Abstract:
An adaptive preceding system is implemented within a data communications system in order to precode data to track changes in a communications channel. The adaptive precoding system initializes a set of precoder values within a precoder filter of a transmitter during an initialization phase. The precoder filter is used to create a precoding signal that is combined with a transmission data signal in order to compensate for distortions on a communications channel. During a communications phase, a decision feedback equalizer generates equalizer coefficients that correspond to changes on the communications channel. The equalizer coefficients are periodically transmitted to a converter within the transmitter over a secondary channel. The converter determines a new set of precoder values that compensate for the changes in the communications channel and slowly updates the values in the precoder filter until they match the new set of precoder values. While the precoder values are being updated, the decision feedback equalizer updates the equalizer coefficient values to reflect the improvements realized by updating the precoder values.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This document claims priority to and the benefit of the filing date of copending provisional application entitled ADAPTIVE PRECODER, assigned serial number 60/053,424, and filed Jul. 22, 1997, and is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to data communications equipment, e.g., modems, and, more particularly, to the equalization of data signals in a data communications system. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a communications receiver employs an adaptive decision feedback equalizer (DFE) to compensate for changes in a communications channel. However, the use of a DFE introduces “error propagation” effects in the receiver. As such, it is known in the art to implement a precoder with modulo arithmetic (e.g., Tomlinson filter) in a remote transmitter in order to mitigate, if not eliminate, the problem of error propagation in the receiver This conventional precoder uses coefficient values generated by the receiver during an initialization phase, sometimes referred to as a training or a start-up phase. 
     However, if the response, i.e., characteristics, of the communications channel changes significantly, the precoder will fail to adequately compensate for the new communications channel. As a result, a disruptive retrain is usually required so that the receiver can generate a new set of precoder coefficients., which is then sent back to the transmitter. Unfortunately, each retrain disrupts the data flow and takes time to both calculate the equalizer coefficients and to communicate them back to the remote transmitter. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the inadequacies and deficiencies of the prior art as discussed herein. The present invention provides an adaptive preceding system and method for preceding an input data signal to compensate for changes in a communications channel. 
     The adaptive precoding system and method of the present invention utilize a precoder filter and a converter. In accordance with the preferred embodiment of the present invention, the precoder filter is configured to maintain precoder values which may be used to precode input data signals prior to transmission on a communications channel. The converter is configured to receive equalizer coefficients periodically generated by a decision feedback equalizer and to slowly update each precoder value as conditions on the communications channel change. 
     As a result of slowly updating the precoder values, the quality of signals received by the decision feedback equalizer slowly improves. Accordingly, the decision feedback equalizer can update the equalizer coefficient values so that a disruptive retrain is not needed 
     The adaptive preceding system and method of the present invention have many advantages, a few of which are delineated hereafter, as examples. 
     An advantage of the adaptive precoding system and method is that they provide for a scheme of compensating for changes in a communications channel. Accordingly, more reliable communication can be established. 
     Another advantage of the adaptive precoding system and method is that they provide for a scheme of continuously updating precoder values as interference conditions on a communications channel change. Since interference typically vary in an unpredictable fashion, more reliable communication can be achieved. 
     Another advantage of the adaptive precoding system and method is that the precoding values may be updated without the occurrence of a disruptive retrain. This allows for more efficient data communication since retrains require time to implement. 
     Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings in the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art communications system; 
     FIG. 2 is a block diagram of a prior art decision feedback equalizer; 
     FIG. 3 is a block diagram of a prior art preceding system; 
     FIG. 4 is an illustrative signal point constellation for use in the preceding system of FIG. 3; 
     FIG. 5 is a block diagram of a communication system implementing the precoding system of the present invention; and 
     FIG. 6 is a block diagram of a preceding system embodying the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An illustrative communications system  9  of the prior art is shown in FIG.  1 . The communications system  9  comprises data communications equipment (DCE)  11 , communications channel  12 , and DCE  14 . For simplicity only a single transmitter/receiver pair is shown as represented by transmitter  15 , of DCE  11 , and receiver  16 , of DCE  14 . Transmitter  15  transmits a precoded data signal, x(n), to receiver  16 , via communications channel  12 . 
     Receiver  16  typically includes a decision feedback equalizer (DFE) to compensate for interference on channel  12 . General information on DFEs can be found in U.S. Pat. No. 5,513,216, entitled “Hybrid Equalizer Arrangement for use in Data Communications Equipment,” filed on Oct. 13, 1994, by Gadot et al., and in U.S. Pat. No. 5,604,769, entitled “Hybrid Equlaizer Arrangement for use in Data Communications Equipment,” filed on Oct. 13, 1994, by Wang, which are incorporated by reference as if set out in full hereinbelow. 
     FIG. 2 depicts a typical DFE  35  of the prior art. In FIG. 2, feedforward filter  49  whitens the noise present in the received data signal  47 . Signal  47  corresponds to x(n) which is transmitted to receiver  16  (FIG. 1) subsequent to preceding by transmitter  15  (FIG.  1 ). The output signal  51  from feedforward filter  49  is applied, via sampler  53  and connection  54 , to adder  55 , which, theoretically, subtracts the inter-symbol interference (ISI) on connections  59  and  61  estimated by ISI filters  64  and  65  (described further hereinbelow). Adder  55  provides a signal  66  to slicer  68 . Slicer  68  selects a particular data symbol as a function of the mapping of the signal  66  into a predefined constellation of data symbols (not shown) to provide a signal  72 , {circumflex over (x)}(n), which is an estimations of transmitted data signal x(n). Signal  72  typically represents a stream of data symbols occurring at a symbol rate of 1/T seconds and is provided for processing by feedback filters  64  and  65  and by other receiver circuitry (not shown) to recover the transmitted data x(n). For example, if trellis coding is used, {circumflex over (x)}(n) is subsequently processed by a Viterbi decoder. 
     ISI filter  65  is a finite-impulse-response (FIR) filter having an impulse response represented by vector  f (n). As mentioned above, ISI filter  65  uses the estimate, {circumflex over (x)}(n), of the transmitted data to predict the amount of ISI to remove from signal  54 . Adaptation of ISI filter  65  is performed by using signal  74  which is an error signal, ê(n), developed by adder  76 . For illustration, it is assumed that a least-mean-square (LMS) algorithm is used to adapt the coefficients of ISI filter  65  which are sometimes referred to as ISI, “equalizer” or “feedback” coefficients. As such, then the I-th coefficient (i=0, 1, . . . , N−1) at the time instant n, f i (n), is given by: 
     
       
           f   i ( n +1)= f   i ( n )+2 μê ( n ) {circumflex over (x)} ( n−i ),  (1) 
       
     
     where μ is the adaptation step size For simplicity, this description assumes the use of real filters and real data. However, the inventive concept is also applicable to complex filters and data as well. 
     The DFE  35  is based on the assumption that {circumflex over (x)}(n) is a good estimation of the transmitted data x(n). As along as {circumflex over (x)}(n) is a good estimation of x(n), there is no significant amount of error. However, if the estimate of x(n) is wrong, then the feedback section adds this error to the next symbol, x(n+1), and error propagation occurs. As a result, a form of non-linear precoding is typically used in transmitter  15  (FIG. 1) to avoid error propagation. 
     In modem communications there are two phases of operation. In the first phase, the “initialization” or “start-up” phase, the DFE  35  (FIG. 2) of the receiver  16  (FIG. 1) adapts to a standard test signal or training sequence received from transmitter  15  (described hereinbelow). This phase is also referred to in the art as a “start-up,” or “training” phase. Typically, there is no precoding of this test signal by the transmitter  15  (FIG. 1) in the initialization phase. Once the DFE  35  (FIG. 2) adapts, the resulting set of coefficients values, f i (n), (i=0, 1, . . . , N−1), of DFE  35  are transmitted back to the transmitter  15  (FIG.  1 ). 
     It should be noted that it is known in the art for receiver  16  (FIG. 1) to generate equalizer coefficient values using a noise predicator (NP) based filter The differences between an ISI based DFE and an NP based DFE in generating the equalizer coefficient values off receiver  16  (FIG. 1) are not significant to the understanding of the present invention and, therefore, will not be discussed in detail herein, However, it should be apparent that NP based filters may be used to implement the principles of the present invention even though ISI filters will be discussed hereafter to illustrate the principles of the present invention. 
     During the initialization phase, the transmitter  15  of FIG. 1 receives the equalizer coefficient values from receiver  16 . Typically, transmitter  15  includes a precoder  85 , as depicted in FIG. 3, to precode a signal  86 , x(n), prior to transmission using any of the well-known precoding techniques, e.g., Tomlinson preceding. As known in the art, the equalizer coefficient values received from receiver  16  (FIG. 1) are stored in precoder filter  94  and will be referred to herein as the “precoder values.” Once the precoder filter  94  is initialized with the precoder values, the initialization phase terminates and the data phase is initiated. 
     In the data phase, data signal  86  is applied to adder  95  which subtracts from signal  86  a signal  98  produced by precoder filter  94 . As known in the art, signal  98  is based on the precoder values within precoder filter  94 . 
     The output signal  102  of adder  95  is applied to modulo  2 L element  99 . which performs as known in the art, to provide an output data symbol stream  104  For example, modulo  2 L element  99  maps the output signal  102  to a position in a signal point constellation. This mapping is performed using modulo  2 L arithmetic, where L is the size of a signal point constellation. FIG. 4 shows an illustrative signal point constellation, where L=7+1. The output data symbol stream  104  of FIG. 4 is precoded and ready for transmission to receiver  16  (FIG.  1 ). The output data symbol stream  104  is also applied to filter  94 , which filters signal  104  in accordance with the polynomial function or filter response vector,  f (n), using the above-mentioned set of precoder values. 
     The precoding technique of the prior art utilizes the above-mentioned coefficient values as determined by the receiver  16  (FIG. 1) during training. If the communications channel  12  (FIG. 1) remains constant for the transmission period, no further adaptation is required since the precoding in transmitter  15  (FIG. 1) is equivalently performing the feedback function. 
     Unfortunately, changes in the communications channel  12  (FIG. 1) induces errors to occur when the precoding is removed by DFE  35  of FIG.  2 . Therefore, another ISI filter  64 , not utilized in the initialization phase, is utilized in the data phase. The equalizer coefficient values in ISI filter  64  are set to zero during the initialization phase. As the data phase is commenced and changes in the communications channel  12  (FIG. 1) begin to cause distortion, the equalizer coefficient values in ISI filter  64  increase in order to compensate for the distortion. In this regard, ISI filter  64  provides a signal  61  to adder  55  based on signals  72  and  74  and based on the equalizer coefficients within ISI filter  64 . When the values of the equalizer coefficients within ISI filter  64  reach a certain level, error propagation begins to occur. When error propagation begins to cause significant error, a retrain occurs wherein the precoder values are replaced by a new set of values through the techniques outlined hereinabove for the initialization phase. Each retrain is disruptive to the communications session in that it interrupts data flow and takes time to both calculate and communicate the necessary equalizer coefficients. 
     However, in accordance with the inventive concept, the foregoing error propagation problem can be solved by adapting the precoder  85  (FIG. 3) to the changes in the response of the communications channel  12  (FIG.  1 ). 
     In accordance with the principles of she present invention, receiver  16  (FIG. 1) is configured to transmit a set of equalizer coefficient values during training to transmitter  15  (FIG. 1) using the techniques described hereinbefore. Transmitter  15  (FIG. 1) is configured to receive the coefficients and to establish a set of precoder values to be used by precoder filter  94  (FIG. 3) as discussed hereinbefore. However, transmitter  15  (FIG. 1) is further configured to adaptively update the precoder values periodically without the occurrence of a retrain. As used herein, the term “adaptively update” shall refer to updating a set of precoder values without the occurrence of a retrain. Furthermore, a “set” of values can refer to one value or a plurality of values. 
     FIG. 5 is an illustrative block diagram of the adaptive communication system  115  of the present invention. After an initial set of precoder values are established in transmitter  15  during training, receiver  16  is configured to periodically transmit equalizer coefficient values within ISI filter  64  to transmitter  15  over secondary channel  119 . Secondary channel  119  may be either an in-band channel or an out-of-band channel. A precoding system  120  within transmitter  15  is designed to use the new coefficient values transmitted over secondary channel  119  to update the current precoder values. 
     In this regard, precoding system  120  includes a converter  130  configured to receive the equalizer coefficient values from ISI filter  64 , as depicted by FIG. 6 Converter  130  is configured to determine a new set of precoder values that will compensate for the changes that occurred on communications channel  12  based on the equalizer coefficient values of ISI filter  64  and the current precoder values. Converter  130  is designed to slowly change the current set of precoder values to the new set of precoder values by periodically incrementing each current precoder value a small amount until the new precoder value is reached or a new set of equalizer coefficients is received. 
     Slowly incrementing the precoder values enables the ISI filter  64  to track the updating of the precoder values. As the precoder values are slowly updated, the quality of the signals received by receiver  16  slowly improves, and, in response, ISI filter  64  is designed to slowly reduce the values of its equalizer coefficient values. As a result, the coefficient values within ISI filter  64  are prevented from reaching values that result in error propagation, thereby preventing the occurrences of retrains. 
     In order to calculate the new precoder values, converter  130  is designed to contain an algorithm utilizing the following transfer function equation in the frequency domain: 
     
       
         H p   (n) ( z )=[1+H p   (n−1) ( z )][1+H ISI   (n) ( z )]−1  (1) 
       
     
     where H p (z) represents the transfer function of a precoder value and H ISI (z) represents the transfer function of a coefficient value from ISI filter  64 . Using Equation (2), converter  130  is preferably configured to update each precoder value. 
     It should be noted that when an NP based filter (not shown) is used to calculate the coefficient values of receiver  16 . converter  130  is configured to use the following equation in place of Equation (1): 
     
       
         H p   (n) ( z )=[1+H p   (n−1) ( z )][1−H NP   (n) ( z )]−1  (2) 
       
     
     where H NP (z) represents the transfer function of an NP equalizer coefficient value. Therefore, whether ISI filters are NP filters are used by receiver  16  does not materially affect the methodology of the present invention. 
     OPERATION 
     The preferred use and operation of the adaptive precoding system  120  and associated methodology are described hereafter with reference to FIGS. 5 and 6. 
     During training, transmitter  15  of FIG. 5 transmits initialization signals to receiver  16 , and receiver  16  formulates a set of equalizer coefficient values as described hereinabove. The equalizer coefficients formulated by receiver  16  are preferably communicated to the precoding system  120  of the present invention located in transmitter  15 . The coefficients are stored within precoder filter  94  as precoder values, and precoder filter  94  uses the precoder values to precode data signals  86  as disclosed hereinabove. Transmitter  15  then enters the data phase, sometimes referred to as the “communications phase,” and communicates the precoded signal  104  to receiver  16 . DTE  35  within receiver  16  removes the precoding thereby recovering the original data signal  86 . During the communications phase, changes in the channel  12  cause a degradation in communication quality. In response, ISI filter  64  begins to increase its equalizer coefficient values. Periodically, receiver  16  transmits the equalizer coefficient values of ISI filter  64  to transmitter  15  across secondary channel  119 . Since the equalizer coefficients are not continuously transmitted, secondary channel  119  may be configured as a low speed channel. 
     Converter  130  (FIG. 6) determines an updated set of precoder values based on the equalizer coefficient values and the current precoder values. In order to enable the ISI filter  64  to track the changes, converter  130  slowly updates the precoder values. This is preferably done by incrementing each precoder value in a plurality of increments until each precoder value corresponds to the new set of precoder values or until the next set of equalizer coefficients is received. 
     For example, assume for illustrative purposes only that converter  130  determines that a current precoder value of five (5) needs to be updated to a new precoder value of six (6). Instead of simply replacing the current precoder value with the new precoder value, converter  130  initially increments the current precoder value (5) to a value between five (5) and six (6). By continuously updating the current precoder value in this manner, the value of six (6) is eventually reached. As the communication on channel  12  slowly improves, due to the slow updating of the precoder values, the ISI filter  64  updates its equalizer coefficients accordingly. As a result, the equalizer coefficient values in ISI filter  64  do not reach a point where error propagation occurs. Accordingly, disruptive retrains are prevented. 
     It should be noted that receiver  16  may transmit the equalizer coefficients one at a time or in sets. Furthermore, it is not necessary for the precoding system  120  to completely update each precoder value before receiving a new set of equalizer coefficients and beginning a new updating phase. 
     In concluding the detailed description, it should be noted that it will be obvious to those skilled in the art that many variations and modifications may be made to the preferred embodiment without substantially departing from the principles of the present invention. All such variations and modifications are intended to be included herein within the scope of the present invention, as set forth in the pending claims.