RF output for vehicle mounting typically 3 W (up to 40 W in GSM) – Medium "bag phones" typically 1. only hand held Mobile Sets of relatively low power are contemplated. and is valuable for large rural cells.
C.) so that changes in one portion do not affect other portions. Some observers have accused the GSM standards of approaching this extreme level of self. etc. you will note that the first 60% of the book is learning jargon. 2000 Logical Channel Taxonomy • Some communication is via shared common physical channel (time slot) – While idle. then 10% is the actual concepts of group theory. we use the logical cha nnel names because they are necessary to refer to the existing documentation effectively. R. it is actually sufficient to know only that certain types of messages are only allowed to be transmitted during certain scheduled time intervals on certain permitted time slots. If you try to read most textbooks on group theory applied to such topics as error protection coding or atomic physics. paging. From your point of view.imposed documentation complexity. In this course. There is great merit in defining the software structure so that it can be divided into relatively independent programming tasks to allow parallel simultaneous development and testing of the software by different programmers. etc.Levine Page 49 . It is also valuable to define different parts of the data message (headers.Levine 49 Today the concept of layered and structured description is generally applied to packet type digital communications systems. to get general system information – broadcast. The rest is jargon! Every profession seems tempted to exalt jargon above meaning. – first access on uplink • Some communication via dedicated channel/time slot – continuation of call setup – conversation. R. and the remaining 30% is application of the theory to the topic at hand! ©1996-2000. data communication • Logical inconsistency regarding logical channels: – some documents categorize various types of messages which can appear on the same physical time slot as different logical channels – in other cases. different types of messages are just categorized as different message types in the same logical channel – part of the "computer science mystique" Revised 2001 ©1996-2001. but we do not dwell on them.C. However.Cellular & PCS April. the general concept of layered description of data communication systems sometimes reaches such a level of complexity that one can spend more time learning the terminology and jargon that in actually understanding how the system works.
Hocquenghem code. RSSI is measured by locating receivers in adjacent cell – Handover begins when RSSI falls below acceptable level • Handover is a co-ordinated change in MS frequency and a switchover of the base channel to another base station and selected frequency *Bose. Chaudhuri. RSSI Radio Signal Strength Indication ©1996-2000.Levine 50 BCH Bose. 2000 Analog Cellular Channels • Specified frequencies (one per cell or sector) used for setup or control – All MSs in cell (except those in conversation) share this frequency – Digital Signaling via binary FSK modulation – Repetitive 5x transmissions (majority logic) with BCH* error detection code • Individual frequency (FDM) used for each conversation in the cell – To make optimum handover choice.C. Hocquenghem error protection code Revised 2001 ©1996-2001.Levine Page 50 . R.Cellular & PCS April.C. R. Chaudhuri. one type of block code with extra error detection bits appended to the end of the message block.
ratio of erroneously received bits to total received bits. R. 8.Cellular & PCS April. Mobile set measurements on adjacent cell RSSI and BER provides data for MSC and BSS to decide which is optimum target handoff cell.Levine 51 BER MAHO Bit Error Rate.Levine Page 51 . 2000 TDMA Cellular Channels • Several (3.) multiplexed conversation channels (time slots) on each frequency • A specified time slot on one frequency used for setup or control in a cell – All MSs in cell (except those in conversation) share this channel – Digital Signaling via same modulation used for coded voice – Mostly convolutional FEC* error protection coding used • Individual scheduled time slot used for each conversation in the cell – RSSI and BER of adjacent cell transmitter measured by MS receiv ers during otherwise idle time slot(s) (mobile assisted handover -MAHO) – Handover when RSSI falls below (or BER rises above) acceptable levels • Handover is a co-ordinated change in MS frequency and time slot with synchronized base switchover • Proper TDMA handover is "seamless" *Forward Error Correction code Revised 2001 ©1996-2001.C. R. which can fool RSSI measurement but which produces more detectable bit errors. Use of BER in addition to RSSI prevents false indication from unduly strong co-channel interference. etc. Mobile Assisted Hand Off.C. MAHO simplifies the system structure and reduces the cost and the data communications traffic load at base stations. ©1996-2000.
Levine Revised 2001 52 One of the objectives of this course is to learn the jargon so you can read and understand the documentation on a cellular or PCS system. but in general the name for a logical channel is a useful thing to know. Access burst. • Others are un-scheduled. physical reserved time is either unused or devoted to a continual background task (such as continuous transmission of digitally coded speech) ©1996-2001. because it indicates unequivocally the physical. – When signals do not appear. R.Levine Page 52 . Paging Channel (PCH). The names and abbreviations originated for GSM are frequently used in the industry. 2000 TDMA Logical Channels • Some logical channels are pre-scheduled uses of the same physical time slot – GSM examples: Broadcast Channel (BCCH). although for some a time is reserved where they may (or may not) appear – examples: Fast Associated Channel (FACCH).C. ©1996-2000. R. scheduled timing. Slow Associated Channel (SACCH). etc. In some cases the authors of the standards may be accused of slight logical inconsistencies.Cellular & PCS April.C. interleaving. and other properties of the information transmitted via that logical channel.
C. and thus labeling the slots from 0 to 15 decimal is also used for some documents.Cellular & PCS April. the IS-54 and IS-136 frame has 6 slots. then waits 2 slots. R. in response to base commands.C. which is the practice in North American standards documents. The basic GSM TDMA frame has 8 slots. When the future half-rate coder comes into extensive use.6 µs/km 2way) radio signal delay Revised 2001 ©1996-2001. R. Remember that the "first" number in a GSM sequence thus is most often 0 rather than 1. to adjust for 3. an alternative numbering identification counting a double frame as one. then waits for 4 "idle" time slots. then repeats • Mobile can do other things in 6 "idle" slots (like MAHO) – Mobile set does not transmit and receive simultaneously when using only 1 time slot for digitally coded voice communication – Mobile can make small Tx timing adjustments. ©1996-2000.3µs/km one-way (6.615 ms 0 5 1 6 2 7 3 0 4 1 5 2 6 3 7 4 Base Tx Base Rx corresponding frame • Base Tx frame start is advanced 3 slots from logically corresponding Base Rx frame start with corresponding slot number – Mobile set using a designated slot first receives. You will note that all GSM/PCS-1900 documents number sequences in time by starting with 0 (zero) rather than 1.Levine 53 Now we get into the time sequence details of the time division multiple access (TDMA) system. then transmits.Levine Page 53 . 2000 GSM TDMA Frame and Slot frame 4.
©1996-2000. broadcast.) Revised 2001 ©1996-2001. The special full duration bursts (frequency correction and synchronization) are only transmitted by the base station. R.Levine 54 The shortened burst is only transmitted by the mobile set. R. 2000 TDMA Time Slot Structures • Two types of burst duration: – Full duration (normal) • Used for Communication of Data – Shortened burst: • Used for a first access RF burst when the distance (and thus the signal delay) between MS and BS is not yet known • Many different types of information content for Normal Burst – Most 2-way exchanges of information – Some 1-way (paging. never by the base station. never by the mobile set.C.Cellular & PCS April.C.Levine Page 54 . etc.
same restrictions on use as Synch Burst above binary bits all zero in F burst 3 142bits 3 8.25 S Burst on slot 0 of pre-designated carrier used to set hyperframe counter in MS Frequency Correction Burst . The base transmitter may adjust the transmit power level separately in each time slot. even of some of them are reserved for traffic and there is no traffic at that particular time. The use of a constant envelope transmit signal.Levine Page 55 . Base stations on the so-called beacon carrier (the one containing FCCH and SCH bursts) must transmit on all 8 time slots at the same power level. The standard does not require the BS to ramp its transmit power down between immediately consecutive time slots.used on all channels (except RACCH) in both directions Information 3 Training Bits Information 3 8.used only on slot 0 of predesignated carrier in downlink direction Information Long Training Sequence Information 3 39bits 64 39 3 8. They use a so-called dummy burst to fill in on the unassigned traffic time slots in such a case. R. while a lower power may be used in a slot transmitting to a MS which is close in. leaving this choice to the various makers of base equipment.Levine 55 Mobile stations must have transmit power off except for the particular time slot which they use.C.25 bits or 576. 2000 Power profile GSM Full Duration Bursts Two 1-bit flag bits (normal burst only) indicate presence of fast associated channel (FACCH) Normal and Dummy Burst . R.Cellular & PCS April. and thus lowers the overall interference level to other frequencies in the system a bit.C. even during the guard times (black intervals on the slide) at a base transmitter makes the RF adjacent carrier frequency emission from the transmitter a little better than required by the standards. but again improves the overall interference level with other cells in the system. except on the beacon frequency. Thus. and therefore they must follow these transmit power profiles. This is optional.25 57bits 1 26 1 57 Normal Burst for all 2-way communications (and BCCH downlink) Synchronization Burst .9 µs Revised 2001 ©1996-2001. a higher power may be used in a time slot which is transmitting to a MS in the outer part of the cell.Entire time slot with ramps and guard period is 156.25 F Burst used to identify physical slot for BCCH and correct the MS radio frequency Most TDMA transmission is full duration. ©1996-2000.
©1996-2000.Cellular & PCS April. the last time slot burst from the old BS is followed by 7 time slots of receiving and other operations. Thus we can achieve a so-called "seamless" handover with no interruption in the TDMA bit stream of the traffic channel.Levine 56 When the MS makes its first transmission to a "new" BS the range and consequent time delay for radio propagation is not yet known. R. Even when there is lost data.Levine Page 56 . and then the next time slot used originates at the new target BS.C. the LPC -type speech coder can bridge over one missing time slot reasonably well. during the handover. A "new" BS situation occurs when a MS begins a call or location update. just for discussion. Therefore the MS transmits a shortened burst.C. R. the same time slot number is used on the new carrier. or when an MS makes a handover between two angular sectors originating from the same BS so that the distance is unchanged. or it could occur when it makes a first transmission after a handover while in a connection. In the case where the two base stations are frame synchronized and. a similar process occurs and in general there is no "lost" digital data or at worst one time slot of data lost. including re-tuning to the new carrier.used on slot 0 of pre-designated carrier in uplink direction and just after handover (optionally) on any time-slot in the uplink direction Training Bits 6 41bits Information 36 3 68. the geography of the cells is known to the MSC/BSC and the use of a shortened burst can be omitted when the size of two adjacent cells is the same within about a km. In many cases of handover.25 bits • • Shortened bursts are used for the first uplink transmission from MS to BS when radial distance and consequent transmission delay are yet unknown – Random access to slot 0 (RACH) – First transmission after handoff (TCH) when distance is unknown (not required when distance is known) Tail bits are shown in gray (all slot types) Revised 2001 ©1996-2001. The BS measures the time of arrival of the burst and then immediately sends a command message to the MS to adjust its timing so that it can subsequently send full duration bursts. When the corresponding time slots are not synchronized at the two base stations (but the timing offset is known) or the handover also involves a change in choice of time slot. 2000 GSM Shortened Duration Burst Power profile Access Burst .
In addition. In most cases. IS-54/136voice traffic is always "full-rate" digitally coded speech at a gross bit rate of 13 kbit/s. This normally designates the use of 3 traffic channels in IS-54. for a frame/carrier which handles a mixture of full-rate and half-rate traffic. and is seldom used because the capacity of most GSM systems is adequate. ©1996-2000.2..C. As examined at the MS. R. Various other special sequences of the 6 synch symbols are designated in IS-136. Also not shown: Half-rate frame with DCCH in slot 1 only and 5 calls.4-2).2. IS-136 prefers the use of all 6 consecutive synch patterns for a 6-channel all-halfrate frame/carrier. the six symbols will be used in the six time slots of a frame in "normal" consecutive order. which requires 2 out of 6 time slots. Synch pattern 1 through 6. Revised 2001 ©1996-2001. This would allow fax or data service at higher bit rates. and which als o can have special synchronizing sequences. R. There are 6 defined synch patterns for IS-136.C. there are future plans for multiple rate traffic channels which carry more bit rate than the full rate channel by using more than 2 of each 6 time slots.2.IS-136 TDMA Frames 40 milliseconds = one frame 4. and the use of only 3 of the synch patterns. each one is 14 symbols in duration (IS136. mobile transmit time slot number 2 coincides with mobile receive time slot number 1. Table 1. Present low bit-rate data traffic (<6.Levine At this time. This permits time division duplex operation (in addition to use of different Tx/Rx frequencies) in fully digital mode. a specific time slot such as mobile transmit slot number 1 immediately precedes mobile receive (and thus base transmit) time slot number 1. which can be indicated by a special sequence of the 6 defined synchronizing b it-field patterns in addition to those shown here. can be supported by using only one time slot out of 6.Rate DTCH (DTCH) in future Call 1 2 3 4 5 6 IS-54/IS-136: 2 Full Rate DTCH (DTCH) with 1 DCCH DCCH Call 1 2 DCCH continued Call 1 continued Call 2 continued Not shown: Mixed use of a frame carrying both full-rate and half-rate traffic.615 ms 8-slot GSM frame for comparison Slot 1 2 3 4 5 6 6. or superior quality digitally coded speech.67 ms 20 milliseconds = 1 block How slots are used: IS-54/IS-136 : 3 Full Rate DTCH (DTCH) one slot Call 1 2 3 Call 1 continued Call 2 continued Call 3 continued IS-54/IS-136: 6 Half.Cellular & PCS April.4-1.5 kbit/s gross rate) and planned future support of a half-rate speech coder. repeated in the sequence 123123etc. 2000 IS-54. Stated another way. However. but it is not popular today because of relatively lower quality.Levine Page 57 42 . when only 3 full-rate traffic channels are supported on one frame/carrier.2 Table 1. and is permitted for use with 3 traffic channels in IS-136. The GSM standards cover one type of accepted half-rate coder.
The IS-136 DTCH is backward compatible with the DTCH of IS-54 with the following two modifications: The IS-54 forward channel has a 12 bit reserved field.2. W=00000000. ©1996-2000. The fast associated control channel (FACCH) is not shown explicitly because it uses the same 260 bits in the slot labeled for DATA. R. Uplink Tx power Coded Digital Verification Color Code Coded Digital Control Ch. Y=0000000000000000 * Bit fields so marked have a different label or function on DCCH.67 millisec Base T x. V=0000. It has no connection with "color" which is a word used here for historical reasons. p.Levine The DTCH was historically used initially in IS-54. X=000000000000. The SACCH is used for slow messages between the BS and MS. the bit fields in the Shortened Burst convey no data or information other than synchronization.C. all zeros. FACCH messages are distinguished fromcoded speech data because of using different error protection code methods. where the 1 bit reserved field (always set to 1) and the 11-bit CDL field are at the right of this figure. D=CDVCC. Reverse. The Abbreviated Burst used on the RACCH (reverse DCCH) is shorter than the Shortened Burst shown here (used only on the reverse DTCH).Levine Page 58 64 . The digital verification color code (DVCC) is an 8-bit code value assigned by the system operator with a unique value in each cell. Also. The FACCH is created by preempting the DATA bit fields when there is a FACCH message to transmit.C. Diagram does not show time duration accurately. and also a different bit field width on the DCCH abbreviated burst compared to the DTCH shortened burst shown here. The Shortened Burst (not always needed) was designed to be used optionally immediately after a handoff for one (or more) frames to control the proper burst timing delay via messages from the BMI (BS) to the MS. Revised 2001 ©1996-2001. The coded DVCC (CDVCC) is the 8-bit code augmented with a 4-bit Hamming code for error protection. The purpose of CDVCC is to detect false reception of co-channel interference from another cell in the system and thus prevent using the wrong signal. R. 84) 6 6 28 Tx power D 12 S 28 D V 12 4 * S 28 D 12 W 8 * S 28 D 12 * X 12 S 28 D 12 Y * 16 S r 28 G2 22 * S=synch.Cellular & PCS April. 2000 IS-136 Digital Traffic Channel (DTCH) Slot 324 bits in 6. Downlink SYNCH SACCH 28 12 * DATA 130 CDVCC 12 * DATA 130 R S V D CDL * 1 11 Normal Mobile Tx. Locator Slow Associated Channel G R DATA SYNCH 66 16 28 * DATA 122 SACCH CDVCC 12 12 * DATA 122 Shortened Mobile Tx for R-DTCH G R S (IS-136. Forward. which retained the ACH for call setup purposes.
Qualcomm revived the idea of CDMA for cellular in 1989 with an improved closed. thus spreading the spectrum of the resulting RF signal. and thus the average value of the product is zero. This was a major problem which caused malfunctio ns of a trial CDMA system used in tests by the Groupe Spécial Mobile in 1986 in Paris.Levine Page 59 . They are the main proponents of CDMA and the IS-95 standard is based on their design. but is rivaled or exceeded by two later enhanced full rate codecs. It converts a low bit rate (narrow bandwidth) digital signal into a high bit rate digital signal. When multiple transmitters send signals with different PN codes to a common receiver. some are unsuitable because they are too obviously periodic (like 10101010…) or contain long sequences of 1s or zeros. ©1996-2000. R.C.loop feedback power control to keep all the received signals from deviating in individual power levels. or the strongest one will dominate all the others and only it can be decoded without errors. To support multiple users each user must have a distinct and separate PN (pseudo-noise) code sequence.C. All of the PN codes used produce minimum mutual interference if they are orthogonal to each other. R. 2000 CDMA: Qualcomm IS-95 • How CDMA works – Speech is digitally coded by a LPC-type codec • The original codec (QCELP) had excellent quality.Levine 59 CDMA was first developed for military point-to-point communications.CDMA is also the basis of some "third generation" (3G) system designs. when one code waveform is multiplied by another (in a NRZ bipolar waveform embodiment) the product is positive and negative in value for equal numbers of PN bit intervals during one data bit interval. the RSSI of all the signals must be very close to equal.Cellular & PCS April. (one codec based on ACELP* used also in other TDMA systems) • voice activity detection (VAD) is used to control transmit power and lower the data bit rate when no voice detected at microphone – Coder bits (at ~10kb/s) are multiplied with PN code (at ~1000 kb/s) to get spread spectrum – Correlation of received signal with duplicate synchronized PN code is used to extract original data Multiple users can theoretically share the same RF spectrum by using orthogonal PN codes – Up to 100 in this following example (64 in actual IS-95 design) • *Algebraic CELP Revised 2001 ©1996-2001. This means that. Although use of a PN bit rate at 100 times the data bit rate provides 100 mathematically orthogonal PN codes.
giving stronger signal and better performance • However. The objective of this design is to give each MS a chip code which can be separated from the combination of all other signals with almost complete randomization of the other signals. Each MS in a cell has a unique chip sequence code which is generated by combining a unique repeated 64 bit code sequence (called a Walsh code) with a very long code unique to the particular MS. A major design objective of CDMA is that all the radio signals arrive at the base receiver with about the same instantaneous RSSI (typically a tolerance of only ± 2 dB is permitted).28 Mb/s chip code – Each chip code chosen for separability (orthogonality) – Desired received signal separated from others by multiplying with properly synchronized replica chip code sequence – Requires approximately equal RSSI at base receiver from all MS transmitters • Soft handoff supports one MS with multiple BSs – Except when near the center of a cell.Levine 60 chip A high bit rate pseudo-random (random appearing but actually deterministic) binary bit sequence used to "chop" or "chip" each signal bit into many intermediate bits. Tight control of MS transmitter power is achieved by a combination of self adjustments (based on MS receiver signal from the BS transmitter) and periodic feedback control signals coming from the BS. The chip code is also called a pseudo-random or pseudo.6 kb/s coded speech (CELP) with 1. this design approach greatly increases the number of BS-MSC links and system complexity • Cannot correct for simultaneous bad RF signal to all base stations Revised 2001 ©1996-2001. 2000 • Multiple code-multiplexed conversation channels on one carrier frequency – Theoretically up to 62 (usually a maximum of 10 to 18 in use simultaneously in one cell/sector) – Also pilot. access codes in each cell for setup channel use CDMA Cellular Channels • Each conversation supported by combining 9. R.C.noise binary code or sequence (PN-PRBS or PRBS) RAKE receiver. This is a type of adaptive radio equalizer which compensates for multipath propagation or for use of multiple base transmitters (at different distances from the MS) by internally delaying and then combining the several delayed versions of the signal which arrive at the receiver antenna.Levine Page 60 .Cellular & PCS April. ©1996-2000. paging. R. the MS is in communication with 2 (or 3) Base Stations all using the same chip code – Better (lower BER) base receiver signal is chosen for each speech block – Internal adaptive equalizer (RAKE receiver) combines all differe ntly delayed base transmit signals at MS receiver.C.
Only one channel used in MS. for example.Levine 61 This simplified block diagram shows the essential parts of the CDMA process.6 kb/s and the actual PRBS bit rate is 1. meaning that the two binary levels are implemented as 1 and -1 volts. The actual data bit rate of the Qualcomm IS-95 system is 9.28 Mb/s.C.g. R. 10 kb/s coded speech a c b One Particular PN-PRBS to RF transmitter (using phase modulation) Other input channels are added at base system. Both signals are in NRZ (non-return to zero) form.Levine Page 61 .Cellular & PCS April. R. ©1996-2000.Pseudo Random Bit Stream) patterns are used. Revised 2001 ©1996-2001. At the base transmitter. several different orthogonal PN-PRBS (Pseudo Noise .C. 2000 CDMA Coder/Transmitter NRZ Data Stream e. one for each separate data (digitally coded speech) signal. At the mobile transmitter there is only one signal and one PRBS pattern. The low bit-rate data signal is multiplied by a much higher bit rate signal.
To decode a CDMA signal. There are other IM problems as well. rather than the 100 described before. Revised 2001 ©1996-2001. This has proven to be a problem. the PN bit rate is drawn at only 10 times the data bit rate to simplify the illustration. thus causing unexpected high levels of BER. or if some of the PRBS codes are not orthogonal to the desired PRBS code.C. This local matching PRBS is again multiplied with the received NRZ waveform. 2000 CDMA Waveforms a NRZ (nonreturn to zero) data stream 1 1 0 1 Waveform d also is a replica of a after error free decoding example shows only 10 PRBS bits per data bit time b PRBS NRZ stream time c Notice the inversion of the NRZ polarity while the data bit is zero.Levine Page 62 . if there are so many of them that they produce a very large random fluctuation of the total signal. it will be the dominant output signal. properly synchronized with the received waveform.Cellular & PCS April. because the other codes are at least partially correlated (non-zero time average product waveform) with the codes in other cells. and the result is to restore the original low bit-rate signal. Also. then the result will not average out to zero over a full data bit interval. The proponents claim that another full 64 codes can be used in all adjacent cells. This latter problem is the one Qualcomm has addressed with their closed loop feedback power control.Levine { Product waveform time 62 In this illustration. one must have available at the receiver a replica of the encoding PN-PRBS bit stream. In the actual IS-95 system. which is the basis of CDMA's original military application.C. However. there are 64 allocated PRBS codes per cell. they will produce zero average disturbance to the decoded waveform. R. ©1996-2000. if one signal is much much stronger (greater amplitude) than all the others. If other orthogonal PRBS coded data streams are also present. on the same carrier frequency as the central cell. It is very similar to encryption. R.
The presence of additional signals causes the decoded waveform to fluctuate about its intended mean/average value. 2000 CDMA Receiver/Decoder Single Broad band RF "front end" receiver d Matching Synchronized PN -PRBS a channel output. should match a from transmitter Receiver has multiple channel capability.Levine 63 The process in the baseband part of the receiver is similar to that at the transmitter. R. R. but in a properly designed and not overloaded system. ©1996-2000. it should be clear whether the data bit value is +1 or -1 volt in each bit interval so the bit error rate (BER) should be low. ©1996-2001.C. MS decodes desired traffic channel and signaling channel(s) Revised 2001 A different channel output Another PRBS etc.C.Cellular & PCS April.Levine Page 63 .
Levine Page 64 . we can load the system with more speakers without producing the same level of instantaneous radio interference that would occur if they all transmitted continuous ly. Other systems have demonstrated VAD and dynamic channel assignment (satellite. undersea cable and fiber. The use of CDMA alone is not more efficient in its use of radio spectrum or in terms of the recommended figure of merit: the number of conversations/unit-area/kHz of spectrum.Cellular & PCS April. thus producing a lower average interference and higher system capacity • Proposed DSI methods for TDMA (such as Hughes Network Systems proposed E-TDMA) has also closed this "capacity gap. because a speaker is normally silent for about 40 to 60% of the time.C. and Hughes Network Systems' E-TDMA digital cellular system) but they all require added coordinating signals. R." • • Relatively easy to configure different data rates for different users MS transmitter is constant envelope phase modulation Class C (high power efficiency) Revised 2001 ©1996-2001. This is the most significant feature of CDMA.) Furthermore.Levine 64 Using VAD (voice activity detection) to turn off the transmit signal during voice silence intervals is the "gimmick" which actually gives CDMA a greater capacity. 2000 Major advantages of CDMA • Voice activity control increases capacity without need for coordinating messages between mobile and base – This is the only distinct source of high spectral efficiency compared to other access methods – Frequency Hopping in GSM also averages silent DTX intervals from other co-carrier cells. in a CDMA system it is not necessary to send additional signals between the base and mobile to coordinate channel assignment when a particular user falls silent for a short while. The use of VAD improves capacity by about 2 to 1 compared to continuous transmitting.C. but the vast majority of cellular/PCS users use voice. By lowering other transmitters' radio power during this interval. R. of course. (This would not be useful for continuous data transmission. ©1996-2000.
and both must transmit the same voice signal with the same PRBS coding. Even using Qualcomm's most optimistic capacity estimates. A great deal has been made by proponents of CDMA regarding the benefits of a wideband signal. the benefit of a wide band signal is significant for both systems. A RAKE receiver adds about as much complexity.Levine Page 65 . which would supposedly not affect the decoding. similar in complexity to adaptive equalizer. In addition. PCS-1900 proponents (particularly Ericsson) have claimed that fast frequency hopping in GSM/PCS-1900 works over 15 MHz or more. trunking capacity.l Revised 2001 ©1996-2001. it raised the equivalent noise level and had to be controlled by means of a RAKE receiver which is. soft handoff cannot be used. IS-95 prescribes a process called soft handoff. etc. The better voice signal is chosen for each 20 ms speech coder time window. mostly very dependent on particulars of site geography. but it is a second order problem. in terms of integrated circuit hardware inside the receiver. R. In fact. but consumes more battery power because it operates at a higher bit clock rate.C. Not to be outdone. etc. and is indeed somewhat better for FH PCS-1900. Well. it greatly adds to the complexity and cost of the CDMA system (both capital cost and monthly recurring operating cost for renting more T-1 links between cells and the MSC). and should not get one embroiled in controversy! ©1996-2000. the cost per customer projected for CDMA is at least 30% higher than PCS-1900. and much of the difference can be attributed to soft handoff hardware.Levine 65 In early claims. another type of adaptive equalizer. in effect. 2000 CDMA Aspects • RAKE receiver. and merely looked like another un-correlated PRBS signal. R. CDMA was said to be immune to the effects of multipath propagation because the delayed signals arrived later than one chip interval. when different carrier frequencies are used in adjacent cells.C. which requires that each cell be under-populated to avoid adjacent cell interference – Soft handoff design incorporates extra processing capacity. multipath statistics.Cellular & PCS April. during which the MS is temporarily in simultaneous communication with two different base stations in two adjacent cells. This requires both BSs to be on the same carrier frequency. etc. • Frequency diversity benefits of CDMA are similar to FH in GSM/PCS-1900 – Very dependent on specifics of each site. corrects multipath • Extra cost of soft handoff when only usable with under-capacity installation – Soft handoff requires n=1 frequency configuration. as does an adaptive equalizer in a PCS-1900 or GSM receiver. since it has less fading. Although this improves the quality of audio if there are many bad radio coverage spots in the handover region. rather than the "measly" 1MHz of IS-95. and both receive and decode signals from the MS.
Levine Page 66 . InterDigital. has also proposed a Wideband CDMA system based on the inventions of Prof. and is consequently much wider bandwidth than IS-95. R.9GHz CDMA PCS service in 14 cities during November. PCS-Primeco started commercial 1. Several 3G system proposals involve CDMA at approximately a 5 MHz chip rate. Donald Schilling. so there is not yet a single accepted third generation technology for cellular and PCS. Actual dependable capacity is likely to be ~6x to 8x analog (comparable to TDMA) Sensitivity to narrow band IM from strong signals – Every receiver has a dynamic range bounded by two power limits • Low power limited by noise effects • High power limited by non-linearity • – CDMA receiver has small dynamic range to receive low level CDMA signal • Availability of contiguous spectrum is sometimes a problem.Levine 66 The wide band receiver used in CDMA can be adversely affected by a strong narrow band radio signal within the pass band. ©1996-2000. 1996. There are also some TDMA 3G proposals as well.Cellular & PCS April. R. they appear to be stalled by lack of capital and there is no announced delivery date for a product.modulation (IM).C. This type of non-linear distortion is called inter. Strong narrowband carriers often exist in band: – On PCS bands: pre-existing point-to-point microwave systems – In 800 MHz cellular: IM products from other cellular carrier frequencies Revised 2001 ©1996-2001. a firm which was primarily in the military electronics fiels. 2000 CDMA's Main Disadvantages • • • Higher cost and complexity of base station (particularly due to soft handoff and its extra needed switch links) Somewhat higher MS cost and receiver power consumption Early promises of 40x or 20x or even 15x analog cellular capacity are not demonstrable in the field. Although they have presented their technical proposals at various standards meetings. Commercial use of CDMA on the 800 MHz band started in several cities in early 1997.C. They also have the problem noted regarding preexisting microwave signals in the middle of their proposed band. Their system uses approximately a 4 MHz PRBS spreading code.
this command requires only 20 ms in TDMA. Thus. to allow for adjustment related to the time delay for the radio signals to travel between the BS and MS. corresponding to a 9 microsecond time interval) or it is known in advance and can be adjusted without sending shortened bursts.C. and do. you need the same size team for CDMA as well. and thus there was an implied promise that the system design and installation could be accomplished more rapidly or with less detailed coverage measurements than for other technologies.Cellular & PCS April. and this loss of one speech coder frame of speech bits is normally interpolated over smoothly by the TDMA speech codec. ©1996-2000. Revised 2001 ©1996-2001. R. – Soft handoff cannot compensate for bad RF coverage or insufficient target cell traffic capacity affecting both cells. etc. which is normally "bridge d" over by the digital codec and is still usually imperceptible. Even when one or two short bursts must be sent for re -adjusting the time advance of the MS. MS is in temporarily in communication with more than one base station for some time interval during handoff. If you need a team of 32 RF engineers and technicians to install. in most real systems the difference in radius between two adjacent cells is either very small (less than 3 km.Levine Page 67 . still occur. in real systems there is seldom any loss of the bit stream at handoff. This has proven not to be true in practice. giving no perception of audio loss. Both analog and TDMA require a brief interruption of the speech prior to handoff to give the command to the MS regarding the new frequency (and time slot in TDMA). bad signal areas. only 20 or 40 ms of bits are lost. R. At one time. test and monitor an analog or TDMA system. 2000 Types of Handoff • Break-before -make handoff – "Hard" analog handoff interrupts audio for about 200 millisec while MS re -tunes to new carrier frequency – "Seamless" TDMA handoff has no perceptible interruption in audio. soft handoff proponents implied that soft handoff somehow compensated for flaws in the radio coverage. there is no similar command. However. • When timing adjustment for distance to new base station is precommanded. In a CDMA soft handoff system. However. There are pros and cons for the way each digital technology does handoff. TDMA handover is truly "seamless" • Make -before -break handoff – "Soft" and "softer" handoff in CDMA.C. Large scale TDMA systems provide for a command to allow the MS to temporarily transmit shortened radio transmission bursts when handing off to a new cell which is very different in size than the old cell. Dropped calls can. which is again covered by the normal interpolation capability of the LPC-type digital speech coder. When this occurs a 20 millisec loss of digitally coded speech occurs. since the MS never changes frequency. MS quickly re-tunes between two normal TDMA bursts.Levine 67 Both CDMA and TDMA systems produce a much better perception of continuity during handoff than analog. • Only in a case of two adjacent cells with extremely large difference in radius is a re-timing adjustment needed (using shortened mobile Tx radio bursts). Good RF coverage is necessary for any type of RF technology to prevent dropped calls.
©1996-2000. Then later we will show the form of the messages used.Levine 68 The signaling over the radio link (Um interface) to accomplish these operations will be described at a general level.C.C.Cellular & PCS April. R. R. with one specific example.Levine Page 68 . mobile destination Handover Release/Disconnect Revised 2001 ©1996-2001. 2000 Call Processing Initialization Call Origination Mobile origin.
it may be provisioned with only one carrier frequency (compatible with the overall frequency plan.C. There are no frequencies which are legally reserved for voice traffic only.C.. An IS-136 control channel always uses slots 1 and 4 (for full rate configuration) or slot 1 only (for half rate configuration). and as many as all 7 if there is minimal call setup.Levine 69 Unlike analog cellular systems. of course) and that frequency may be used as a beacon frequency without restriction. Then at least 4 of the remaining 7 time slots may be used as traffic channels. if there is a particularly low traffic cell/sector. – A "brand new" MS scans all the frequencies • Scans all carriers when turned on in a "new" area and can't find the "old" control channels • Only 21 carriers to scan in 800 MHz analog cellular • IS-136 base stations transmit the carrier number of the setup channel on all frequencies as a CDL "helping pointer" MS Initializes – A previously used MS saves the last known control frequencies found in the city in its memory • Usually provides faster initialization (seconds v s. so long as there is only one (for GSM designs) carrier per cell/sector which is used in that way. IS-136 Revised 2001 ©1996-2001. For example. Particularly helpful for GSM.) This gives the operator great flexibility. however. ©1996-2000. R. 2000 • A non-conversation-state MS "looks" for a control or setup frequency (and time slot for TDMA or pilot code for CDMA) when: – Power is turned on – Signal on the present control channel is weak or has bad bit errors (usually when leaving a cell) – A periodic timer in MS initiates a re-scan • Then the MS scans all relevant carrier frequencies looking for a control channel. nor any reserved for beacon use.Cellular & PCS April. R. Remember that a GSM beacon carrier can be configured to use one time slot for the broadcast and other generally used channels.Levine Page 69 .. and SMS activity. The other slots on that carrier must be used as traffic channels. (IS-136 permits multiple control channels in the same cell/sector. and then several other slots may be optionally configured for the stand-alone channels used for the intermediate messages during call setup. the "beacon" carrier (containing the control channel time slot in a TDMA system) may be any frequency that the operator wants to use for that purpose. location update. minutes to be ready to operate).
2000 Idle MS in a cell • When a power-on but non-conversing MS is in a cell. R. it normally locks onto the control channel of that cell – automatically scans and finds new best RSSI.Levine Page 70 .Cellular & PCS April. best BER control channel as it moves from cell to cell MS could start a call if user dials a number and presses START/ TALK button MS could "ring" if it is "paged" by means of a message broadcast on the control channels of all cells in the vicinity – User can then answer.Levine 70 The MS must find the nearest (or more accurately. and MS can be commanded to change to a voice or traffic channel for a conversation • • • Analog systems normally page all cells in city • Digital systems have multi-cell "location areas" (LAs) which broadcast distinct identification numbers periodically • MS identifies when it crosses a boundary from one LA to another • MS eventually tunes to a "voice" or "traffic" channel (commanded by the BS) after the preliminaries above Revised 2001 ©1996-2001. the best signa l) beacon carrier rapidly. so it can stay in contact with the base system in case of a page. or so that the end user can originate a call. ©1996-2000.C. R.C.
©1996-2000.C. R. then: • MS can originate calls – HLR and active VLR know where the MS is currently • HLR can cause "call forwarding" (in North America) to other cities if pre-arranged In original 1980 analog cellular network designs.Levine Page 71 .Cellular & PCS April. just before power-down it "detaches" • In some of these cases. Fraud was rampant. R.Levine 71 No notes on this page. 2000 Preliminary Registration • In current cellular and PCS networks. much of this did not happ en. Call delivery was not easily done. the MS "authenticates" to prevent fraud • This leads to infra -structure network messages which eventually update the home HLR and the current VLR – After authentication of the MS.C. the MS must register or "attach" to the network (identify itself via a radio transmission on the uplink control channel) – At first power up for the day – When entering a new base system – When entering a new LA • Conversely. Revised 2001 ©1996-2001.
R. R.Levine 72 In the North American cellular system. there was originally no firm plan regarding networks of MSC equipment. which performs each operation with a separate message • North American IS-54 and IS-136 combine several data message elements into a smaller number of longer messages – Also encryption is not done on everything at IS-136 registration time. but rather optionally at calling time Revised 2001 ©1996-2001. the GSM system was designed from the beginning with all its network features pre-specified.Cellular & PCS April. ©1996-2000.C.C. in the various releases of the IS-41 standard. In contrast. Specific features were designed and added as required.Levine Page 72 . 2000 Registration Process • A number of transactions occur during a modern digital cellular or PCS system Registration • Next example is from GSM.
SDCCH 7.) assigned Request location update (send IMSI. the practice of most operators is to establish a new cipher key for each telephone connection. slot. encryption mask Revised 2001 ©1996-2001.5.Cellular & PCS April.) Authentication request (random challenge value) Authentication response (challenge response value) Request to go into cipher mode Acknowledge cipher mode These messages Confirm update. the messages and data elements required for establishing ciphering need not be repeated.C.Levine 73 Details of cipher mode are explained on another page. In a GSM system. a ciphering key sequence number (CKSN) is set in the base and mobile. if required.C. If the MS CKSN value agrees with the corresponding value in the base system. R.SDCCH 8. VLR know MS location ("city" & LA) • MS has TMSI. The purpose and result of this is that a handover can be done. no steps 4. describing which nearby beacon frequencies to scan. Periodic timer caused. when cipher mode is established. call setup or short message transmission. This is not possible in older analog cellular systems. encryption After update: • HLR. assign TMSI are encrypted Acknowledge Release channel Base also communicates with HLR to perform update. Channel (carrier.SDCCH Note 3 types:Normal or Forced by LA change.Levine Page 73 . such as short message service (SMS). new Channel request with random 5-bit code TMSI optional in last two.Attach for power-up in previous MNC system.AGCH 3. 2000 GSM Location Update Direction Logical Channel MS-BS Message 1.SDCCH 9. In that way a new cipher key need not be established just because the MS is doing a location update. the MS sends a message with the CKSN value as a data element.SDCCH 6. during a location update. However.SDCCH 10. Please note also that there is a GSM SACCH channel associated with the SDCCH channel used for location updating and other pre-connection type exchanges of information. and then report back the signal quality on each such beacon frequency (about one report per second). etc. This SACCH permits the MS to receive a list. all the steps to establish new cipher keys in other operations following this should be viewed as optional operator choices without explicitly labeling each one thus. R. Cipher. On subsequent contacts by the MS with the BS (for further location updates or to begin a call). In summary. from the BS. authentication. etc. ©1996-2000.RACH 2.SDCCH 4.SDCCH 5.
Cal Berkeley group indicates no flaw in the algorithm. and can reject the connection if the mobile cannot respond with the correct num ber. such as customers presenting false identity (e. 2000 Ki MS Authentication RAND RAND SRES correct value compare bits MSC (base) A3 algorithm ("black box") SRES • authentic or wrong? Authentication is a process which proves that the MS contains a secret key value Ki – Calculations in A3 are similar to NIST DES.Levine Page 74 . This process has proved to be technologically unbeatable to date. it will not permit imitation of the valid set. IS-136 Revised 2001 ©1996-2001. The base station transmits a random "challenge" number RAND (different value on each occasion when authentication is to be performed for any reason) to the mobile set. since it reduced the security of their system.C. but even if it is known to a criminal. but rather intentional assignment of only small number values as Ki key values when longer constants are feasible. a stolen driver's license) to get service but never paying their bill (subscription fraud). The question "why was this done by the GSM system operators?" remains to be answered.Levine 74 PCS-1900 authentication involves a two-way transaction. nobody can determine which TMSI was assigned to the MS. aside from not being able to "read" the coded speech or call processing data. R. ©1996-2000. The base system also knows what the correct result will be. The other operation involves XOR (ring sum or modulo 2 sum) of the data bits with an encryption mask or key value. but they can be reversed by a person who knows both the algorithm and the secret key value..g. The April 1998 reported "cracking" of A3 by U. Even if the entire radio link transaction is copied by a criminal. This transaction also generates some other secret numbers which are used in subsequent transmissions for encryption of the data. the criminal cannot get the right answer without also knowing the internal secret number Ki as well. The mobile set performs a calculation using that number and an internal secret number Ki and returns over the radio link the result of the computation SRES. The A3 algorithm used for the calculation is not published. Therefore. There is still nontechnological fraud.C. Lucifer or other encryption codes (repeated bit permutation and XOR with distinct secret number) – Performed in separate secure SIM chip (processor and memory) in GSM – SIM may be packaged in a "smart card" • Similar process in North American IS-95. These operations are repeated a number of times (rounds) to thoroughly scramble the data. One is the permutation or rearrangement of the data bits. because the base system begins the next authentication with a different challenge value RAND. R. and there is no technological fraud similar to the major problem with analog cellular.Cellular & PCS April. The mathematical processes involved in DES and Lucifer encryption consist of two repeated operations.
95 and 136) there is an authentication transaction very similar to that shown on the previous page for GSM. in the North American systems (IS-54. and the MS performs a calcula tion using it and an internal secret number called SSD-A. Authentication and encryption setup in North American systems are more often performed at call setup time rather than at registration time.Levine 75 With regard to authentication. IS-136 use only the control channel for these processes Revised 2001 ©1996-2001. A random challenge number is transmitted from the BS to the MS.C. as shown for the GSM type system on these pages. The A-key can be set at the factory or entered by the end user via the keyboard of the mobile set. ©1996-2000. but using fewer (multi-parameter) messages – GSM uses an "intermediate" standalone SDCCH channel for these processes (8 MSs scheduled to use one time slot for lower average bit rate) – IS-54.Cellular & PCS April.C. R. since the MS begins at a point corresponding to the middle of the previous case • Following example is from GSM – Again. it can be changed by means of radio messages to a new value. Only a base system which knows the proper value of the A-key can perform these operations. 94.Levine Page 75 . 2000 Call Setups • A mobile destination call is the most lengthy to set up because the MS must first be paged and then respond • A mobile origination call is simpler. IS-54 or IS-136 perform similar functions. 91. and then returns the result in a reserved data field which is part of the paging response or call setup message (for mobile destination or mobile origination respectively). If the operator suspects that the SSD-A in the MS has been compromised. R. The secret number SSD-A is derived from a second internal number called the A-key.
R.SDCCH 11. For a mobile destination (also called mobile terminated or answered) call. which is used for short intervals by each MS in turn which is involved in a call setup.SDCCH 10. the MS must make radio access to the base system.15. Only messages 1.Cellular & PCS April.13.C.10. most of the messages are exchanged using a SDCCH channel.Levine 76 The steps involved in setting up a connection are similar in all cellular and PCS systems. etc. location update or short message trans mission.Levine Page 76 . To do this. slot.C. Other sequences of call setup are possible in a system with SS7 signaling to the PSTN.) assigned (access grant) Answer paging (or origination request for mobile orig.4 and 9 differ in any details. Accept connect msg. etc.TACH Revised 2001 ©1996-2001. R.SDCCH 9. 2000 Mobile Destination Call Direction Logical Channel MS-BS Message Scheduled paging to MS (using TMSI) Channel request with random 5-bit code Channel (carrier. release.SDCCH 6. message 4 is Acknowledge on TACH/FACCH origination containing Alerting/ringing message dialed digits.PCH 2. 9 is Connect (MS off-hook) outgoing call setup. Therefore. ©1996-2000.AGCH 4. so the voice channel does not need to be connected until the called person answers. the base stations in the last known LA must page the MS. After the connection on the TACH is established.RACH 3.FACCH 14.SDCCH 8.SDCCH 7.) in the same way regardless of whether it is mobile originated or mobile destination. Following the receipt of a paging message (which contains the TMSI for identification of the proper MS).) Authentication request (random challenge value) Authentication response (challenge response value) These messages Request to go into cipher mode are encrypted Acknowledge cipher mode Setup for incoming call Note: Mobile Originate Confirm call omits messages Assign TACH (mobile "re-tunes") 1.FACCH 15.SDCCH 12.3. but are not feasible with present MSC to PSTN signaling which do not have SS7 signaling. A mobile originated call involves primarily the same operations as a mobile destination call. thus prolonging the interval until battery recharge is needed for the MS. Two-way conversation 1. In GSM/PCS-1900 the paging for certain IMSI numbers occurs during prescheduled time windows only.FACCH 16.SDCCH 5. the call is processed for all subsequent steps (handover. the MS can "sleep" (operate with several internal modules turned off) in a low power-consumption state until a paging window time. and then an exchange of messages leads to directing the MS to the correct TACH. reverse arrows 9.FACCH 13.
Threshold handover start BER • Note that either of above could start handover independently – "Delta" of above parameters to cancel handover • In general these delta parameters are used to minimize "ping-pong" effects via intentional hysteresis • Values may be set to independent and distinct values in each sector of the system.Cellular & PCS April.C. ©1996-2000.Levine Page 77 . R. A lot of experimentation is used to "fine tune" the value of each threshold in each cell or sector.Levine 77 Setting these handover thresholds is one of the few parameters which the operator (as opposed to the manufacturer of the base system) has under control.Threshold handover start RSSI 2. and different for different MS power classes Revised 2001 ©1996-2001. downlink channel) 1.C. 2000 Handover Control • Operator has generally 4 measurable parameters to control handover (2 each on uplink. R.
Cellular & PCS April. a properly provisioned digital cellular system should not have any unexpected delays in performing a handoff.C.C.g. more carriers in a cell) – Data processing delays (unusual!) • Install more (parallel) processing capacity *Dependence on traffic leveling implies need for extra cell overlap Revised 2001 ©1996-2001. R. although analog cellular systems suffer from problems arising from handoff delay. R.Levine Page 78 . 2000 Why A Too Slow Handover? • Handover may be unintentionally delayed due to: – Locating receiver measurement delay in analog systems • Fix: install more base station locating receivers (costly!) – Queuing of MSC-BS data link control messages • Fix: More total bit rate on these channels – No available traffic channels in destination cell(s) • Temporary Fix: traffic leveling handovers where feasible* – Delay waiting for a cascade of traffic leveling handovers may drop the call! • Permanent Fix: increase cell capacity (e. ©1996-2000.Levine 78 In most cases.
TACH These messages are encrypted Steps 2-5 may be omitted (and usually are. both of these systems may have a brief interruption of the digitally coded speech data in order to send the handoff command (which requires about 0. R.C.FACCH 7.2 seconds). R.TACH 3. so a "seamless" handoff can usually be accomplished with no lost coded speech. In a TDMA system with adjacent cells of approximately the same size.C.2 seconds of speech during a handoff since they have no designed. ©1996-2000. 2000 Handover Direction Logical Channel MS-BS Message Handover command (old cell) shortened access burst (may be repeated) Timing adjust command Trial burst (optional to operator) unacknowledged confirmation handover confirmation Two-way conversation 1.FACCH 6.Levine Page 79 .Cellular & PCS April. in fact) when cells are same or preknown size so timing adjustment can be made via the handover command. Revised 2001 All steps except 1 are on the TACH in the target cell. Analog systems always lose about 0.FACCH 2.Levine This shows the commands which are sent AFTER the system has already determined that a handoff is necessary and which cell (and carrier frequency and time slot) should be the target of the handoff.FACCH 5.in way to save and repeat prior speech during that time. it is not necessary to make timing adjustments for the transmission delay. Of course. CDMA systems also have continuity of speech during a "soft" handoff. Handover may occur while MS is on a TACH channel or while on a SDCCH channel. but the missing data is interpolated over because of the design of the speech coding process. 79 ©1996-2001. Produces "seamless" handover with minimal speech effects. Example shows TACH.FACCH 4.
In addition. it is better to drop a call attempt which is still at the dialing stage.FACCH 3.FACCH 2. 2000 Release (disconnect) Direction Logical Channel MS-BS Message Disconnect request Release (response) Release confirm Physical channel release Disconnecting Unacknowledged ack 1.FACCH 5. If you have to drop something due to an uncontrolled situation (bad signals. ©1996-2000. This does not exist in the design of other non-GSM systems -you must manually redial if you are accidentally disconnected.). R. there is a whole procedure designed for GSM/PCS-1900 to automatically re-establish a call which was accidentally or unintentionally disconnected due to bad radio channel errors or other problems. lack of channel capacity.FACCH These messages are encrypted Example shows MS requests disconnect first.Cellular & PCS April.C.FACCH 6. since the customer will not be so irritated (and often needs to only press the SEND/TALK button again). then BS follows. Release may occur while MS is on a TACH channel or while on a SDCCH channel.Levine 80 The number of confirming acknowledgments used in this exchange leading up to a disconnect is testimony to how important it is to never. R. Revised 2001 ©1996-2001.C. and not shown explicitly in the diagrams.Levine Page 80 . Example shows TACH (messages all sent on FACCH part of TACH). Opposite sequence is also supported. never disconnect a call already in progress.FACCH 4. etc.
– Today GSM standards provide for 14.2 kb/s ) is also supported using 2 or more time slots for the same TDMA connection. This linking of two channels for higher bit rate is described in the IS-130 standard. 2000 Circuit-Switched Data or FAX • In addition to voice.) use a simple serial data (RS232) connector • The MSC contains a modem pool for PSTN connection* • GSM data was originally limited to 9. ©1996-2000. used with the IS-136 North American TDMA standard. IS-136. R.4 kb/s in GSM or 13 kb/s in IS-54.6 kb/s is the present maximum customer data bit rate.4 or more kb/s (such as 19.6 kbit/s since significant amount of error protection code must be added and available total bit rate is 22.C. 14.C.Cellular & PCS April. R.Levine 81 The actual PCS-1900 system data throughput for the low bit rate data streams allows some extra bits for data overhead used or originated by the terminal. digital cellular systems provide for circuit-switched data and FAX • Special connectors (Terminal Adapters) are provided on some MSs for a FAX or data terminal – The FAX adapter contains a FAX MODEM which extracts the binary FAX information – Other devices (laptop PC.Levine Page 81 . CDMA systems also support a so-called "Rate 2" connection with 14.4 kb/s data.4 kb/s by linking two time slots. *Modem at MSC is an example of inter-working functionality (IWF) Revised 2001 ©1996-2001. Although 9. etc.
Cellular & PCS
April. 2000
Packet Data in 3G, 2.5G
• 3G and 2.5G technology enhancements of cellular and PCS technology support packet data • Subject of later lecture in EETS8304/MS708N • Packet data allows efficient use of a logical channel by numerous bursty data flows to/from distinct subscribers.
– Unlike circuit switched connections, which dedicate a channel to one subscriber even when there is no data to be transmitted
Revised 2001
©1996-2001, R.C.Levine
©1996-2000, R.C.Levine
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April. 2000
Error Protection Codes
• Convolutional codes
– Similar to multiplication*
• Cyclic Redundancy Check (CRC)
– Similar to transmitting long division* remainder, recalculating it and comparing at Rx end
• Block codes
– Similar to matrix multiplication* – Fire code used for call processing messages in GSM/PCS-1900
* Binary arithmetic is performed without usual carry or borrow for these codes (so called modulo-2 or ring-sum arithmetic)
Revised 2001
©1996-2001, R.C.Levine
Now we have an idea how a message is put together. When the message or some speech coding bits are ready for transmission, they need to have error protection applied to them. Several different types of error protection codes are used in GSM. The major types are listed above with examples on following pages. There are also some specific types not explicitly listed which are used only in one context such as the shortened burst. IS-54 and IS-136 systems use convolutional and CRC codes as the GSM system does, but not block codes like the Fire code. In addition, interleaving of the bits over a number of consecutive time slots is used in both GSM and IS-54 and IS-136. After the bits are re-assembled in the order they had before interleaving, the number of consecutive bit errors due to a radio channel fade is thus reduced. This allows various error correction codes to work more effectively since they have a limitation on the number of consecutive errors which they can correct.
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April. 2000
Convolution Code
• Analogous to multiplying* by a pre determined constant before transmission • 10110•10101=100101110, transmit product • Divide* received bit string by the predetermined constant at receiver
– Non-zero remainder indicates errors – Some patterns correspond to limited numbers of bit errors at identifiable bit positions
• correct error(s) by reversing those bits (0 <->1)
– Other patterns correspond to more than one error condition * Ring sum or
• errors are detected but not correctable
Revised 2001
modulo 2
©1996-2001, R.C.Levine
The two binary numbers shown in the example correspond to decimal 22 and 21, respectively. If we did ordinary arithmetic multiplication with carry, the product would be decimal 462. Examination of the result above shows that it corresponds to decimal 302, because we did not carry in cases where there were two or more binary 1 values in a bit column at intermediate stages of the multiplication process. The convolutional code is used on most (but not all) of the bits from the speech coder, and on all of the bits from data sources, but (in GSM and PCS-1900 only) in conjunction with a Fire block code as well for call processing messages.
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C.C. ©1996-2000. in combination with a convolutional code. The CRC is used on only some of the most important bits in the speech coding. Of course.Levine Page 85 . R. an can fix a very limited number of errors as well. and the computed and received CRC compared – Non-zero difference indicates errors – Some patterns correspond to limited numbers of bit errors at identifiable bit positions • correct error(s) by reversing those bit values (0↔1) – Other patterns correspond to more than one error condition • errors are detected but not correctable * Ring sum or modulo 2 Revised 2001 ©1996-2001. The exact properties depend on the length and type of divisor used to calculate the CRC. 2000 Cyclic Redundancy Check Code • Analogous to dividing* by a pre-determined constant and appending the remainder (the CRC) to the data before transmitting • The division* is repeated at the receiver. a longer divisor will produce a longer CRC remainder as well.Cellular & PCS April. R.Levine 85 CRC is a good error detecting code.
used only in GSM and PCS-1900. Fire. named after its inventor P. It is a very good error detecting code. and is used only for data which can be retransmitted with some delay.E. is the Fire code.Levine Page 86 . an incorrectly received packet is retransmitted repeatedly until it is received error free and is successfully acknowledged. ©1996-2000. R. In this way.C.C. ARQ (automatic request to retransmit) protocol uses an acknowledgement signal from the receiver back to the transmitter to prevent automatic re-transmission of each message packet. R.Cellular & PCS April.Levine 86 The particular block code. computed parity bits are compared to received parity bits – Non-zero difference indicates errors – Some patterns correspond to limited numbers of bit errors at identifiable bit positions • correct error(s) by reversing those bits (0 ↔ 1) – Other patterns correspond to more than one * Ring sum or error condition • errors are detected but not correctable Revised 2001 modulo 2 ©1996-2001. without affecting the system too adversely. which are appended to data and then transmitted • Matrix process is repeated at receiver. 2000 Block Code (only in GSM) • Data is matrix multiplied* by a pre-defined matrix to generate parity check bits. It is not a forward error correcting code and is not used for speech coding or customer data. by means of an ARQ protocol.
Most of the bits in Classes 1a and 1b are most significant bits of filter coefficients and other numerical bit quantities which have an obvious significant effect on the sound output if they are wrong.1a and the first bit in Cl.Levine 87 The bits are arranged in order of importance to speech quality by the simple but tedious method of intentionally introducing 50% BER into one selected bit in a recorded voice sample. since there is a observed larger change in quality between having errors in the last bit in Cl. 2000 Speech Coder Selective Error Protection Coding • GSM RELP coder example Original 260 bits produced by RELP coder from 20 ms speech sample time window arranged in order of effect of bit error(s) on speech quality. Class. R. and detect any bursts of errors which are within a consecutive group of 5 bits.2. Class 1a are most important. By ranking the quality of all such samples.2 bits have no error protection 189 bits produce 378 bits due to convolution with 189 bit constant. and likewise with the last bit in Cl. and comparing the subjective quality with a similarly processed recording which has 50% BER introduced into another bit.1b Convolutional Code 50 3 189 bits 132 4 r = 1/2. Tail bits appended to Cl. R. Most Cl. 1b and the first in Cl. then encrypted and transmitted Revised 2001 ©1996-2001.2 bits are least significant bits of numeric quantities and some bits describing the excitation waveform.Levine Page 87 . 1b. (Don't ask me why the three classes are not labeled as Classes 1. compared to the difference between consecutive bits within the classes.2. ©1996-2000. The convolution code can correct several bit errors. and the bit with the least importance to quality is placed at the right in Class 2. The bits are then divided into three classes of importance.C. the bit with the most importance to good quality is placed at the left in Class 1a. and 3 !?!) The 3-bit CRC permits correction of single bit errors in the 50 most protected bits all by itself. 1a 50 bits Class.Cellular & PCS April.C. K = 5 378 bits 78 bits Class. 2 78 bits cyclic redundancy code (CRC) parity bits appended zeros CRC appended to 50 most important bits. 456 bits total will be distributed over 8 time frames via interleaving. 1b 132 bits Class.
and the interleaving method is different from the interleaving used for digitally coded speech. The 24 extra bits allowed in each 20 ms time interval in addition to the 48 data bits.4 kb/s example is shown here because its interleaving method is exactly the same as the one used for speech. of course!). which the coding will carry through the GSM system. R. or a data terminal with a keyboard and display. The 2. FACCH and SACCH data bits on the TACH channel.Levine 88 This a method only applicable to 2.4 kb/s data. or a point of sale terminal. K=5 456 456 bit are distributed by interleaving appropriate to the channel. may be used by the terminal equipment for packetizing the data (header and terminal.C. a different rate convolution code is used. The gross data throughput (due to the extra bits) is really 3. etc. ©1996-2000.8 or 9.6 kb/s. 2000 Data Error Protection-I Used for GSM customer 2.Levine Page 88 .4 kb/s data 48 Bits data + 24 bits for any terminal use 72 Terminal bit stream is partitioned into 48 bit blocks. Such data could be from a FAX machine (running slower than normal. R. then encrypted and sent Revised 2001 ©1996-2001. one for each 20 ms time interval.6 kb/s here. The actual net bit rate is 3. The terminal may generate an additional 24 bits as well. When higher bit rate data such as 4.C. appended zeros 72 4 76 bits Convolutional Code r=1/6. The GSM/PCS-1900 system is designed to eventually deliver the entire 72 bits at the other end.6 kb/s is used.related error protection codes) or any other purpose desired by the terminal. and the terminal can use this in any way desired.Cellular & PCS April.
the 76 bit data block is "multiplied" by a 380 bit predetermined constant. The description of the convolutional code on each figure shows its rate r and its constraint length K.C. Each block is protected overall by a 40 bit Fire code parity sequence. to produce a 456 bit result. A longer cluster of errors will not be properly detected. K=5 456 456 bit are distributed by interleaving appropriate to the channel. there is no carry used in this modulo 2 or ring sum multiplication. Of course. The Fire code using this particular implementation can detect any combination errors of up to 11 bits total in error. The constraint length K is the longest cluster of error bits that this particular code can disclose as an error. 2000 Data Error Protection-II Used for Call Processing messages 184 Bits Messages are partitioned into 184 bit blocks. The rate is merely the ratio of the number of bits of data to the total number of resulting bits. in principle. in the r=1/6 code used previously for customer data. Thus. so it is not true multiplication in the everyday sense of that word. but they are transmitted in blocks of 184 bits.Cellular & PCS April.Levine Page 89 . The predetermined multiplier contains a number of bits equal to the difference between the two bit string lengths. ©1996-2000. Fire-Code 184 40 appended zeros 4 228 bits Convolutional Code r=1/2. and if necessary are reassembled at the other end. R.Levine 89 The GSM call processing messages may be of any length. regardless of their arrangement (consecutive or otherwise). then encrypted and sent Revised 2001 ©1996-2001. R. This is similar to the general rule in decimal arithmetic that the number of digits in the product is the sum of the number of digits in the two numbers which are multiplied. although multiple error bursts separated by a section of good data will all be detected properly.C.
tail zero bits. In practice. It was only designed as a low level privacy method in order to meet US export restrictions on cryptography." but the algorithm used in the North American systems was intentionally designed to be simple and can be "cracked" by analysis of samples of data using a reasonably powerful computer.Levine Page 90 . and generating a mask bit sequence that is truly random appearing and cannot be reconstructed by an eavesdropper. the encryption mask is generated by different algorithms in these two families of system designs.136.Levine This same method is used in both GSM/PCS-1900 and IS-54. depending instead upon the digital coding of the speech to protect the conversations against interception by undesired listeners who only have an analog radio receiver or "scanner. R. most North American system operators have not implemented even optional encryption yet. 11001100 information bits +10101010 mask 01100110 result seen at Um +10101010 mask again 11001100 info restored Revised 2001 Locally generated and properly synchronized copy of encryption mask.Cellular & PCS April. also generated by A8 in GSM. The European A8 algorithm is not known to be "crackable. 90 ©1996-2001. 2000 Encryption The last binary process before modulation encryption mask from algorithm A8 in GSM example information bits Um radio interface replica of original information crypto mask bits XOR XOR information bits (already error protected and interleaved)." ©1996-2000. etc. R.C. We skip non-info bits like training. The major technical problems are synchronization of the Tx and Rx mask bits.C. However.
R.Levine 91 SMS makes a MS into both the functionality of a pocket alpha. etc. 2000 Short Message Services (SMS) • IS-136. R." etc. or to a class of recipients • Automobile traffic reports. PCS-1900 are capable of sending a short message (up to 160 characters in GSM -.C. IS-95. weather warnings." "Your message understood. SMS has the promise of lower overall cost than the use of two separate services using separate customer units for each purpose and separate infrastructure for each type of signal. IS=95 or PCS-1900 service.C. • Arbitrary message from keypad or attached PC. GSM. Revised 2001 ©1996-2001. ©1996-2000. – Messages can be broadcast to all. It is viewed by many industry observers as a very important customer motivator to change over to or begin IS-136.Cellular & PCS April. etc.Levine Page 91 .numeric message pager along with its voice telephone capability.7-bit ASCII code characters) – Individually addressed or broadcast – Appears (scrolls) on alpha display of handset – MS can send short messages as well • Select from menu of "canned"or "cliché" messages – "Let's have lunch.
CDMA.Levine Page 92 . these are all data network infrastructure developments which are beyond the scope of the GSM or IS. CCS7 (which also has numerous other abbreviations) is almost universally used for telephone networks in North America and most other industrialized nations. 2000 SMS Connections SMS messages may originate in many ways – Call-back number. or caller-entered touch tone – Alphabetic message from many sources • • • • E-mail to MS user Internet messages to SMS server Dial-in MODEM for SMS messages Attendant typist transcribes verbal telephone messages into text of SMS message – MS may reply to any of these specifically – But SMS is not a real-time 2-way dialog Revised 2001 ©1996-2001.Levine 92 While digital PCS systems offers many sophisticated ways to deliver short messages to an MS.C. coming from calling line ID of PSTN.xx standards documents. All of the methods described here are equally applicable to GSM and competitive services such as IS-136 (which was openly modeled after GSM). The only direct interaction is the transmission of these messages via the MAP common channel signaling message set. ©1996-2000.C. which is a subset of common channel number 7 signaling. R. and also the so-called NPCS (narrow band PCS) paging systems recently licensed on the 900 MHz band.Cellular & PCS April. R.
etc. may change this and give MSCs full access to SS7 signaling and provide MAP implementation on the PSTN (or at least on part of it). Local Number Portability (LNP).C. Neither system gives the MSC access to all the network capabilities which are designed into the European implementation of MAP. It is likely that the advent of landline local service competition from companies which also own long distance (inter-exchange carrier or IXC) networks as well. 2000 Network Interactions with Public Switched Telephone Network (PSTN) • Until the present. The objection by the public telephone companies to using SS7 all the way to the MSC has been that it opens up possible methods of fraud such as placing calls which are not billed because they are identified as test calls.Levine Page 93 . which has been mandated for cellular/PCS telephones as well as wireline telephones by the FCC is another reason why SS7 signaling will eventually be available to cellular switches. R. the set of messages for handling mobile customers via the SS7 signaling network. but some operators feel that otherwise it costs a lot of money for no additional capabilities. R. ©1996-2000.. which provides better answer supervision – The "vanilla" signaling system is A. It does not indicate specifically when the distant called destination answers the ringing call attempt.B bit signaling. PRI does better on that score.Cellular & PCS April.C.Levine 93 The "vanilla" PBX signaling uses so called "in band" signals to indicate when a call is originated and disconnected. There have been some cases of fraud with existing PBX signaling which lead to this concern. like AT&T. Both systems can provide caller ID by means of different signaling mechanisms. etc. MCI-Worldcom. which has lower cost! • Roamer call delivery can be provided by forwarding calls through the home MSC – This is part of the IS-41 infrastructure signaling standard used by North American cellular operators Revised 2001 ©1996-2001. connections between the PSTN and MSC switches for PCS & cellular used signaling originally developed for PBX (private branch exchange) – The best version of this is primary rate interface (PRI).
C.Cellular & PCS April. to an MS which is visiting Boston from a home location in Los Angeles. a call placed from Boston. the traffic load on the long distance networks using the IS-41 call forwarding method will increase exponentially. although some MAP data messages go back and forth to Los Angeles as part of the call setup. As the number of roaming cellular and PCS users increases. R. ©1996-2000. will be routed entirely within the city of Boston. In principle.C. and the voice channel in the PSTN will never get out of the city. since with positive answer supervision the call could be connected on a TACH only after it is answered by the called destination.Levine 94 MAP and SS7 signaling were specified in detail as a part of the GSM standards and are being implemented in Europe. 2000 Promises of MAP • MAP (mobile application part) is an extension of SS7 signaling for mobile services – Uses the same SS7 network to carry MAP messages as other telephone signaling • Compare IS-41 which normally requires a separate parallel data network as well as the public telephone network for voice • Allows calls to be routed directly to the present MS location – NOT via the home MSC.Levine Page 94 . R. there is a possible saving in air time. And the customer annoyance at paying for unnecessary long distance connections when calls actually originate and are answered in the same city will also increase exponentially! In addition. One would not need to use the TACH channel to listen to busy or ringing tone. as IS-41 does • Adoption of MAP in North America is a business/ political issue to be settled in the future – FCC requirement for local number portability by cellular/PCS carriers ultimately make SS7 signaling interworking between MSC and the PSTN mandatory Revised 2001 ©1996-2001. with a fully developed MAP system.
The inherent traffic capacity of all 3 technologies is about the same Proper Measure= conversations/kHz/km2 2. the very shortest intervals are not always utilized effectively. When optional frequency hopping and discontinuous transmit is used with GSM. so the probability of all channels being in actual use instantaneously is very low. etc.TASI) is a technology which re-assigns a physical channel to a different user when the current user pauses during speech. since the average RF interference is reduced in proportion to the amount of audio silence. an increase in capacity similar to that achieved with DSI is also apparent. in general. due to pauses between syllables. capital or recurring costs) Revised 2001 ©1996-2001.C.Levine 95 A lower bit-rate digital speech coder of equal quality obviously permits more conversations to share an overall link digital data transmission capacity having a fixed number of total bits per second. It is only helpful for speech or highly bursty data. b) A good engineer can make any access technology into a working system 3. DSI is used extensively on both undersea cable and satellite telephone channels. Significant differences in implemented system capacity arise from secondary non-access features. in an older analog version Time Assignment Speech Interpolation -.Cellular & PCS April. The decisions regarding competing technologies depend on overall performance and economic criteria: Measure: conversations/kHz/km2/$ (equiv. Otherwise there will be "clipping" of the beginning of syllables because an idle channel is not always immediately available. TDMA (and later CDMA) are also called the "religious wars" My own conclusions: "Levine's Laws of RF Access Technologies" 1. It is also effectively part of the design of the Qualcomm CDMA system (IS-95) and the Hughes Network Systems E-TDMA system. an increase in capacity of almost 2. In actual systems.or.Levine Page 95 . 2000 TDMA vs. Aside from silence on the part of one participant in a two-way conversation when the other participant is speaking. FDMA • • Access technology debates between FDMA. R. R.5 could be achieved by completely utilizing all these silent intervals. Voice activity controlled channel assignment (also known as Digital or dynamic Speech Interpolation -. The application of DSI also requires a large number of conversations to chose from. ©1996-2000. a typical "continuous" stream of speech is actually about 50 to 60% silence. which is an enhancement to IS-54 and which can also be applied to GSM/PCS-1900 as well. In theory.DSI -. CDMA vs.C. TASI) Corollaries: a) there is a corresponding technological problem in every technology for each technological advantage. phrases. Note that DSI is not useful for digital data communication of long data transfers. for example: • • Speech coder Voice activity control (DSI combined with Frequency Hopping. so the improvement is under a factor of 2.
Levine Revised 2001 96 Truly TDMA handsets. The slow roll out of features leaves this in question. It is apparently not being promoted as a general cellular or PCS system. about 1/4 the cost/channel • Mobile Assisted Handoff – Mobile station can tune to nearby RF carrier frequencies during idle TDMA time slots. 2000 TDMA (GSM) Advantages • Economy of TDMA Base Station – 8-channel transceiver costs about twice the cost of a single channel analog transceiver – Therefore. The PIN diode is smaller in size. and it has been installed in the 800 MHz cellular system in Las Vegas. There are two systems in existence which meet this description. This is not yet available for CT 2+. R. and is intended as a low-tier (short range) semi-public PCS system. The IS-54 dual-mode cellular mobile sets must be able to operate in a simultaneous Tx/Rx mode for the analog-type control channel and the analog voice channel. R. the cost of base stations for other technologies which provide public/private service is dropping so that this advantage is eroding. the full public capability of CT-2+ is only realized when there is an infrastructure of switching software which can provide both mobile destination calls as well as mobile originate calls. NV. In addition. Its main advantage arises from the present relatively low cost home cordless base station available with the private/public handset.C. Cost. systems like IS-91 (GTE Telego.C. Motorola's N-AMPS technology uses narrow band analog FM voice.) provide both public and private answering and origination of calls. report signal strength to system – No extensive "locating receiver" system as used in analog cellular • No simultaneous receive/transmit – Bulk. as used in GSM. CT-2+. although the handsets are designed to be capable of this when it is available. and thus have this feature before CT -2+. but their respective manufacturers appear to be supporting them to a much lower degree than other standardized approaches of the TDMA or CDMA variety. or IS-136 (digital mode only) can use an electronic antenna switch (usually implemented with a PIN diode) rather than a filtertype antenna duplexer. One FDMA system is CT-2. and its Canadian second-generation version.Cellular & PCS April. and slightly more power efficient than a filter. ©1996-2000. However. no major systems currently use FDMA in the form of one conversation per narrow-band carrier. Power Savings of RF antenna switching in MS – Better compared to duplexer filter ©1996-2001. somewhat lower in cost. CT-2+ uses digital speech coding.Levine Page 96 . PCS-1900. so they do not use any type of antenna switching. Several MicroTAC™ handsets are N-AMPS capable. Meanwhile. etc. In the debate between TDMA. CDMA and FDMA.
– 3G allows you to view a movie like "Gone With the Wind" on your handset or mobile PC. – GPRS/EDGE allows you to see a complicated web page with no perceptible delay on your handset or mobile PC. R. R.C. theoretically able to provide up to 2 Mbit/s.Levine 97 ©1996-2000. as well as other GSM radio bands elsewhere • 3G technology. discussed in later lecture. will migrate GSM technology into the North American 800 MHz band. – GPRS/EDGE is likely to be more cost effective for users needing higher bit rates up to ~384 kbit/s Revised 2001 ©1996-2001. or download a large data base.C. 2000 Is GSM the Future "winner?" • In the instructor's opinion. if carried out. may prove to be too costly to be popular.Cellular & PCS April.Levine Page 97 . GSM is the most likely candidate for 2G worldwide adoption as an ultimate "universal" cellular technology standard • GPRS/EDGE is most easily co-supported in the same base system with GSM • EDGE Compact strategy.
CT2+ • Historical Outgrowth of British CT-2 systems – Short Range due partly to low RF power – Short range also implies simpler design • No timing adjustment in TDMA • Little or no error protection code • No adaptive equalization in handset • CT-2 was not a business success – But main problems were not technology • Late development of a common air interface (CAI) contributed to demise in UK – Bad marketing. such as wireless PBX in an office. Germany. or limited use in special areas like the airport or a shopping center. R. Revised 2001 ©1996-2001.Cellular & PCS April.C. they can be a direct competitor to PCS-1900 if sufficient number of networked closely spaced base stations are installed to cover the public areas of the city. timing – Still in limited use in Singapore. R. WACS/PACS.Levine Page 98 . They have the advantage that the handset is then a dual. 2000 Low-tier Systems: DECT/DCT/PWT. Hong Kong. and all but CT-2+ have already demonstrated network capability to locate the handset and deliver calls to it. ©1996-2000. etc. All the new low-tier technologies have designed in capability to both answer and originate calls in the public domain. The original British CT-2 system had the limitation that one could only originate calls in the public domain. but not answer calls.C.use public/private handset for both office/home and also for pub lic networks. The original CT-2 handset could both originate and answer when used with a special base unit as a home cordless telephone only. However.Levine 98 Low tier systems are presently sold now only for limited range use. pricing. due to the significantly lower cost of their infrastructure. because there was no network facility for call delivery. France.
C.Cellular & PCS April. but some installations do not support it at this time • Most low -tier systems have theoretical originate -answer capability. R. but some installations (CT-2+) do not support public domain answer at this time • Vendor support for low tier systems comes and goes. etc. with intentionally less comprehensive service than cellular or high-tier PCS • Use of same handset as both home/office cordless set and public low -tier handset • Shorter range of coverage. ©1996-2000. incomplete geographical coverage in some systems – Service only in heavily used public areas: airports.C. R. • Most low -tier systems have theoretical handoff capability. shopping malls. Currently (2001) on the "back burner" • Several Low-tier systems use Time Division Duplex (TDD) Revised 2001 ©1996-2001.Levine Page 99 . 2000 Low-tier Technology survey – Objective is low cost.Levine 99 Low tier systems are generally designed to provide limited coverage for a high geographical user and traffic density at lower cost than public domain or high tier cellular or PCS systems.
2000 Advantages of time division duplex (TDD) • TDD utilizes short RF bursts in alternate directions – – – – Digital storage buffer at each end produces continuous output Burst digital bit rate is twice the continuous bit rate for single channel 8 or 12 times for TDMA/TDD with 8 or 12 channels respectively Paired spectrum channels (as in FDD) not required • any adequate contiguous single spectrum chunk will work – Use of same frequency in both directions permits use of a base station diversity method only • shared base duplexer for multi-channel system is less costly than an equalizer in every handset – Handset receiver benefits via base transmitter diversity. These problems can be overcome in future designs.Levine Page 100 . R.C.C.Levine 100 TDD has many beneficial properties. • • quality of performance dependent on excellence of base diversitymethodology Tx diversity parameters based on prior time slot Rx properties – Most low tier systems consequently limited to pedestrian handset speeds • ~5 km/h (3 mi/h) is fast walking speed • Low tier systems perform well up to about 40 km/h (24 mi/h) Revised 2001 ©1996-2001... since they both transmit on the same freque ncy. Its most notable negative property is a limited ability (in existing implementations) to make timing adjustments for changes in propagation distance. Note for example tha t the ETSI W-CDMA system includes TDD as one feature. but. and some mutual interference between base and mobile stations. R.Cellular & PCS April. ©1996-2000.
Some involve radio.) • Telephone service via cable TV facilities – could be serious economic competitor Revised 2001 ©1996-2001.C. R. Most of the radio systems based on cellular or other higher cost technology cannot compete reasonably with ordinary wire landline telephone.C. 2000 Fixed wireless systems and related technologies: • Too costly via most existing high-tier technologies – special short-term or interim uses – legal protection of service area to prevent loss of exclusive franchise • Some business exceptions: • BETRS and other rural-radio-telephone systems (Ultraphone. etc.Levine 101 Many other technologies have been proposed for general public PCS use. others are alternative methods of wire transmission. R. Only in special short term applications is high cost wireless useful. PWT?) • Specially designed fixed wireless systems (Ionica. ©1996-2000. It is often installed only to temporarily provide service (such as after a hurricane or other widespread disaster) until replaced permanently by wired telepho ne service. etc.) • Possible use of low-tier systems (DECT.Levine Page 101 .Cellular & PCS April. DSC-Alcatel.
R. ©1996-2000.km • And more important Economic geographic spectral efficiency: conversations/kHz/sq.C. 2000 • Remember Geographic Spectral Efficiency: conversations/kHz/sq. R.Cellular & PCS April.C. the most economical system with adequate capacity will ultimately dominate • Perhaps the industry will gravitate to one main PCS technology after the year 2001 Revised 2001 ©1996-2001.km/$ – $ represents equivalent capital or equivalent total recurring cost of all operations Review of technical and economic points of comparison: • Delivery calendar and product roll-out are very important: availability beats theoretical advantage in this game • When heavy price competition breaks out.Levine Page 102 . although that depends in turn on the amount of traffic load offered by each customer. which is beyond the scope of this presentation). or even for comparison of two vendors selling the same technology! What counts is the proportional cost per conversation (and ultimately per customer.Levine 102 Remember these figures of merit for comparison of different systems.
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