CHANNEL TRAINING OF MULTI-CHANNEL RECEIVER SYSTEM

Abstract: In a multi-channel receiver system (100) embodying the invention, a "trained" receiver channel which is active and propagating data signals may be used to train any other receiver channel. Each receiver channel of the system has an input (105) and an output (123), with each input for receiving a different portion of an original data signal. Each receiver channel includes adaptive filter device (10) coupled between its input and its output for propagating the signals received at its input to its output. In accordance with the invention, a first trained receiver channel is used to train a second receiver channel by coupling the output of the first receiver channel to the output of the second receiver channel for training the adaptive filter device of the second receiver channel. The training of the second receiver is completed by transmitting the same data to the first and second receiver channels.

Description: CHANNEL TRAINING OF MULTI-CHANNEL RECEIVER SYSTEM BACKGROUND OF THE INVENΗON This invention relates to data communications systems and, in particular, to a system for receiving different portions of a transmitted data signal on different channels and recombining the different portions to reconstitute the original data signal. For example, in a high bit rate digital subscriber line (HDSL) system, it is desirable to divide the high rate data stream to be transmitted into two portions to enable a higher effective transmission rate. An original single data stream to be transmitted is first divided into two portions at the transmitter. Then, the transmitter transmits one portion via one transmission channel (e.g., channel A) and transmits the other portion via a second transmission channel (e.g, channel B). As used herein, a "transmission channel" or a "communication channel" is defined as the path or loop from a transmitter to its receiver. However, the two different transmitted portions of the signal received by the receiver must be recombined at the receiver. R___ombining the two received portions to reconstitute the original high rate data stream signal is problematic because the characteristic of the two transmission channels may be significantly different. That is, the two channels may be subjected to different degrees and levels of distortion, where the distortion includes different propagation delays, different phase shifts, and different amounts of noise. In particular, since the data received on one channel may be delayed with respect to the data received on the other channel, the two different data portions received by the two receiver channels must be aligned and synchronized to be properly recombined into a single high rate data stream which corresponds to the original single data stream. For ease of illustration, assume an HDSL system having two receiver channels and that one portion of the data signal is transmitted via a transmission channel A to a corresponding "receiver channel A" and another, different, portion of the data signal is transmitted via a transmission channel B to a corresponding "receiver channel B". The term "receiver channel", as used herein, refers to the portion of a receiver channel extending from its input terminal, at which it receives transmitted signals, to the output of its equalizer network, which output is then fed to a merging circuit for combining the various channels of the receiver. Each receiver includes circuitry to process received data signals in order to align and synchronize the two different data portions relative to each other so they can be recombined to reconstitute the original single data stream. The processing and adjustment of a receiver channel to compensate for transmission channel distortion and to align it to another channel of a reciever is generally referred to herein and in the appended claims as the "training" of the receiver channel. Thus, in establishing communication between a transmitter and its associated receiver there is first a "start-up" or "training" phase followed by a "communication" or "actual data transmission" phase. A presently known technique for training a dual channel HDSL system includes means for applying a special training sequence to both receiver channels simultaneously during a start up period. The sequence includes an accurate "time-stamp" and training signals for "adjusting" the receiver channels and the transmitter sequences. Once the two receiver channels are trained, customer data is transmitted from the transmitter to the two receiver channels. Subsequent to being "trained", if any channel fails, it must be "retrained" before being restored to service. A problem with the existing training schemes is that if one of two channels fails, both channels (i.e., the failed channel and the still operative channel) must be "taken down" and die two channels retrained before full recovery of the transmitter-receiver can be established. This is disadvantageous in that it interrupts the operation of an operative channel and slows down the operation of the system due to the need to retrain both channels. SUMMARY OF THE INVENTION In a multi-channel receiver system embodying the invention, a "trained" receiver channel may be used to train any other receiver channel. In accordance with the invention, the trained receiver channel may continue operating and propagating data signals while it is used to train another receiver channel which is being placed in, or restored to, service. This avoids the need to disrupt an operative receiver channel, as in the prior art, when another channel is being trained. Also, using an already trained and operative receiver channel to train another receiver channel, shortens the required training time since the training sequence is simpler than in the prior art (e.g., it may not require a "time-stamp" sequence). A receiver embodying the invention includes at least two receiving channels, with each receiving channel having an input and an output, with each input for receiving a different portion of an original data signal. The outputs of the receiving channels are connected to a merging circuit for combining the different portions of the received data signal and reconstituting the original data signal. Each receiving channel includes an adaptive filter means coupled between its input and its output for propagating the signals received at its input to its output. Each adaptive filter means is adjustable to compensate for distortions and delays along its associated transmission channel. In accordance wid the invention, a first already "trained" receiver channel may be used to train a second receiver channel by coupling the output of the first receiver channel to the second receiver channel for training the adaptive filter means of the second receiver channel. To complete the training of the second receiver channel, the same data signal that is being transmitted to the first receiver channel is transmitted to the input of the second receiver channel. In one embodiment of the invention, the receiver includes a switching arrangement for coupling an output of a "trained" receiver channel to a receiver channel to be "trained". An associated transmitter includes a switching arrangement for transmitting the same data signals to the trained receiver channel and the receiver channel being trained. In an embodiment of the invention, each receiver channel includes an adaptive feed-forward filter (FF) and an adaptive feedback filter (FB), with the input of FF being coupled to the input of the receiver channel and the input of FB being connected to die output of the receiver channel. The outputs of FF and FB are supplied to an adder which produces a difference signal apphed to the input of a sheer circuit. The output of the sheer circuit is connected to the output of its receiver channel when the receiver channel is in a data transmission phase. When the receiver channel is in a training phase, the sheer output is disconnected from its receiver channel output and instead, a training signal is applied thereto. A first receiver channel which is trained and active (i.e., it is receiving data signals and the output of its sheer circuit is connected to its receiver channel output) may be used to train a second receiver which is placed in the training mode (i.e., it's sheer ouφut is disconnected from its channel output) by connecting the output of the first receiver channel to the output of the second receiver channel. Concurrentiy, the data being transmitted to the first receiver channel is also transmitted to the second receiver channel. In a reveiver channel embodying the invention, the input to the sheer circuit and the receiver channel output are apphed to: an adder to produce an error signal which is apphed to: (a) a first multiplier whose ouφut is used to adapt the feed-forward (FF) filter; and (b) a second multiplier whose ouφut is used to adapt the fedback (FB) filter. RWTFF TWSΓRIFΠON OF THE DRAWINGS In the accompanying figures like reference characters denote like components; and FIGURE IA is a block diagram of part of a receiver system embodying die invention; FIGURE IB is a block diagram of a transmitter embodying the invention; FIGURE 2 is a block diagram of a multiplexing arrangement for coupling signals to, and between, the two receiver channels. FIGURE 3 is a block diagram of a portion of the adaptive equalizer of a receiver channel as configured during training; FIGURE 4 is a block diagram of a portion of the adaptive equalizer of a receiver channel as configured during customer-data transmission; and FIGURE 5 is a block diagram of an adaptive filter of one receiver channel being trained by another receiver channel in accordance with the invention. np.τ _π FT_ D SCRIFΠQN OF THE INVENTION Fig. IA shows portion of a of a dual-duplex receiver 100 embodying the invention adapted for use in an illustrative dual-duplex system, and Fig. IB shows a transmitter embodying the invention for use with receiver 100. In Fig. IA, receiver channel A data is received on lead 101 and channel B data is received on lead 102. The signals on leads 101 and 102 are respectively coupled through analog-to-digital (A/D) converters 103a and 103b to form input samples A and B on leads 105a and 105b, respectively. Each one of the A/D converters 103a and 103b is strobed by a common sampling clock signal supphed on line 1071 by a timing recovery generator 107. Timing generator 107, which may include a master clock source and a timing recovery circuit, provides the same sampling clock signals to A/D 103a, 103b and to receiver channels A and B. The rate of the sampling clock signals supphed to A/D converters 103a and 103b causes each of these converters to generate line samples at n/T samples per second, where n is a predetermined integer equal to or greater than one. The channel A input signal samples on line 105a are apphed to a time stamp detector 180, a carrier detector CDA, and d e input (FFIA) of its associated adaptive filter 10a. The channel B input signal samples on line 105b are apphed to time stamp detector 180, a carrier detector CDB, and the input (FFIB) of its associated adaptive filter 10b. As is known in the art, to varying degrees, inter-symbol interference (ISI) is always present in a data communications system. ISI is the result of the transmission characteristics of die communications channel, i.e., the "channel response," and, generally speaking, causes neighboring data symbols, in a transmission sequence, to spread out and interfere with one another. In Fig. IA, each one of the adaptive filters 10a, 10b is configured, during data transmission, to form an adaptive decision-feedback equalizer (DFE) which functions to remove ISI in the receiver. Each one of filters 10a and 10b includes an adaptive feed¬ forward filter section (FFA, FFB), a decision circuit (DECA, DECB), an adaptive feedback filter section (FBA, FBB), a first adder (15a, 15b) and a second adder (16a, 16b). Each feed-forward filter receives and processes n inputs for each symbol interval; whereby each equalizer may be referred to as a fractionally-spaced equalizer and more specifically as a T/n type of fractionally spaced equalizer since each receives and processes n samples per symbol interval. The outputs of filters FFA and FFB are generated once per symbol interval and are respectively coupled through decision circuits DECA and DECB. Each one of decision circuits DECA and DECB may be any one of a number of "sheer" circuits which functions to quantize its coupled equalizer ouφut to the closest one of a plurahty of permissible transmitted signal levels. Accordingly, the decisions circuits (DECA and DECB) are also referred to herein and in the appended claims as sheers. Each of the feed-forward (FFA, FFB) and feedback (FBA, FBB) filters tends to remove a portion of the ISI. Each of these filters includes a plurahty of adjustable coefficients to compensate for the distortions in transmission channels A and B, respectively. Since the distortion in the transmission channels varies with time, these filters are preferably of the automatic or adaptive variety and their coefficients have values which may be varied. The ouφut (AO1, BO1) of each feed-forward filter is applied to one input of its associated first adder (15a, 15b). The ouφut (AO3 and BO3) of each feedback filter (FBA, FBB) is apphed to a second input of its associated first adder circuit (15a, 15b). The ouφut (Ela, Elb) of each first adder (15a, 15b) is apphed to the input of a corresponding decision circuit (DECA, DECB) and to an input of a corresponding second adder (16a, 16b). In receiver channel A, the ouφut (AO2) of decision circuit DECA is connected to a terminal 117. A second input of adder 16a and the input to feedback filter FBA are connected to the wiper arm of a switch Sl A. In receiver channel B, the ouφut (BO2) of decision circuit DECB is connected to a terminal 118. A second input of adder 16b and the input to feedback filter FBB are connected to the wiper arm of a switch SIB. Ouφut lines 117, 118 and 119 are selectively coupled via switch S1A to line 123a and via line 123a to the second input of adder 16a and to the input of FBA. Similarly, ouφut lines 117, 118 and 119 are selectively coupled via switch SIB to line 123b and via line 123b to the second input of adder 16b and to the input of FBB. Switches S1A and SIB are switched to their various positions (lines 117, 118, 119) by means of a control circuit 140 which, in turn, is responsive to a control signal SCI generated by a controller (e.g. a microprocessor or other controlling agency, which is not shown) of the receiver. The signal SCI determines the interconnections and configurations of the receiver channels during various operating phases. With respect to channel A, when its customer-data is being processed, switch S1A is connected to terminal 117 and d e sheer ouφut of DECA is apphed to the second input of adder 16a and to the input of FBA. With respect to channel B, when its customer-data is being processed, switch SIB is connected to terminal 118 and the sheer ouφut of DECB is apphed to the second input of adder 16b and to the input of FBB. During customer data transmission, each equalizer 10a, 10b is configured as shown in Fig. 4. When channels A and B are being trained using ideal reference generator 182, switches SIA and SIB are connected to line 119. The resulting configuration of equalizers 10a, 10b during a training phase is shown in Fig. 3. As discussed below, in accordance with the invention, when channel A is being trained using channel B, switch SIA is connected to line 118 and when channel B is being trained using channel A, switch SIB is connected to line 117. The resulting configuration of an equalizer of one receiver channel (e.g., B) being trained by another receiver channel (e.g., A) is shown in Fig. 5. Switches SIA and SIB may be implemented using multiplexing gates or any appropriate logical gate arrangements to provide the required switching function. A typical implementation is shown in Fig. 2 where SIA and SIB are shown to be implemented by means of a three input multiplexer (MUXA, MUXB). Referring to Fig. IA, adder 16a functions to subtract the signal (REF A) on lead 123a from the ouφut (Ela) of comparator 15a on lead 127a to produce an error-A (eA) signal on lead 129a. The error-A signal is used to adapt (i.e., update and adjust the coefficients) filters FFA and FBA. In a similar fashion, adder 16b functions to subtract the signal (REFB) on lead 123b from the ouφut (ElB) on lead 127b to produce the error-B (eB) signal on lead 129b. The error-B signal is used to adapt (i.e., update and adjust the coefficients) filters FFB and FBB. In Fig. IA, the error signal eA is apphed to a multiplier Ilia, and die resultant signal provided on line 136a is used to adapt FFA. The error signal eB is apphed to multiplier 111b, and the resultant signal provided on line 136b is used to adapt FFB. Multipliers Ilia and 111b, and 131a and 131b, multiply tiieir respective error signals by a constant or step size (al, a2). It is normally preferable to speed up die "training" of the adaptive filters. This entails using large steps which provide higher speed but at the expense of accuracy. During data transmission, it is normally preferable to use smaller steps, providing greater accuracy, but with decreased speed. Accordingly during a training phase, the step size of al and a2 are chosen so that the circuit responds more quickly (but less accurately) to adjust the coefficients of the f eed-forwaid and feedback filters FFA, FFB, FBA and FBB. During die customer data transmission phase of operation, the step size of al and a2 are chosen to provide smaller steps so that the speed of response is decreased; but die accuracy is increased to drive error levels towards zero. Time stamp related sampled signals on lines 105a, 105b are supphed to time- stamp detector 180 which, when detected, supplies a START signal to ideal reference (IDR) generator 182, which initiates a training sequence. In response to the START signal, IDR generator 182 generates IDR signals on line 119 which are used in the training of adaptive equalizers 10a and 10b. The START signal also functions to cause LDR generator to start its operation coincident widi the generation and transmission by transmitter 200 of an IDR signal. IDR generator 182 may be a pseudo random symbol generator which functions to produce a signal sequence (referred to as "ideal") which is similar to a signal sequence generated by die transmitter. The transmitted IDR signals and the receiver generated IDR signals are supphed to die receiver channels to train the coefficients of the adaptive equalizers during a standard training cycle. The data ouφuts on lines 123a and 123b generated by DECA and DECB, respectively, are supphed to a data merge circuit 190 which functions to combine their ouφuts and to reconstruct on line 192 the original digital data as it existed prior to its division and transmission via the two transmission channels. Referring to FIG. IB, the transmitter 200 includes a data base 201 for generating a high speed digital data bit stream (which may be comprised of baseband or passband signals) at some predetermined rate and dividing die stream into two portions; portion "A" or customer data-A and portion "B" or customer data-B. Portions A and B are supphed to respective circuits 202a, 202b which function to scramble the data, convert it into symbols and map it. The ouφut of circuits 202a which forms customer-A data (CDA) to be transmitted to the receiver is supphed to a terminal pl of transmission switches STA and STB. In a similar manner, the ouφut of circuit 202b which forms the customer-B data (CDB) is supphed to a terminal p2 of switches STA and STB. An idealized reference generator 204 generates a reference signal which is supphed to terminal p3 of switches STA and STB. A time stamp generator 206 produces a time stamp signal supphed to terminals p4 of switches STA and STB. Also, a carrier signal generator 208 supphes carrier indicator signals to terminals p5 of switches STA and STB. Switches STA and STB may be any one of a number of logic gates or multiplexers suitable to select any one of the signals present at their pl through p5 portion. Each one of switches STA and STB is shown to have an ouφut line (210a, 210b) which is selectively coupled to (and makes contact to) any one of its corresponding pl dirough p5 input terminals. STA and STB are controlled by means of transmitter switch control circuit 209 which determines which one of the input terminals gets connected to die output (210a, 210b) of the switches. The ouφut (210a, 210b) of each switch STA, STB is tiien connected to a ttansmitter filter (212a, 212b) and the ouφut of each transmitter filter is then supphed to a digital-to-analog converter (D/A) 214a, 214b. The ouφut of D/A 214a is transmitted via transmission channel A to receiver channel A and the output of D/A 214b is transmitted via transmission-channel B to receiver channel B. The transmission channels A and B linking die transmitter 200 to receiver 100 may be any known type of loop or link used to transmit data. Each of transmission channels A and B is typically bidirectional but, for purpose of simphcity, the description which follows only discusses signal processing in one transmission direction. Each of the transmission channels has an associated signal propagation delay and phase shift which is generally different from that of the other channel and which may vary with time. It should be understood that the present invention is applicable to transmission systems using any of a number of suitable modulation schemes as well as to baseband systems and can be used in systems using virtually any baud or bit rate. The signal transmitted dirough each transmission channel represents a succession of data symbols transmitted at a rate of 1 T symbols per second. The transmitter shown in HG. IB is assumed to be located in a central office (CO), and die receiver shown in FIG. IA is assumed to be located in a customer's premises (CP) or in any location remote from the transmitter. Communication between transmitter and receiver is initiated by setting transmitter switches STA and STB to their position p5 and sending a carrier signal CSG (which may be, for example, a pseudo¬ random special signal sequence) from the carrier signal generator 208 of the transmitter over channels A and B to receiver channels A and B. The carrier signal is detected at die receiver by carrier detectors CDA and CDB. Upon detection of the carrier signals, the carrier detectors CDA and CDB send signals back to the transmitter indicating tiiat receiver channels A and B are ready and a connection is tiien made ("handshaking" occurs) between the transmitter and the receiver. Following the establishment of the connection between the transmitter and receiver, signals are transmitted between diem for establishing timing recovery and appropriate signal level amplitude via AGC and timing circuitry (not shown). Then, the training of the receiver channels is instituted to compensate for "distortion" (i.e., differences in propagation delay, phase shifts and noise) in the transmission channels. Switches STA and STB are set to their position p4, and a time stamp signal produced by transmitter time stamp generator 206 is transmitted over transmission channels A and B to receiver channels A and B. When receiver time stamp detector 180 senses the receipt of the time stamp signal, it produces a START signal which is apphed to receiver E R generator 182. Concurrent with the generation of the START signal, receiver switches SIA and SIB are set to make contact to lead 119 in order to couple the IDR signals generated by receiver IDR generator 182 to adders 16a, 16b and feedback filters FBA, FBB. Concurrent with the generation of die START signal at the receiver DR generator 182, transmitter switches STA and STB are set to enable transmission of an IDR signal symbol sequence generated by IDR generator 204, via transmission channels A and B to receiver channels A and B. The intent of the START signal is to cause IDR generator 182 to generate a symbol sequence which is intended to be nearly identical to the symbol sequence being transmitted by IDR generator 204 and for both to occur at nearly the same time. Therefore, a similar signal sequence is being transmitted to channels A and B as is being generated in the receiver and apphed to the adders 16a, 16b and the feedback filters. The transmitted IDR symbols received at the receiver channel A input are coupled via A D converter 103a and FFA to adder 15a and d e transmitted IDR symbols received at the receiver channel B input are coupled via A/D converter 103b and FFB to adder 15b. At the same time, d e receiver generated IDR signals are coupled via switch SIA and line 123a to adder 16a and feedback filter FBA and via switch SIB and line 113b to adder 16b and feedback filter FBB. The transmitted IDR signals are apphed to the inputs of FFA and FFB and die receiver generated IDR signals are apphed to the input of FBA and FBB and to an input of adders 16a and 16b. The configuration of equalizers 10a and 10b for this condition is as shown in FIG. 3. With respect to receiver channel A, the transmitted IDR signals received at the input of receiver channel A and the ouφut of FBA responsive to the receiver generated IDR signals are apphed to first adder 15a, to generate a first difference signal (Ela). Ela and the receiver generated IDR signals are then apphed to adder 16a to produce an error-A (eA) signal. The eA signal is used to update and adjust die coefficients of feed-forward and feedback filters FFA and FBA to compensate for the distortion of transmission channel A. Likewise, in receiver channel B, the transmitted IDR signals received at d e input of receiver channel B and die ouφut of FBB responsive to the receiver generated IDR signals are apphed to adder 15b to produce a difference signal Elb. Elb and d e receiver generated IDR signals are then apphed to adder 16b to produce an error-B (eB) signal. The eB signal is used to update and adjust die coefficients of FFB and FBB to compensate for the distortion of transmission channel B. Thus, the transmitted and receiver generated IDR signals are used to "train" the system. By transmitting known signal sequences to receivers A and B and by comparing "known" transmitter signals to similar ("ideal") signal sequences generated by d e receiver, the coefficients of the adaptive filters FFA, FFB, FBA and FBB are updated so as to compensate for the distortion and impairments in the two different transmission channels. Note that during die training period the step size of multipliers 111 and 131 may be made larger or smaller to increase or decrease d e speed of updating die filter coefficients, but with decreasing accuracy for increasing speed. After an agreed upon period of time has elapsed, d e "training" of the adaptive filters 10a, 10b is assumed to be completed and die system is ready to transmit and to receive customer data and actual data transmission begins. [Alternatively, the level of the error-A and error-B signals may be monitored and when die error level is below some value, training is deemed completed.] Once the receiver channels are "trained", switch SIA is connected to lead 117 which functions to connect the sheer ouφut (AO2) of DECA to line 123a and to data merge circuit 190, and also via line 123a to an input of adder 16a and to the input of feedback filter FBA. Likewise, switch SIB is connected to lead 118 which functions to connect the sheer ouφut (BO2) of DECB to line 123b and to data merge circuit 190, and also via hne 123b to an input of adder 16b and to the input of feedback filter FBB. The configuration and interconnection of these components is then as shown in Fig. 4 to form an adaptive decision-feedback equalizer (DFE). Following this operation, transmitter switch STA is connected to its position pl to transmit customer data- A over transmission channel A and switch STB is connected to its position p2 to transmit customer data-B over transmission channel B. During the data transmission phase, switch SIA is connected to lead 117 and couples the sheer ouφut (AO2) of DECA to hne 123a and data switch SIB is connected to lead 118 and couples the sheer ouφut (BO2) of DECB to line 123b. The outputs of the decision circuits (DECA, DECB) on lines 123a and 123b are tiien supphed to data merge circuit 190 which functions to recombine the two receiver channel ouφuts into one synchronized and ordered data stream on line 192. As is evident from the previous description receiver channel A and its adaptive filter 10a is similar to the structure channel B and its adaptive filter 10b. Hence, these respective structures and functions are identified widi like reference characters with a prefix or suffix "A" or "a" denoting components and functions of channel A and a prefix "B" or "b" denoting components and functions of channel B. To better understand die operations of the equalizers (10a, 10b) reference is made to Fig. 4. In Fig. 4, the ouφut signal from feed-forward filter FF is apphed to adder 15 which, theoretically, subtracts the remaining portion of ISI removed by feedback filter FB from the ouφut signal of FF. Adder 15 provides a signal on line 127 which is apphed to adder 16 and to sheer DEC. Sheer DEC selects a particular data symbol as a function of the mapping of the signal on line 127 to a point in a predefined constellation of data symbols (not shown). Sheer DEC provides a data symbol every T seconds, where 1/T is the data symbol rate. This data symbol is an estimate of die received symbol and is provided by sheer DEC on line 123 for processing by other receiver circuitry (not shown) to recover the actually transmitted data, in this case the data representative of the training signal. The ouφut of sheer DEC is also provided to (ISI) feedback filter FB and adder 16. Feedback filter FB predicts the amount of ISI present in the received signal and provides an ISI prediction signal to adder 15, via line 126. Adder 15, as described above, removes the remaining portion of ISI from the received signal by subtracting the ISI prediction signal from the ouφut signal of feed-forward filter FF. Adder 16 subtracts the estimated data symbol provided by sheer DEC from the ISI-reduced signal present on line 127 to provide an ERROR signal on line 129. The ERROR signal represents the amount of ISI error and channel noise that has not been corrected by the operation of either feed-forward filter FF or feedback filter FB. The ERROR signal is used to adapt both feed-forward filter (FF) and feedback filter (FB) via multipliers 111 and 131. It is assumed that the adaptation algorithms (not shown) of the feed-forward and feedback filters conform to the use of ntinimum mean square error (MMSE), zero forcing, or its variations, as known in the art. Multiplier 111 multiplies the ERROR signal by a constant or step size, al. The resulting signal provided on line 136 is used to adapt feed-forward filter FF. Further, the ERROR signal on hne 129 is provided to multiplier 131, which effectively multiplies the ERROR signal by the step size a2. The resulting signal provided on hne 141 is used to adapt feedback filter FB. In the prior art systems, if one of the two transmission channels fails for any reason (e.g., break in a line, or excessive noise), upon resumption of operation of the failed channel, the operation of die continuously active channel is interrupted in order to retrain and realign both channels. In systems embodying de invention, a continuously active and trained channel may be used to train any other channel or channels, or to retrain a failed channel and restore it to service. The operation of a system in accordance widi die invention is explained as follows. Assume, for example, that receiver channel A is trained and operative and tiiat receiver channel A will be used to train receiver channel B, as shown in Fig. 5. In accordance widi die teachings of die invention assume that customer data continues to be transmitted to channel A and to be processed via receiver channel A throughout the period of time that receiver channel B is being trained by channel A. To complete the training of channel B, the transmitter and receiver system are operated and/or function as follows: The same customer data being transmitted via transmission channel A is also being transmitted via transmission channel B. That is, transmitter switch STB is set to position pl whereby die customer data being propagated on channel A is also transmitted via transmission channel B to receiver channel B. Receiver channel A continues operating as before. However, a training and alignment cycle will be conducted for channel B using customer-A data and die sheer- A determined ouφut AO2 of receiver channel A. For the training of channel B, channel A customer-A data is transmitted and received on line 102. Concurrently, switch SIB is connected to lead 117 whereby the sheer ouφut A02 of the channel A decision circuit DECA is apphed to the inputs of feedback filter (FBB) and adder 16b, as shown in FIG. 5. Channel B is also preferably switched to a training mode in which multiplier elements 11 lb and 131b are set to allow a faster response of the feed-forward and feedback filters of channel B to the "training" signals produced by receiver channel A. The channel A data at the ouφut of DECA thus functions as the reference input signal for 16b and FBB of channel B to adapt (adjust) die coefficients of FFB and FBB. For the condition of STB and SIB described above, the channel A customer data being transmitted over transmission channel A is also being transmitted over transmission channel B. Channel A data is thus received at 102 and after conversion in A/D 103b is supphed to die input of FFB. The ouφut of FFB is compared to die ouφut of FBB. The ouφut of FFB which is now responsive to the transmitted channel A data is subtracted at die adder 15b from the ouφut of FFB which is driven by data which was equalized and adjusted, as discussed above, and represents transmitted channel A data which has been corrected via the circuitry of channel A. The ouφut Elb of adder 15b is tiien also compared in adder 16b to A02 data to produce an error-B signal which is coupled to lines 136b and 141b to update and adjust die coefficients and parameters of FFB and FBB. The coefficients of FFB and FBB are thus set to compensate for distortions in the transmission channel B and any delay in channel A. The training phase of a receiver channel (e.g., B) by means of another channel (e.g. A) may be assumed to be completed after a preselected period of time has elapsed. Alternatively, the error signal (e.g., B) of the receiver channel being trained may be monitored until die error level is below a predetermined acceptable level. It should be noted that during a training phase the ouφut of die sheer circuit of a receiver channel being trained is disconnected from its receiver channel ouφut (i.e. line 123). Instead, either: (a) the receiver generated (or based) IDR training signal is apphed to die receiver channel ouφut (see Fig. 3); or, (b) the receiver channel ouφut (e.g., A02) of another active receiver channel (e.g., A) is apphed to die ouφut (123b) of die receiver channel (e.g., B) as shown in Fig. 5. A significant advantage of using an already trained and active receiver channel (e.g., A) whose sheer ouφut is connected to its channel ouφut, to provide the reference signal to train other receiver channel (e.g., A02 apphed to line 123b with sheer B ouφut disconnected from line 123b) is that, in addition to adapting (adjusting) the feed-forward and feedback filters of the otiier receiver channel (e.g., FFB and FBB) to compensate for distortion in transmission of the other receiver channel (e.g., B) the adaptive filters (FFB, FBB) of the other receiver channel (e.g., B) are compensated for the overall delay in the trained channel (e.g., A). In order to compensate for the delay in channel A, the fractional spaced equalizer (FFB) must have sufficient coefficients and a delay line which is long enough so the filter response position of channel B can be adjusted for the right amount of channel delay. Since the channel A (doing die training) filter response is "centered in its delay line, the feed-forward filter size should be set to accommodate die optimum feed-forward filter (FF) length and the maximum delay difference between the receiver channels. Once channel B has been realigned and retrained, the receiver switch SIB makes contact to lead 118 (switch SIA remains in contact with lead 117) whereby die ouφut of decision circuit B is now fed back to the input of FBB and to adder 16b. Concurrentiy, at the transmitter, switch STB is switched to contact position p2 to now send customer- data B signals via transmission channel B. The use of customer channel A data for aligning channel B is valid since the error for the active channel receiver is low. Thus, in accordance with the invention, when a transmission channel fails, its corresponding receiver channel can be trained and placed in service using an active trained receiver channel without interrupting its operation. The signal channel training is also much faster than in die prior art scheme since it does not require an ideal reference time stamp or timing recovery training. The invention has been illustrated using two channel (A,B). However, it should be evident that more tiian two transmitting channels and more than two receiving channels may be used in systems embodying die invention; i.e., there may be "N" channels where N is an integer greater than one (1). Also, the invention has been illustrated using a particular type of DFE. It should be evident that any suitable adaptive filter arrangement can be used instead.

Claims: WHAT IS CLAIMED IS: 1. A combination comprising; N receiver channels, each receiver channel having an input and an ouφut; where N is an integer greater than 1; each receiver channel being adapted to receive, at its input, a different portion of an original signal to be reconstituted from the various portions; means for coupling the ouφuts of the N receiver channels to a merging circuit for combining the different portions of the received signals and reconstituting the originally transmitted signal; each one of said N receiver channels including adaptive filter means coupled between the input and die ouφut of its receiver channel and along which the signal is propagated; each adaptive filter means being adjustable to compensate for distortions in the signal received at its receiver channel input; each receiver channel including switching means for selectively switching its receiver channel to a first, filter adaptor, configuration during a training phase and to a second, data processing and propagating, configuration during a data transmission phase; and means for selectively coupling the ouφut of a receiver channel switched to said second configuration to the ouφut of another receiver channel switched to said first configuration. 2. A combination as claimed in claim 1 , wherein the adaptive filter means of each receiver channel includes an adaptive feed-forward filter, an adaptive feedback filter, a sheer circuit, and first and second adders; wherein the feed-forward filter has an input coupled to the input of its receiver channel and has an ouφut coupled to an input of the first adder; wherein the feedback filter has an input coupled to the ouφut of its receiver channel and has an ouφut coupled to another input of the first adder; wherein the first adder has an output, for producing a difference signal, coupled to an input of the sheer circuit and to an input of the second adder; wherein the ouφut of die receiver channel is coupled to die input of the second adder; and wherein the ouφut of the sheer circuit of a receiver channel is connected to the ouφut of its receiver channel for forming said second configuration and is disconnected from the ouφut of its receiver channel for forming said first configuration. 3. A combination as claimed in claim 2 wherein said second adder of each receiver channel produces an error signal which is coupled: (a) via a first multiplier to the feed-forward filter of its receiver channel to adjust it; and (c) via a second multiplier to the feed-back filter of its receiver channel to adjust it. 4. A combination as claimed in claim 3, wherein N is equal to two. 5. A combination as claimed in claim 1 wherein each adaptive filter means is a decision feed-back equalizer (DFE). 6. A combination as claimed in claim 1 further including means for selectively coupling to the ouφut of a receiver channel switched to said first configuration a receiver generated training signal. 7. A combination comprising; N receiver channels, each receiver channel having an input and an ouφut; where N is an integer greater than 1 ; each receiver channel being adapted to receive, at its input, a different portion of an original signal to be reconstituted from the various portions; means for coupling the ouφuts of the N receiver channels to a merging circuit for combining the different portions of the received signals and reconstituting die originally transmitted signal; each one of said N receiver channels including adaptive filter means coupled between the input and die ouφut of its receiver channel and along which the signal is propagated; each adaptive filter means being adjustable to compensate for distortions in the signal received at its receiver channel input; and means for selectively coupling the ouφut of a first one of the N receiver channels to a second one of die N receiver channels for adjusting ti e adaptive filter means of said second one of said N receiver channels. 8. A combination as claimed in claim 7, wherein the adaptive filter means of each receiver channel includes an adjptive feed-forward filter, an adaptive feedback filter, a sheer circuit, and first and second adders; each having two inputs and an ouφut, and first and second adders; each adder having two inputs and an ouφut; wherein the feed-forward filter is coupled at its input to the receiver channel input and at its ouφut to an input of the first adder and die feedback filter is coupled at its input to its receiver channel ouφut and at its ouφut to another input of the first adder; wherein the ouφut of the first adder is connected to an input of the second adder and to the input of the sheer circuit; and wherein the ouφut of the receiver channel is connected to another input of the second adder. 9. A combination as claimed in claim 8 wherein said means for selectively coupling the ouφut of a first one of the N receiver channels to a second one of the N receiver channels includes means for selectively coupling the ouφut of die first one of the N receiver channels to the ouφut of said second one of die N receiver channels. 10. A combination as claimed in claim 8 wherein the ouφut of each receiver channel includes a switching means for selectively coupling the channel ouφut to one of: (a) the sheer circuit ouφut; (b) the ouφut of a receiver based generator producing training signals; and (c) die ouφut of anotiier receiver channel. 11. A combination as claimed in claim 10, wherein N is equal to 2 and wherein the ouφut of said first receiver channel is selectively apphed to the ouφut of the second receiver channel for training the second receiver channel. 12. A combination as claimed in claim 10, wherein each adaptive feed-forward filter is a fractionally spaced equalizer. 13. A combination comprising; N receiver channels, each receiver channel having an input and an output; where N is an integer greater than 1 ; each receiver channel being adapted to receive, at its input, a different portion of an original signal to be reconstituted from the various portions; means for connecting the outputs of the N receiver channels to a merging circuit for combining the different portions of the received signals and reconstituting the originally transmitted signal; each one of said N receiver channels including adaptive filter means coupled between its input and its ouφut; each adaptive filter means including: (a) a feed-forward filter having an input coupled to the input of its receiver channel, and having an ouφut coupled to an adding means; (b) a feedback filter having an input coupled to the ouφut of die receiver channel, and having an output coupled to d e adding means; (c) a sheer circuit having an input and an ouφut; and (d) said adding means for supplying a first difference signal which is a function of the difference in the ouφuts of said feed-forwaid and feedback filters to the input of the sheer circuit; and each one of said N receiver channels including switching means for selectively couphng the receiver channel ouφut to one of: (a) the ouφut of its sheer circuit; (b) the sheer circuit ouφut of another receiver channel; and (c) die output of a training signal generator. 14. A combination as claimed in claim 13 wherein each one of said adaptive filter means includes: (a) additional adder means for generating an error signal indicative of die difference between the signal at its receiver ouφut and d e first difference signal; (b) a first multiplier for multiplying said error signal by a preselected first constant and for producing an ouφut signal coupled to the feed-forward filter for adjusting its coefficients; and (c) a second multiplier for multiplying said error signal by a preselected second constant and for producing an ouφut signal coupled to the feedback filter for adjusting its coefficients. 15. A combination as claimed in claim 13, wherein the switching means of one receiver channel couples the ouφut of its receiver channel to the ouφut of another channel for causing the ouφut of the other channel to be apphed to the adaptive filter means of the one receiver channel for training the adaptive filter means of die one receiver channel. 16. A method for using a first already trained and active receiver channel to train a second receiver channel, where each receiver channel has an input and an ouφut and includes a adaptive filter means connected between its input and its ouφut for compensating for distortion in the transmission channels of the receiver channels, comprising the steps of: (a) applying the ouφut of the first receiver channel to the output of d e second receiver channel; and (b) transmitting the same data signal to the first and second receiver channels. 17. A method as claimed in claim 16 wherein the step of transmitting the same data signal to the first and second receiver channels includes d e step of setting a transmitter to transmit to the second receiver channel the same data being transmitted to die first receiver channel. 18. A receiver comprising: first and second receiver channels each having an input and an ouφut, and each receiver channel including an adaptive feed-forward filter having an input and an ouφut, an adaptive feedback filter having an input and an ouφut, a sheer circuit having an input and an ouφut and adder means: each receiver channel having its feed-forward filter connected at its input to its receiver channel input and its feedback filter connected at its input to its receiver channel ouφut with the ouφut of the feed-forwaid and feedback filters having coupled to its adder means for producing a difference signal at the input of its sheer circuit; and means for selectively couphng the sheer circuit ouφut of die first receiving channel to the ouφut of the first receiver channel and to die ouφut of die second receiver channel for enabling the first receiver channel to adapt and train the feed- forward and feedback filter of the second receiver channel. 19. A communication system comprising: a transmitter for normally transmitting a first portion of a data signal via a first transmission channel and a second portion of the data signal via a second transmission channel; a receiver having first and second receiver channels for respectively receiving the transmission from the first and second transmission channels, each receiver channel having an input and an ouφut; and each receiver channel including adaptive filter means coupled between its input and its ouφut; being means for using the first receiver channel to train the second receiver channel comprising: (a) means included in the transmitter for selectively transmitting the same data signal information via the first and second transmission channels to the input of the first and second receiver channels; and (b) means for selectively couphng the ouφut of the first receiver channel to the ouφut of die second receiver channel for adapting the response of the filter means of the second receiver channel. 20. A communication system comprising: a transmitter for: (a) selectively transmitting a first portion of a data signal via a first transmission channel and a second portion of the data signal via a second transmission channel during a data transmission interval; (b) selectively transmitting the same training signal via the first and second transmission channels during standard training periods; and (c) selectively transmitting the same data signal via said first and second transmission channels during selected training periods; and a receiver having first and second receiver channels for respectively receiving the transmission from the first and second transmission channels; each receiver channel having an input and an ouφut; and each receiver channel including an adaptive filter means coupled between its input and its ouφut along which the data signal it receives is propagated; and means for selectively coupling (a) d e ouφut of each channel to a merging circuit during said data transmission intervals; (b) the ouφut of each receiver channel to a receiver based training signal generator; and (c) the ouφut of one channel to the ouφut of anotiier channel for causing die one channel to train the other channel.

Inventors: LANGBERG EHUD, LANGBERG, EHUD
Applicants: GLOBESPAN TECHNOLOGIES INC, GLOBESPAN TECHNOLOGIES, INC.
Application year: 1996
Main classification: H04B1
IPC codes: H03H21, H04B1, H04B3, H04L25