Abstract:
There is disclosed, for use in a receiver receiving from a transmission channel an incoming stream of known symbols and unknown symbols distorted by intersymbol interference (ISI), an apparatus for reducing a precursor ISI signal. The apparatus for reducing the precursor ISI signal comprises: 1) a decision feedback equalizer for receiving the incoming stream of distorted known symbols and distorted unknown symbols and generating a sequence of detected symbols; and 2) a known symbol generator for generating a copy of a first known symbol prior to an estimation of the first known symbol by the decision feedback equalizer, wherein the decision feedback equalizer uses the copy of the first known symbol to reduce a first precursor ISI signal in a second symbol transmitted prior to the first known symbol.

Description:
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/165,321 entitled “Precursor Cancellation DFE” filed Nov. 12, 1999. 
   CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to those disclosed in the following United States Patent Applications:
         1. Ser. No. 09/627,191, filed concurrently herewith, entitled “SYSTEMS AND METHODS FOR OPTIMAL SYMBOL SPACING TO MINIMIZE INTERSYMBOL INTERFERENCE IN A RECEIVER”;   2. Ser. No. 09/627,453, filed concurrently herewith, entitled “DUAL EQUALIZER FOR USE IN A RECEIVER AND METHOD OF OPERATION”; and   3. Ser. No. 09/627,190, filed concurrently herewith, entitled “SYSTEMS AND METHODS FOR OPTIMAL DISTRIBUTION OF SYMBOLS IN A FIXED SIZE DATA PACKET TO IMPROVE RECEIVER PERFORMANCE”.       

   The above applications are commonly assigned to the assignee of the present invention. The disclosures of these related patent applications are hereby incorporated by reference for all purposes as if fully set forth herein. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention is directed, in general, to wireless and wireline receivers and, more specifically, to a system and method for cancelling precursor intersymbol interference (ISI) in a receiver. 
   BACKGROUND OF THE INVENTION 
   The rapid advance of digital technology has created a great demand for, and corresponding advances in, wireless and wireline technology for communicating voice and data traffic. Much of this traffic is carried by the public switched telephone network over fiber optic cable and copper wire. Computers and other data equipment communicate over the Internet and a variety of proprietary local area networks (LANs) and wide area networks (WANs). Increasingly, various types of digital subscriber line (DSL) service or cable modem service are bringing broadband data into homes and offices. Many third generation cellular telephones and wireless PDA devices are also equipped to handle broadband data traffic and Internet capable. 
   However, even the most modern of wireless and wireline data communication equipment still must contend with the age-old problems inherent in transmitting data through a channel from a transmitter to a receiver. Data is often transmitted as a series of pulses (or symbols) through a wire or the atmosphere. The data symbols may become distorted due to intersymbol interference (ISI), which is an overlap of adjacently transmitted symbols. In a wireless network, ISI may be caused by reflections of the transmitted symbols off natural objects (e.g., tress, hills) and man-made objects (e.g., buildings, brides) in the environment. The reflections cause multiple time-delayed, partially overlapping copies (echoes) of the same signal to arrive at the receiver. ISI also may occur in a non-linear, bandwidth limited channel if the symbol transmission rate is comparable to or exceeds the channel bandwidth, W. 
   Receivers frequently use a well-known technique, adaptive decision-feedback equalization, to minimize the effects of ISI. An adaptive decision-feedback equalizer (DFE) consists of a feedforward (or forward) filter, a feedback filter, and a decision circuit that decides or detects the value of each symbol in the received signal. The input to the forward filter is the received distorted sequence of data symbols. The input to the feedback filter is the sequence of previously decided (detected) symbols at the output of the decision circuit. The feedback filter removes from the symbol presently being estimated that portion of the ISI that is caused by previously detected symbols. 
   There are limitations, however, to the performance of decision feedback equalizers. Even under the best of circumstances, a DFE occasionally makes an incorrect decision regarding the value of a received data symbol. The incorrect estimate is then propagated back to the feedback filter, thereby affecting decisions regarding subsequent symbols. Furthermore, a DFE almost always does not perform detection on the first copy of a symbol as it is received. Because of the performance of the channel, symbol reflections may combine in such a way that the peak power of the transmitted symbol occurs after the first echo of the symbol enters the DFE. Thus, some reflections of a symbol (postcursors) are received by the DFE after a symbol is detected, but other reflections of a symbol (precursors) are received by the DFE before the symbol is due to be detected. A conventional DFE is unable to compensate for precursor ISI in the detection of the present symbol because of the causal nature of the feedback filter. 
   For example, in a sequence of ten symbols, the DFE may be working on detecting (deciding) the fifth symbol. However, precursor ISI from the sixth and seventh symbols and post-cursor ISI of the third and fourth symbols may contribute to distortion of the fifth symbol. Since the third and fourth symbols have already been decided by the decision circuit, the feedback loop can be used to remove the postcursor ISI. However, since the sixth symbol has not been detected yet, the feedback filter does nothing to remove the precursor ISI. 
   There is therefore a need in the art for improved receivers and transmitters for use in communication networks. In particular, there is a need in the art for improved decision feedback equalizers that have a lower detected symbol error rate. More particularly, there is a need for receivers containing decision feedback equalizers (DFEs) that are capable of at least partially minimizing precursor ISI due to symbols that have not yet been detected. Moreover, there is a need for improved transmitters and data networks that are capable of maximizing the performance of receivers that contain decision feedback equalizers capable of reducing precursor ISI. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an apparatus for reducing a precursor ISI signal for use in a receiver capable of receiving from a transmission channel an incoming stream of known symbols and unknown symbols distorted by intersymbol interference (ISI). In an advantageous embodiment of the present invention, the apparatus for reducing a precursor ISI signal, comprises: 1) a decision feedback equalizer capable of receiving the incoming stream of distorted known symbols and distorted unknown symbols and generating a sequence of detected symbols; and 2) a known symbol generator capable of generating a copy of a first known symbol prior to an estimation of the first known symbol by the decision feedback equalizer, wherein the decision feedback equalizer uses the copy of the first known symbol to reduce a first precursor ISI signal in a second symbol transmitted prior to the first known symbol. 
   According to one embodiment of the present invention, the decision feedback equalizer comprises a forward filter capable of receiving the incoming stream of distorted known symbols and distorted unknown symbols and generating an equalized output comprising a first estimated sequence of known symbols and unknown symbols. 
   According to another embodiment of the present invention, the decision feedback equalizer further comprises a first feedback filter capable of receiving the sequence of detected symbols and generating a first feedback signal capable of reducing in the second symbol a first postcursor ISI signal caused by a first detected symbol transmitted prior to transmission of the second symbol. 
   According to still another embodiment of the present invention, the decision feedback equalizer further comprises a first summation circuit capable of summing the equalized output from the forward filter and the first feedback signal from the first feedback filter to produce a combined output. 
   According to yet another embodiment of the present invention, the decision feedback equalizer further comprises a symbol estimator capable of quantizing the combined output from the first summation circuit to thereby generate the sequence of detected symbols. 
   According to a further embodiment of the present invention, the decision feedback equalizer further comprises a second feedback filter capable of receiving the first known symbol from the known symbol generator and generating a second feedback signal capable of reducing the first precursor ISI signal in the second symbol. 
   According to a still further embodiment of the present invention, the decision feedback equalizer further comprises a second summation circuit capable of summing the first feedback signal and the second feedback signal. 
   According to a yet another embodiment of the present invention, the transmission channel is one of a wireline channel and a wireless channel. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
       FIG. 1A  illustrates an exemplary wireline network according to one embodiment of the present invention; 
       FIG. 1B  illustrates an exemplary wireless network according to one embodiment of the present invention; 
       FIG. 2  illustrates selected portions of exemplary transmitter circuitry disposed in the transmitting stations and, for two-way systems, the receiving stations in  FIGS. 1A and 1B ; 
       FIG. 3  illustrates selected portions of exemplary receiver circuitry disposed in the receiving stations and, for two-way systems, the transmitting stations in  FIGS. 1A and 1B ; 
       FIG. 4  illustrates an exemplary precursor cancellation decision feedback equalizer in a receiver according to one embodiment of the present invention; and 
       FIG. 5  is a flow diagram illustrating the operation of the transmitters and receivers in the exemplary wireline and wireless networks according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged receiver. 
   Many wireline and wireless communication systems transmit a known sequence of symbols, called a training sequence, along with the unknown sequence of user data symbols in order to synchronize and adjust the receiver. The timing and values of the symbols in the training sequence are known by the receiver, thereby making the training sequence relatively easy to detect. An adaptive algorithm controller in the receiver analyzes the received training sequence, compares it to the known sequence, and uses the statistical properties of the received signal to adjust the values of the weighting coefficients in the forward filter and feedback filter of the DFE. When the unknown symbols are received, the DFE is better able to detect the correct values of the user data symbols. The present invention takes advantage of the transmission of known symbols to provide an improved receiver that at least partially reduces precursor ISI. Furthermore, the present invention also provides an improved transmitter that transmits the known symbols in an optimum manner to take advantage of the ability of the receiver to reduce the precursor ISI. 
     FIG. 1A  illustrates exemplary wireline network  100  according to one embodiment of the present invention. Wireline network  100  comprises transmitting station  110  and receiving stations  121 ,  122  and  123 . Transmitting station  110  communicates with receiving station  121 - 123  via wirelines  111 ,  112  and  113 . The words “transmitting” and “receiving” with respect to transmitting station  110  and receiving stations  121 - 123  are exemplary only and should not be construed to limit the scope of the invention to one-way communication. In fact, in advantageous embodiments of the present invention, transmitting station  110  may comprise transceiver circuitry capable of transmitting data to, and receiving data from, receiving stations  121 - 123 . Accordingly, in such embodiments, receiving station  121 - 123  also may comprise transceiver circuitry capable of transmitting data to, and receiving data from, transmitting station  110 . Both transmitting station  110  and each of receiving stations  121 - 123  transmit data to a receiving device as mixture of known symbol sequences (e.g., training sequences) and unknown symbols (i.e., user data). 
   By way of example, in one embodiment of the present invention, transmitting station  110  may comprise a server in a local area network (LAN) or wide area network (WAN) that communicates bidirectionally with client nodes (i.e., receiving stations  121 - 123 ). In an alternate embodiment of the present invention, transmitting station  110  may comprise a cable television broadcast system that primarily transmit video signals to cable set-top boxes (i.e., receiving stations  121 - 123 ) in subscriber homes. However, transmitting station  110  may also receive upstream data traffic transmitted by the cable set-top boxes (STBs). 
     FIG. 1B  illustrates exemplary wireless network  150  according to one embodiment of the present invention. Wireless network  150  comprises transmitting station  160  and receiving stations  171 ,  172  and  173 . Transmitting station  160  communicates via the air interface with receiving station  171 - 173 . Again, the words “transmitting” and “receiving” with respect to transmitting station  160  and receiving stations  171 - 173  are exemplary only and should not be construed to limit the scope of the invention to one-way wireless communication. In fact, in advantageous embodiments of the present invention, transmitting station  160  may comprise transceiver circuitry capable of wirelessly transmitting data to, and wirelessly receiving data from, receiving stations  171 - 173 . Accordingly, in such embodiments, receiving station  171 - 173  also may comprise transceiver circuitry capable of wirelessly transmitting data to, and wirelessly receiving data from, transmitting station  160 . Both transmitting station  160  and each of receiving stations  171 - 173  transmit data to a receiving device as mixture of known symbol sequences (e.g., training sequences) and unknown symbols (i.e., user data). 
   By way of example, in one embodiment of the present invention, O transmitting station  160  may comprise a server in a wireless LAN or WAN that communicates bidirectionally with client nodes (i.e., receiving stations  171 - 173 ). In an alternate embodiment of the present invention, transmitting station  160  may comprise a base transceiver station in a cellular network that transmits voice and data traffic to mobile stations (i.e., receiving stations  171 - 173 ) and receive voice and data traffic from the mobile stations (e.g., cell phones). In still another embodiment of the present invention, transmitting station  160  may comprise a high definition television (HDTV) broadcast facility that transmits high definition video signals to HDTV receivers (i.e., receiving stations  171 - 173 ) in its local coverage area. 
   In both wireline network  100  and wireless network  150 , each transmitted data symbol arrives at the receiving device distorted by postcursor and precursor intersymbol interference (ISI) caused by the band-width limited transmission channel and or reflections off objects. To minimize the effects of ISI, receiving stations  121 - 123  and receiving stations  171 - 173  comprise adaptive decision feedback equalizers (DFEs) capable of reducing both postcursor ISI and precursor ISI in accordance with the principles of the present invention. Furthermore, in bidirectional communication networks, transmitting stations  110  and  160  also may comprise adaptive DFEs capable of reducing both postcursor ISI and precursor ISI in signals transmitted by receiving stations  121 - 123  and receiving stations  171 - 173 . 
     FIG. 2  illustrates selected portions of exemplary transmitter circuitry disposed in transmitting stations  110  and  160  and, for two-way systems, receiving stations  121 - 123  and  171 - 173 . The exemplary transmitter circuitry comprises outgoing data source  205 , calibration/training bits generator  210 , known symbol distribution controller  215 , multiplexer  220 , symbol encoding circuitry  225 , up-converter/modulation circuitry  230 , and transmitter front-end circuitry  235 . Outgoing data source  205  generates the user data that is to be transmitted to a receiving device. For example, outgoing data source  205  may be a cell phone vocoder that converts an analog voice signal to digital data to be transmitted to a base transceiver station. Alternatively, outgoing data source  205  may be an application executed on a server that transmits data to a client work station via a wireline LAN. 
   The user data generated by outgoing data source  205  are unknown data bits that are combined with known data bits generated by calibration/training bits generator  210 . Calibration/training bits generator  210  may generated a training sequence that precedes the unknown user data. Calibration/training bits generator  210  also may generate additional training sequences that are distributed among the unknown data as a single block or in a plurality of smaller blocks at known intervals or locations among the unknown user data. For example, in a GSM mobile phone network, a GSM data packet comprises 148 bits, including 116 user data bits. The GSM data packet also comprises 3 start bits at the start of the user data, a 26-bit training sequence in the middle of the user data bits, and 3 stop bits at the end of the user data bits. 
   The size and location of the training/calibration sequences of known data bits are controlled by known symbol distribution controller  215 , which selectively switches the source of data that is output by multiplexer (MUX)  220 . Thus, the output of MUX  220  is a stream of known calibration/training bits interspersed at known locations among unknown user data bits. In an advantageous embodiment of the present invention, known symbol distribution controller  215  is an adaptive device that is capable of modifying the size and location of groups of calibration/training bits according to the known characteristics of the receiver. More particularly, known symbol distribution controller  215  is capable of modifying the size and location of groups of calibration/training bits in order to maximize the performance of the improved precursor ISI cancellation decision feedback equalizer (DFE) in the receiving device. 
   Symbol encoding circuitry  225  encodes the data bits according to any known symbol encoding scheme. Thus, a Logic 1 bit received from multiplexer  225  may be encoded as a first unique sequence of pulses and a Logic 0 bit received from multiplexer  225  may be encoded as a second unique sequence of pulses. Thus, the output of symbol encoding circuitry  225  is the sequence of known and unknown symbols that must be detected by the receiver DFE. 
   Up-converter/modulation circuitry  230  converts the baseband sequence of known and unknown symbols to a modulated (up-converted) signal capable of being transmitted through the transmission channel (wireline or wireless). For example, in a wireless network, up-converter/modulation circuitry  230  may comprise an RF mixer that converts the baseband sequence to a modulated radio frequency (RF) signal capable of being transmitted through the air channel to a receiving device. Finally, transmitter front-end circuitry comprises RF amplifiers, duplexer circuitry, and antennas that transmit the output of up-converter/modulation circuitry  230  into the corresponding wireline or wireless channel. 
   The arrangement and interconnection of known symbol distribution controller  215 , calibration/training bits generator  210 , and symbol encoding circuitry  225  is exemplary only. Those skilled in the art will recognize there are numerous other circuit arrangements capable of inserting known symbols at known locations in a sequence of outgoing unknown symbols. The arrangement in  FIG. 2  is suitable for those applications in which a symbol is used to represent no more than one data bit. This ensures that a known bit from calibration/training bits generator  210  is encoded only as a known symbol and that an unknown bit from outgoing data source  205  is encoded only as an unknown symbol. 
   However, in other systems, a single symbol may be used to represent more than one data bit. For example, four different symbols may be used to represent the bit pairs 00, 01, 10, 11. In such a system, precautions must be taken to ensure that symbol encoding circuitry  225  does not encode a known bit and an unknown data bit as a single symbol. To accomplish this, known symbol distribution controller  215  and calibration/training bits generator  210  may be coupled directly to symbol encoding circuitry  225 , and multiplexer  220  may be omitted. In such a circuit arrangement, symbol encoding circuitry  225  would encode all unknown data bit pairs from outgoing data source  205  as a sequence of unknown symbols and known symbol distribution controller  215  would cause symbol encoding circuitry  225  to insert known symbols representing known data bit pairs into the outgoing sequence of unknown symbols. 
     FIG. 3  illustrates selected portions of exemplary receiver circuitry disposed in receiving stations  121 - 123  and  171 - 173  and, for two-way systems, transmitting stations  110  and  160 . The exemplary receiver circuitry comprises receiver front-end (F-E) circuitry  305 , down-converter/demodulation circuitry  310 , bandpass (BP) filter  315 , sampler circuitry  320 , precursor decision feedback equalizer (PC-DFE)  325 , timing synchronization circuitry  330 , adaptive algorithm controller  335 , and receiver processing circuitry  340 . Receiver front-end circuitry  305  typically comprises a low noise amplifier and filters that receive the transmitted known and unknown symbols from the wireline or wireless channels and amplify and isolate the frequencies of interest (i.e., receive band). 
   Down-converter/demodulation circuitry  310  demodulates (down-converts) the incoming modulated signals to produce an analog baseband signal comprising a sequence of known and unknown symbols that are distorted to postcursor and precursor ISI. Sampler circuitry  320  converts the analog baseband signal to a digital baseband signal. The digital baseband signal is filtered by PC-DFE  325  to minimize the postcursor and precursor ISI distortion. Ideally, the output of PC-DFE  325  is the original sequence of known and unknown symbols. 
   Timing synchronization circuit  330  receives the output of PC-DFE  325  and uses it to synchronize (align) the analog-to-digital converter in sampler circuitry  320  and to synchronize the filtering circuitry in PC-DFE  325 , as described below in greater detail. Adaptive algorithm controller  335  also receives the output of PC-DFE  325  and compares it to the input sequence of distorted symbols from sampler circuitry  320 . From this comparison, adaptive algorithm controller  335  can determine and modify the weighting coefficients in the forward filter section and the feedback filter section of PC-DFE  325  in order to minimize ISI distortion. Finally, receiver processing circuitry  340  converts the sequence of known (i.e., calibrations/training) symbols and unknown symbols back to data bits and extracts the user data bits according to the algorithm used by known symbol distribution controller  215 . 
     FIG. 4  illustrates exemplary precursor cancellation decision feedback filter (PC-DFE)  325  in greater detail according to one embodiment of the present invention. PC-DFE  325  comprises forward filter  405 , summer  410 , symbol estimator  415 , feedback filter  420 , summer  425 , feedback filter  435  and known symbol generator  430  according to one embodiment of the present invention. Those skilled in the art will recognize that forward filter  405 , summer  410 , symbol estimator  415 , and feedback filter  420  constitute a conventional decision feedback filter capable of reducing postcursor ISI from previously estimated (or decided) symbols. The present invention differs from a conventional decision feedback filter due to the addition of summer  425 , feedback filter  435 , and known symbol generator  430 . 
   As noted above, adaptive algorithm controller  335  determines the values of the weighting coefficients, Ci, of forward filter  405 , the weighting coefficients, Cm, of feedback filter  420 , and the weighting coefficients, Cn, of feedback filter  435 . Adaptive algorithm controller  335  estimates the channel impulse response during receipt of the known training symbols and during receipt of other known symbols, such as known synchronization symbols and known packet identification symbols. If a training sequence is used, forward filter  405 , feedback filter  420  and feedback filter  435  may be adaptively adjusted using the recursive least square (RLS) algorithm or the least mean square (LMS) algorithm. 
   Forward filter  405  receives the sequence of ISI-distorted symbols, Y k , from sampler circuit  320  and produces an equalized output, Y′ k , that is an estimate of the input sequence. Adder  410  add the Y′ k  output to a composite (precursor and postcursor) ISI correction signal (explained below in greater detail) from summer  425  to produce the symbol estimate, V k . Symbol estimator  415  quantizes the V k  symbol estimate to the nearest symbol value to form a sequence of decided (i.e., detected) symbols, Ŝ k-d , that is transmitted to timing synchronization circuitry  330 , adaptive algorithm controller  335 , and receiver processing circuitry  340 . The quantity k is the index of the current symbol and d is the decision (detection) delay associated with symbol estimator  415 . The decided symbol sequence also is transmitted back to feedback filter  420 , which removes that part of the intersymbol interference from the present estimate caused by previously detected symbols (i.e., post-cursor ISI). 
   Known symbol generator  430  receives a timing signal from timing synchronization circuitry  330  and transmits a sequence of known symbols, S′ k , through feedback filter  435  at the proper location in the sequence of known and unknown symbols that are being processed by symbol estimator  415 . In an advantageous embodiment of the present invention, feedback filter  435  is a L 2 -tap transversal filter chosen to minimize precursor ISI from the known symbols. The output of known symbol generator  430  is normally zero. However, known symbol generator  430  generates known symbols during the time periods when one or more preceding unknown symbols are being estimated by symbol estimator  415 . In this manner, the effect of the precursor ISI of the known symbol can be removed from the present estimate, even though the known symbol has not been detected yet. 
   For example, if the sixth symbol in a sequence is known, known symbol generator  430  can output the sixth symbol during the estimation of the unknown fourth symbol and the unknown fifth symbol. The precursor ISI of the sixth symbol can therefore be removed from, for example, the fifth symbol estimate, just as feedback filter  420  removes from the fifth symbol estimate the postcursor ISI of the fourth symbol. 
     FIG. 5  depicts flow diagram  500 , which illustrates the operation of exemplary transmitting stations  110  and  150  and exemplary receiving stations  121 - 123  and  171 - 173  in wireline network  100  and wireless network  150  according to one embodiment of the present invention. Transmission of user data begins when, for example, transmitting station  110  transmits a training sequence of known symbols to receiving station  121  (process step  505 ). Next, adaptive algorithm controller  335  in receiving station  121  adjusts the coefficients of the filters in PC-DFE  325  to achieve, for example, minimum mean square error (process step  510 ). By the end of the training sequence, PC-DFE  325  uses the output of symbol estimator  415  in feedback filter  420  to minimize postcursor ISI in subsequent unknown user data symbols (process step  515 ). At the same time, PC-DFE  325  uses the output of known symbol generator  430  in feedback filter  435  to minimize precursor ISI in subsequent unknown user data symbols (process step  520 ). 
   Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.