BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to power line communication systems that transmit and receive signals via power lines. 
     2. Description of Related Art 
     Power lines are commonly connected to residential units using two power lines and one ground line. Communication signals may be transmitted using these power lines without requiring additional cabling. The number of physical channels has been limited by the number of wires, each of the physical channels requiring at least two wires. Previously, only one channel has been defined using the three wires. There is a need to increase the number of channels without requiring additional cabling. 
     SUMMARY OF THE INVENTION 
     This invention provides two duplex channels using only two power lines and one neutral line. The transmitter/receiver of each duplex channel includes adaptive filters to cancel cross-coupling between the channels introduced by using the same neutral line for each of the duplex channels. Additionally, the channels are equalized by adaptive filters to compensate for channel distortions. 
     Each of the duplex channels is coupled to its respective power-neutral line pair using a hybrid line driver/receiver device. The hybrid line driver/receiver device includes an echo canceller and a bridge circuit to allow transmitting and receiving signals to/from a single power-neutral line pair over the same frequency bands. The hybrid line driver/receiver device also includes a line driver and a line receiver. The echo canceller cancels transmitted signals that are coupled to the line receiver due to imperfections of the bridge circuit and the line driver. 
     The adaptive filters and the hybrid line driver/receiver device increases signal-to-noise ratios of each duplex channel providing two duplex channels over three power lines. Thus, the communication bandwidth of power line communication systems is greatly increased over that previously obtained with power line carrier communication devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in relation to the following drawings wherein like numerals represent like elements, and wherein: 
     FIG. 1 is a diagram of a power line system connecting residential units; 
     FIG. 2 is a schematic diagram a communication system using power line connections between a residence and a transformer as a transmission medium; 
     FIGS. 3 and 4 are schematic diagrams of transmitters/receivers in two directions using the same set of power lines; 
     FIG. 5 is a schematic diagram of a first adaptive equalizer configuration used in the transmitter/receivers shown in FIGS. 3 and 4; 
     FIG. 6 is a schematic diagram of a typical least-mean-square adaptive equalizer; 
     FIG. 7 is a diagram of a second adaptive equalizer configuration; 
     FIG. 8 is a schematic diagram of a combination of echo canceller and a hybrid line driver/receiver device for transmitting and receiving data; 
     FIG. 9 is a schematic diagram of a hybrid circuit as presented in FIG. 8; and 
     FIG. 10 is a diagram of a power-neutral line pair. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows power line 118 connected to individual residential units 102, 104, 106 and 108. The power line is connected to a transformer 120 which converts power from high voltage lines to power to be supplied to the residential units 102, 104, 106 and 108. Each of the residential units 102, 104, 106 and 108 are individually connected to the power line 118 through connection lines 116, 114, 112 and 110, respectively. A central transmitter/receiver 122 is coupled to the power line near the transformer 120. 
     FIG. 2 shows a schematic diagram of the connection of transformer 120 to the house 108. The secondary side of the transformer 120 includes three lines 208, 210 and 212. The lines 208 and 212 are power lines and the line 210 is a neutral line. For a conventional power line connection, the voltage between each of the power lines 208 and 212 and the neutral line 210 is at about 110 volts AC. However, the phase difference between the power lines 208 and 212 are about 180 degrees apart so that the voltage between the power lines 208 and 212 is about 220 volts AC. 
     Signal choke blocks 214 and 216 are connected in series with the power lines 208 and 212, respectively at the transformer. The lines 210, 236 and 238 form a power line drop from the transformer 120 to the residential unit 108. The signal choke blocks 218 and 220 are connected in series to the power lines 236 and 238, respectively, at the residential unit end of the power line drop. The signal choke blocks 218 and 220 are also connected to power lines 240 and 242 that supply power to the residential unit 108. The signal choke blocks 214, 216, 218, and 220 are intended to isolate RF signals on the power line drop from impedance variations and noises from the transformer and the residential unit 108. 
     The transmitter/receiver 122 and a transmitter/receiver 202 are coupled to the power lines 236 and 238 via coupling capacitors 222-232. The transmitter/receiver 122 is also coupled to a communication network such as telephone network 234. The transmitter/receiver 202 is coupled to terminal(s) located within the residential unit 108. Thus, communication between the terminal(s) within the residential unit 108 and other terminal(s) coupled to the telephone network 234 may be accomplished. 
     The signal choke blocks 214 and 216 decouple noise signals on the lines 208, 210 and 212 from the power lines 236 and 238. The signal choke block 218 and 220 decouple noise signals on the lines 240, 242 and 210 from the power lines 236 and 238. Thus, noise occurring in the communication bands generated by home appliances in the residential units 102, 104, 106 and 108 are blocked from interfering with the communication between transmitter/receivers 122 and 202. 
     The transmitter/receivers 122 and 202 provide two duplex channels transmitting and receiving signals through the two power lines 236 and 238 and the neutral line 210. A first duplex channel transmits and receives signals in the same bands through the power line 236 and the neutral line 210 while a second duplex channel transmits and receives information in the same bands using the power line 238 and the neutral line 210. Thus, the neutral line 210 is shared between the first and the second duplex channels and the same frequency bands are used on both power-neutral line pairs 236 and 210, 238 and 210 for both transmit and receive signals. Because the neutral line 210 is shared, signals in the first duplex channel may be cross-coupled to the second duplex channel and vice versa. This cross-coupling reduces the signal-to-noise ratio so that transmission using two duplex channels is difficult without compensation. Thus, the cross-coupling through the neutral line 210 must be canceled before the two duplex channels can operate independently. 
     The transmitter/receivers 122 and 202 cancel the cross-coupling between the first and second duplex channels as well as performing equalization of the transmission channels by using adaptive filters. Thus the transmitter/receivers 122 and 202 are able to provide two duplex channel communication between the residential unit 108 and the telephone network 234 over the same frequency bands. The adaptive filters are described in detail below. 
     FIGS. 3 and 4 show a schematic diagram of the connection between line drivers and line receivers of the transmitter/receivers 122 and 202. The transmitter/receiver 122 includes differential line drivers 302 and 304 corresponding to the first and the second duplex channels forward inputs. The line driver 302 receives an input signal from signal line 318 and the line driver 304 receives an input signal from input signal line 320. The line driver 302 outputs a differential signal between the power line 236 and the neutral line 210 through the coupling capacitors 222 and 224. The line driver 304 outputs a differential signal to the power line 238 and the neutral line 210 through coupling capacitors 226 and 224, respectively. 
     Line receivers 306 and 308 of the transmitter/receiver 202 receives the signals transmitted by the transmitter/receiver 122 through the power lines 236 and 238 and the neutral line 210. The output of the line receivers 306 and 308 are connected to an adaptive filter unit 309 through signal lines 322 and 324. The adaptive filtering unit 309 performs line equalization and cross-coupling cancellation to improve the signal-to-noise ratios of the signals output to the signal lines 326 and 328 corresponding to the first and the second duplex channels forward outputs. 
     FIG. 4 shows a similar diagram as FIG. 3 but with the signals flowing in the opposite direction. Reverse inputs on 327 and 329 connected to line drivers 314 and 316 of the transmitter/receiver 202 transmit signals onto the power lines 236 and 238 and the neutral line 210. Line receivers 310 and 312 of the transmitter/receiver 122 receive the transmitted signals and output the received signals onto signal lines 330 and 332 to be input into adaptive filter unit 311. The adaptive filter unit 311 performs line equalization and cross-coupling cancellation to output received signals onto signal lines 319 and 321 corresponding to the first and the second duplex channel reverse outputs. 
     FIG. 5 shows a block diagram of an adaptive filter unit such as the adaptive filter unit 309. The adaptive filter unit 309 includes four adaptive filters H 11  402, H 12  404, H 21  406 and H 22  408. The adaptive filter H 11  402 receives an input signal from the signal line 322 and outputs a filtered signal to a positive input of a summer 414 through signal line 340. The adaptive filter H 21  406 receives an input signal from signal line 324 and outputs a filtered signal to a negative input of the summer 414 through signal line 350. Thus, the summer 414 subtracts the output of the adaptive filter H 21  406 from the output of the adaptive filter H 11  402 to generate an output signal on the signal line 326. The adaptive filters H 11  402 and H 21  406 receive the output of the summer 414 as a feedback error signal through signal lines 342 and 348, respectively. 
     In addition, data reference signals d 1  are input to a negative terminal of the summer 414. The data reference signals d 1  are required for H 11  402 to converge and may be data training sequence signals and/or decision feedbacks from final channel data decision outputs of a downstream receiver (not shown), for example. 
     The adaptive filter H 22  408 receives an input signal from the signal line 324 and outputs a filtered signal to a positive input of a summer 416 through signal line 354. The adaptive filter H 12  404 receives an input signal from the signal line 322 and outputs a filtered signal to a negative input of summer 416 through signal line 344. Thus, the summer 416 subtracts the output from the adaptive filter H 12  404 from the output of the adaptive filter H 22  408 to generate an output signal on the signal line 328. The adaptive filters H 22  408 and H 12  404 receive the output of the summer 416 as a feedback error signal through signal lines 352 and 346, respectively. Similar to summer 414, data reference signals d 2  are input to a negative terminal of the summer 416 and are required for H 22  408 to converge. 
     The adaptive filters H 11  402 and H 22  408 provide line equalization of the input signals received from the signal lines 322 and 324, respectively. The adaptive filters H 12  404 and H 21  406 adaptively cancel cross-coupling of the input signals between the signal lines 322 and 324 and the neutral line 210. 
     FIG. 6 shows an exemplary embodiment of a linear adaptive filter such as the adaptive filters H 11  402, H 12  404, H 21  406 and H 22  408 in greater detail. While different adaptive filters may be used such as linear and decision feedback equalizers, FIG. 6 shows a finite-impulse-response (FIR) least-mean-square-error (MMSE) adaptive filter as an example. The filter portion 502 receives the input signal from signal line 322 and delays the input signal by delay units 520, 522 and 524. As indicated by the dotted portion of signal line 536, any number of delay units 520, 522 and 524 may be used depending on particular design requirements of the least-mean-square adaptive filter. If the adaptive filter is a digital adaptive filter the delay units 520, 522 and 524 delay the input signal by some integer fraction of one symbol sub-unit time Ts/K. Thus, the delay unit is Ts/K for some integer K=1,2,3 . . . depending on implementation requirements. This is a fractionally spaced equalizer. 
     The delay units 520, 522 and 524, multiplier units 506, 508, 510 and 512 and adder units 514, 516 and 518 together form a filter that filters the input signal on signal line 322 and outputs the filtered input signal through signal line 328. The filter characteristics are determined by the weights W 1 , W 2 , W 3 . . . . W M , where M is the length of the filter. Depending on the particular values of the weights, the frequency characteristic of the filter will be determined. 
     The weights W 1 , W 2 , . . . W M  are adaptive weights. The adaptive weights are updated and the updated weights are used by the multipliers 506, 508, 510 and 512 as indicated by the lines connecting a weight update unit 504 and the multipliers 506, 508, 510 and 512. 
     The weight update unit 504 calculates new weights based on the current set of weights adjusted by a factor which is a product of a constant g multiplied by the error signal ε(j) multiplied by the input signal h(j), where j is the current time. The constant g is selected so that the weight update algorithm converges to generate an optimum set of weights W 1 , W 2 . . . W M  that results in the best signal-to-noise ratio within the constraint of adaptivity to changing system conditions. The error signal ε(j) is the output of the summer 414 to adapt H 11  402 shown in FIG. 5, for example. The input signal h(j) is delayed corresponding to the total number of delays applied to the input signal by delay units 520, 522 and 524. Thus, the input signal used to update W 1  is an undelayed input signal h(j). W 2  is updated by using the input signal delayed by the delay unit 520, and so on. The weight update unit 504 generates weights adaptively to minimize, in the mean-square-error sense, the difference between the output of H 11  402 and the desired reference d 1 . A similar adaptive filter is used for H 21  406, and this is also adjusted by the error output of summer 414 to minimize the cross-coupled component in the final output 326. 
     FIG. 7 shows an adaptive filter unit 609 that is an alternative embodiment to the adaptive filter unit 309. The adaptive filter unit 609 includes the adaptive filters H 11  410, H 12  404, H 21  406 and H 22  412. The adaptive filter unit 609 also includes two summers 418 and 420 for cross terms and two summers 422 and 424 for referencing to desired data d 1  and d 2  The adaptive filter H 11  410 is connected to the output of the summer 418 instead of between the signal line 322 and the summer 418 as in the adaptive filter unit 309. Similarly, the adaptive filter H 22  412 is connected to the output of the summer 420. Thus, the adaptive filter unit 609 performs line equalization after canceling the cross-coupling between the signal lines 322 and 324. There may be practical performance advantages to using the two-stage method of FIG. 7. 
     FIG. 8 shows a hybrid line driver/receiver device 500 that includes an echo cancellation device 530 between signal lines 318 and 330 and a transmitter/receiver hybrid 532. The hybrid line driver/receiver device 500 performs a similar function as the line driver 302 and the line receiver 310 shown in FIG. 4. The transmitter/receiver hybrid 532 includes a differential line driver 513, a differential line receiver 509, and a balanced bridge 506. The echo cancellation device 530 includes an adaptive filter 514, a summer 510 and a buffer 511. The differential line driver 513 drives the balanced bridge circuit 506 at top and bottom nodes 518 and 520, respectively. The differential line receiver 509 receives signals from right and left bridge nodes 524 and 522. The output of the differential line receiver 509 is input to a positive input of the summer 510 through signal line 518. The adaptive filter 514 receives an input signal from signal line 318 and outputs a filtered signal to a negative input of the summer 510 through signal line 516. The summer 510 subtracts the filtered signal from the output of the differential line receiver 509 and outputs the result to the buffer 511 which generates an output to signal line 330. The adaptive filter receives the output of the summer 510 through signal line 512 as a feedback error signal. 
     The adaptive filter 514 may be implemented using techniques such as linear and decision feedback equalizers and echo cancellers that are well known to one of ordinary skill in the art. For example, the finite-impulse-response (FIR) least-mean square-error (MMSE) adaptive filter as shown in FIG. 6 may be used. The function of the adaptive filter 514 is to remove a signal correlated with the signal on line 318 that has been coupled to the signal line 518 via the differential line driver 513 and any imbalance in the balanced bridge circuit 506. The adaptive filter 514 adaptively adjusts weights so that the echo cancellation may account for variations in specific conditions encountered during operation. 
     FIG. 9 shows a circuit diagram of the differential line driver 513 and the balanced bridge circuit 506. The differential line driver 513 may include two buffers 602 and 612 connected in voltage follower configurations together with resistors 606, 608 and 616. The positive input terminal of the buffer 612 is connected to a circuit ground reference via signal line 614. Thus, the output of the buffer 612 is maintained at the voltage of the ground reference so that the signal line 504 is at this reference voltage. The positive terminal of the buffer 602 is connected to the signal line 318. Then a signal on the signal line 318 is applied differentially across the bridge nodes 518 and 520 and hence differentially across the power-neutral line pair 236 and 210. Resistors 606, 608 and 616 control the gain of the differential amplifier. 
     The balanced bridge circuit 506 includes two legs between the nodes 518 and 520. The left leg includes a resistor 522 and an impedance 524. The right leg includes a resistor 520 and an impedance that is the driving point impedance looking into the power-neutral line pair 236 and 210. The resistors 520 and 522 have the same value and the impedance of 524 is assumed to have the same value as the line impedance across the frequency bands of interest for the RF signal transmission. 
     When a voltage is developed between the nodes 518 and 520, the voltage between nodes 522 and 526 is unaffected when the balanced bridge circuit 506 is balanced. If a signal is received across the power-neutral line pair 236 and 210, the signal will be imaged across nodes 526 and 522. Thus, the voltage difference between the nodes 522 and 526 corresponds only to the signal received across power line 236 to neutral line 210 and does not contain a component of the signal between nodes 518 and 520 that is to be transmitted to the power-neutral line pair 236 and 210. 
     The voltage between the nodes 522 and 526 is coupled to the differential line receiver 509 through signal lines 508 and 507. Thus, the balanced bridge circuit 506 permits transmitting signals out to the power-neutral line pair 236 and 210 and receiving signals from the power-neutral line pair 236 and 210 in the same bands and at the same time with minimal interference between the transmitted signal and the received signal. 
     The value of the impedance 524 is determined based on the transmission line impedance seen looking into the power-neutral line pair 236 and 210. FIG. 10 shows impedance values observed from arrows 702 and 704. For the case of an application at the transformer 120 and of the system in FIG. 2, the impedance 524 is set to the value of the impedance as observed from arrow 704 which includes: 1) the impedance of the transmission line as seen from arrow 702; 2) the equivalent impedance represented by the coupling capacitors 222 and 224 as well as the signal choke block 214; and 3) the impedance between power line 208 and the neutral line 210 representing the load of the transformer 120. Similarly, 524 would be determined by the impedance observed looking into power-neutral line pair 236 and 210, the signal choke block 218, and impedance across power-neutral line pair 240 and 210 for application at the transmitter/receiver 202. 
     While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations would be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention as set forth herein are intended to be illustrative not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.
Two duplex channels are provided using only three lines, two power lines and one neutral line. The transmitter/receiver of each duplex channel includes adaptive filters to cancel cross-coupling introduced by the common neutral line. Each channel is coupled to a power line using a hybrid line driver/receiver device. The hybrid line driver/receiver device includes an echo canceller and a bridge circuit to allow transmitting and receiving signals to and from a single power line with minimized echoing effects. The signal-to-noise ratios of each duplex channel is significantly increased so that two duplex channels may be implemented using only three wire connections. Thus, the communication capacity of power line communication systems is greatly increased compared to that for a single 2-wire channel.