Abstract:
A receiver has an equalizer with plural equalization settings, which compensates for distortion in a received signal, and an adapter for selecting one of those settings which optimally compensates for the distortion. The adapter employs a trial and error procedure for evaluating equalizer performance for each such setting by first observing multiple levels of the incoming signal and defining therefrom valid regions, encompassing each of the multiple levels, and invalid regions. For each setting, the adapter computes first and second metrics respectively consisting of a count of samples within each of the invalid regions, and differences that are less than a predetermined threshold between pairs of samples falling within that valid region. For each setting, the adapter combines the metrics to produce a combined metric. The adapter then compares all of the combined metrics to determine the best metric and chooses the setting corresponding thereto.

Description:
RELATED APPLICATION 
     This application contains subject matter related to co-pending, commonly assigned U.S. application Ser. No. 09/777,080, filed concurrently herewith entitled PHASE DETECTOR FOR BAUD RATE-SAMPLED MULTI-STATE SIGNAL RECEIVER. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention is related to signal processing of received signals of the type having a set of allowable states or amplitudes, such as pulse amplitude modulated signals, such as signal processing employing equalization. In a particular application, the invention concerns the adaptive control of an equalizer employed in the processing of multi-state signals. 
     2. Background Art 
     Multi-state signals are employed in high speed (e.g., gigabit-per-second) network communications, such as local area networks of computers. While the present invention may find application in processing various types of multi-state signals, such as pulse amplitude modulated signals, phase modulated signals and so forth, the detailed description presented below concerns application of the invention to processing of pulse amplitude modulated signals. 
     Many high speed computer networks transmit ultra-high frequency signals (gigabit-per-second data) over a coaxial conductor cable. The cable introduces signal distortion, arising from certain characteristics of the cable such as its reactance. Signal distortion also arises in channels that do not employ an electrically conductive cable. Signal processing is employed to correct for such distortion. For example, the signal processing distortion correction may be performed by an equalizer of the type which introduces a certain reactance that compensates for the reactance of the cable. A conventional equalizer suitable for digital signal processing introduces a transfer function whose representation in the complex plane has appropriate poles and zeroes corresponding to the desired reactance, as is well known to the skilled worker. Various reactances may be stored in the equalizer, and one of them is selected at any one time. The problem is that the cable reactance is not known a priori, and therefore the equalizer must have a large number of settings (e.g., reactances) one of which is chosen only after actual testing in the field of the cable. Since the cable characteristics may not be constant and/or the cable may be changed by the user, the choice of equalizer setting must be made periodically during actual use of the network. This is accomplished by adaptive techniques in which the signal distortion is periodically or constantly monitored and the equalizer setting is periodically or constantly adjusted in a manner calculated to minimize the distortion. 
     Numerous conventional techniques have been employed to carry out such adaptive equalization. Such techniques include recursive algorithms such as a Recursive Least Squares adaptive algorithm and a Least Mean Square adaptive algorithm. A significant problem with such techniques is that these adaptive algorithms are mathematically intensive, involving large numbers of multiply and accumulate steps. Implementing a very large number of multiply operations in a circuit is very expensive and complex, making it difficult to provide such a product on a cost-competitive basis. Therefore, there is a need to provide adaptive equalization without requiring such a mathematically intensive algorithm or without requiring multiply and accumulate operations. 
     SUMMARY OF THE INVENTION 
     The invention is embodied in a receiver that receives a modulated signal having multiple levels. The receiver has an equalizer with plural equalization settings for compensating for distortion in the received signal. The receiver further includes an adapter for selecting one of the plural equalization settings that provides an optimum compensation for the distortion. The adapter employs a trial and error procedure for evaluating the equalizer performance for each one of the equalizer settings by first observing the multiple levels of the incoming signal and defining therefrom valid regions encompassing each of the multiple levels and invalid regions not encompassing the multiple levels. Next, the adapter computes a first metric consisting of a count of samples within each of the invalid regions. It also computes a second metric consisting of the differences that are less than a predetermined threshold between pairs of samples falling within the same valid region. Finally, the adapter combines the first and second metrics to produced a combined metric for said one equalizer setting. The adapter then compares all of the combined metrics to determined the best metric and chooses the equalizer setting corresponding to the best combined metric. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a histogram of received 3-level pulse amplitude modulated signal samples transmitted over such a short cable that no distortion is visible. 
         FIG. 2  is a histogram of received 3-level pulse amplitude modulated signal samples transmitted over a very long cable so that significant distortion manifest as deviations from the three valid signal levels is apparent. 
         FIG. 3  is a block flow diagram illustrating the operation of a cable feedforward adapter in accordance with a preferred embodiment of the invention. 
         FIG. 4  is a block diagram illustrating a receiver system embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention involves a recognition that in a multi-state signal, distortion causes a significant number of samples of the signal to be detected in unallowed states. For example, in a three-level pulse amplitude modulated signal, a severely distorted signal would appear to the receiver to have a large number of samples of the signal at amplitudes between the allowed levels. Thus, if the three levels are 5 volts, −5 volts and 0 volts (with a tolerance of +0.2 volts), then a severely distorted signal would have a preponderance of samples in the invalid region between +4.8 and +0.2 volts and in the invalid region between −4.8 and −0.2 volts. For example, a sample at 2.5 volts would be in the middle of an invalid region. The correct amplitude of such a sample is one of the two valid levels on either side of the sample, but it is impossible to determine which one. Thus, samples lying between the two valid levels are anomalous, and the information they represent is lost. The closer a sample is to the middle of an invalid region, the more difficult it is to resolve this anomaly and in many cases it is impossible, leading to failure of the communication system. If the appropriate equalizer setting is found that corrects the distortion, then no received samples lie within either invalid region. 
     In the invention, this fact is exploited to construct a simple way of evaluating each equalizer setting to find the optimum equalizer setting that best removes distortion. During a training period, a received signal is sampled and then each sample is equalized by an equalizer. The equalizer settings are changed in a trial and error procedure to discover the optimum equalizer setting by evaluating the corrected signals as they are produced. The efficacy of each equalizer setting is evaluated from the resulting corrected signal in accordance with the present invention. Specifically, the efficacy of an equalizer setting is evaluated in accordance with the population of samples produced by that equalizer setting lying within the three allowed regions relative to the population of such samples lying outside the allowed regions. 
     More particularly, the invention involves first determining for a given equalizer setting the number of samples produced by that equalizer setting lying within each one of the allowed regions (for example, within the region between +4.8 and +5.2 volts) and the number of such samples lying within each one of the unallowed region between +4.8 volts and +0.2 volts. The size (tolerance) of the allowed region may be enlarged to provide more rapid convergence. For example, the tolerance may be 10% of the maximum amplitude absolute value, in which case the allowed regions are 4.5 to 5.5 volts, −0.5 to 0.5 volts and −4.5 to −0.5 volts. In this case, the unallowed regions include the region between 0.5 and 4.5 volts and the region between −0.5 and −4.5 volts. The metric here is either the number of samples falling within the unallowed regions or, alternatively, the percentage of all samples falling within the unallowed regions. Such a metric is referred to herein as a “white box” metric. The smaller this metric, the less ambiguity between allowed levels and therefore the better the equalization. 
     The next determination is a “tightness” metric. This metric involves determining, for each one of the three valid regions, differences between successive samples lying within the one valid region. This computation is made for each sample lying within the one valid region by computing the difference between the sample and the chronologically last sample that fell within the same valid region. Samples falling within another one of the three valid regions are ignored. First order differences are computed separately for each one of the three valid regions. The smaller the first order differences, the less deviation and therefore the less distortion there is in the processed signal. Thus, the tightness metric is a measure of the smallness of the first order differences. A preferred way of computing the tightness metric is to count the number of first order differences that are less than a small percentage, e.g., 5%, of the peak amplitude deviation. In the foregoing example, the number of first order differences less than 0.25 volts is counted. The more first order differences that are 0.25 volts or less, the tighter the distribution of samples within an allowed region and therefore the better the equalization. 
     In summary, the equalizer setting having the smallest white box metric and the largest tightness metric is the optimum equalizer setting. All equalizer settings are therefore evaluated by determining their white box metric and their tightness metric. Then, in one implementation, the tightness metric is subtracted from the white box metric, and the equalizer having the least metric (least positive or most negative) is deemed to be the best. 
     A significant advantage of the invention is that very little arithmetic power is required, apart from a few add operations. In comparison, a least means square algorithm or similar recursive algorithm capable of finding an optimum equalizer setting requires a large number of multiply operations and is therefore far more expensive to implement. The present invention only requires an adder capability, and therefore is far less expensive to implement. 
       FIG. 1  illustrates a histogram of received samples of a three-level pulse amplitude modulated signal with no distortion. The three signal levels are given as percentages of peak amplitude, specifically 100, 0 and −100. In  FIG. 1 , each signal sample falls exactly on one of the three allowed signal levels. 
       FIG. 2  illustrates a histogram of received signal of the same signal in the presence of distortion attributable to the reactance of a long (150 meter) coaxial cable over which the signal was received.  FIG. 2  shows that the samples tend to cluster around the three allowed levels, but some of the samples deviate as much as 25% from the nearest allowed level. A 50% deviation is completely anomalous, since at that deviation the sample is equidistant from two allowed levels and therefore it is not known which level is the true level that was transmitted. To avoid such a failure, equalization is necessary to reduce the deviation of the sample population and gain a tighter distribution closer to the ideal case of FIG.  1 . 
     Referring to  FIG. 2 , it is seen that a fairly large fraction of the samples deviate more than 10% from the nearest allowed level of 100, 0 or −100. Thus, one practical choice for the white box metric is to define one of the invalid regions as lying between 10 and 90 and the other as lying between −10 and −90. In this case, a practical choice for the tightness metric is to define all first order differences that are 5 or less as satisfying the tightness criteria.  FIG. 3  illustrates how these choices would be carried out in implementing an equalizer adapter process of the invention. 
     Referring to  FIG. 3 , a stream of 3-level pulse amplitude modulated signal samples is received and their peak positive and peak negative amplitudes are detected to determine the actual amplitudes of the three levels (block  310  of FIG.  3 ). An equalizer having a number of settings is set to the next equalizer setting in a predetermined sequence of settings (block  320 ). The process then proceeds along two parallel branches  330 ,  335 . In branch  330 , the white box metric is computed by counting the number of samples in each of the two invalid regions, namely the region lying between 10 and 90 and the region lying between −10 and −90, respectively, of the graph of  FIG. 2  (block  340 ). In branch  335 , the tightness metric is computed by first identifying the samples lying within each valid region (block  350 ). The valid regions include the region above +90, the region below −90 and the region between +10 and −10. These regions encompass deviations of 10% from the allowed or valid amplitudes of 100, −100 and 0. Of course, wider regions (e.g., encompassing 15% deviations) or narrower regions (e.g., encompassing 5% deviations) may be chosen. The next step is to compute the amplitude difference between each pair of chronologically successive samples lying within the same valid region (block  360 ). For this purpose, a pair of samples is considered to be successive even though an intervening sample occurred but fell outside of the region. Such a sample is ignored. Once the differences between each pair of successive samples have been computed for one region, the same computation is performed for another valid region, until all valid regions have been accounted for. Next, for all valid regions, the number of differences not exceeding a threshold amount (such as 5% of the peak amplitude) is counted, the total count being the measure of tightness of the present equalizer setting (block  370 ). The total metric for the present equalizer setting is then computed (block  380 ) by combining the whitebox metric of block  340  with the tightness metric of block  370 . Preferably, this is done by subtracting the tightness metric from the whitebox metric. Then, if not all equalizer settings have been evaluated (branch  382  of block  380 ), the next equalizer setting is selected (block  320 ) and the foregoing process is repeated for the next equalizer setting. Once all equalizer settings have been evaluated (branch  384  of block  380 ), then the metrics for all of the equalizer settings are compared and the equalizer setting having the best metric is selected (block  390 ) 
     The “best” metric is the least positive (or most negative) metric in the preferred embodiment where the metric is defined as the whitebox metric minus the tightness metric. Other definitions could be employed, however. For example, the metric could be the ratio of the whitebox metric to the tightness metric, in which case the smallest metric would be the best. 
       FIG. 4  illustrates a receiver system embodying the present invention. The receiver system forms a part of a 3-level pulse amplitude modulation gigabit-per-second computer network. In such a system, the same cable (the cable  400  of  FIG. 4 ) carries the transmitted and received signals simultaneously. Therefore, in order to isolate the received signal, an analog subtractor  402  subtracts the analog transmitted signal (the input labeled “analog tx”) from the signal on the cable, producing the received signal (“rx”) at the output of the subtractor  402 . An analog-to-digital converter  404  samples the analog received signal rx in synchronism with a recovered clock signal produced by a clock recovery circuit  406 . The analog-to-digital converter  404  converts each analog sample to a digital word (e.g., an eight-bit digital word) in accordance with an analog reference level from a conventional reference generator  408 . The digital output of the analog-to-digital converter  404  is processed by a feed-forward equalizer  410  having a transfer function specified in accordance with industry standards to remove a predetermined bias imposed on the signal by the node that transmitted the signal. 
     In order to compensate for distortions imposed on the received signal during its transit over the cable  400 , such as those attributable to reactance of the cable discussed above in this specification, a cable feedforward equalizer  412  imposes a selected transfer function on the signal output by the equalizer  410 . The equalizer  412  is of the conventional type whose transfer function may be represented in the complex plane with plural poles and zeroes corresponding to a desired reactance. Preferably, the equalizer stores a number of such transfer functions, one of which may be selected at any one time. A cable feed forward equalizer adapter  414  carries out the function illustrated in  FIG. 3  for choosing the best one of the transfer functions or settings of the cable feedforward equalizer  412 . 
     The equalized digital signal produced by the cable feedforward equalizer  412  is combined in an adder  416  with a crosstalk correction signal produced by a crosstalk correction circuit  418 . The crosstalk correction circuit  418  produces the crosstalk correction signal so as to compensate or cancel crosstalk from the transmitted signal when combined with the equalized digital signal in the adder  416 . The crosstalk correction circuit has two inputs, namely the corrected signal from the output of the adder  416  and the transmitted signal tx, as indicated in FIG.  4 . The crosstalk correction circuit  418  consists of a near end crosstalk (“NEXT”)/echo canceller  420  and a NEXT/echo adapter  422  that controls the canceller  420 . The crosstalk correction circuit  418  including the canceller  420  and the adapter  422  are described in U.S. patent application Ser. No. 09/636,047 entitled “ADAPTER FOR NEAR-END CROSSTALK AND ECHO CANCELLER FOR BI-DIRECTIONAL DIGITAL COMMUNICATIONS” filed Aug. 10, 2000, by Duy Pham et al and U.S. patent application Ser. No. 09/636,042 filed Aug. 10, 2000, by Duy Pham et al, both applications being assigned to the assignee of the present application, the disclosures of which are incorporated herein by reference. 
     The output of the adder  416  is fed back as an input to the crosstalk correction circuit  418 , as referred to above, and to the cable feedforward adapter  414  at feedback input  414   a . Referring again to  FIG. 3 , the receipt of the succession of samples of step  310  refers to the successive digitized samples furnished to the input  414   a  of the adapter  414 . The adapter  414  performs the function illustrated in  FIG. 3  so as to maximize the number of digitized samples received at the input  414   a  falling within the three allowed levels discussed above. 
     The digital signal output by the adder  416  is also applied as a feedback signal to the clock recovery circuit  406 , and specifically to a phase detector  432 . The phase detector  432  is described in co-pending commonly assigned U.S. application Ser. No. 09/777,080 filed herewith by Duy Pham et al entitled “PHASE DETECTOR FOR BAUD RATE-SAMPLED MULTI-STATE SIGNAL RECEIVER”, the disclosure of which is incorporated herein by reference. The output of the phase detector  432  is applied to the input of a conventional loop filter  434  whose output controls a voltage controlled oscillator  436 . The voltage controlled oscillator  436  generates the recovered clock signal applied to the analog-to-digital converter  404 . The phase of the voltage controlled oscillator  436  is incremented or decremented depending upon the polarity of the phase error detected by the phase detector  432 . 
     A conventional slicer  450  makes a decision for each digital sample as to which one of the allowed levels the sample represents (i.e., is closest to). It does this in accordance with a conventional threshold generator  452 . It should be noted that during the training period of the equalizer adapter  414 , 3-level pulse amplitude modulation is employed, but the actual data may be transmitted using a different number of levels, such as 5-level pulse amplitude modulation. 
     A peak detector  454  determines the prevailing or current peak amplitude (positive and negative) of the digital samples output by the adder  416 . The positive and negative peak amplitudes define the upper and lower valid levels of the 3-level signal used during training of the adapter  414 . In the example described above, the positive peak was 100, the negative peak was −100, defining the upper and lower valid levels, while the middle level between them was 0. The adapter  414  deduces the three valid levels of the 3-level pulse amplitude modulation signal by assigning the positive peak value sensed by the peak detector  454  to the upper valid level, the negative peak value sensed by the peak detector  454  to the lower valid level and the amplitude midway between the two peaks as the middle valid level. Conventional circuitry is employed to carry out this task, which is part of the step of block  310  of FIG.  3 . 
     The output of the peak detector  454  is also utilized in conventional well-known fashion by the conventional analog-to-digital reference generator  408 . The reference generator  408  deduces from the peak magnitudes sensed by the detector  454  the current analog range of the incoming signal, and in conventional manner cause the maximum digital range of the analog-to-digital converter  404  to match the sensed analog range of the incoming signal. 
     The output of the peak detector  454  is also applied to a phase detector reference circuit  430  of the clock recovery circuit  406 . The phase detector reference circuit  430  uses the peak magnitudes sensed by the peak detector  454  to deduce the allowable levels of the digitized signal at the output of the adder  416 . The allowable levels thus deduced are then provided to the phase detector  432 . The phase detector  432  compares each digital sample received from the adder  416  to the allowable levels provided by the phase detector  430  in order to deduce the current phase error. It does this in the manner described in co-pending commonly assigned U.S. patent application Ser. No. 09/777,080 filed herewith by Duy Pham et al entitled “PHASE DETECTOR FOR BAUD RATE-SAMPLED MULTI-STATE SIGNAL RECEIVER”, the disclosure of which is incorporated herein by reference. 
     Having now described the entire system, the operation of the adapter  414  illustrated in  FIG. 3  will now be reviewed with more particular reference to FIG.  4 . Initially, and then at periodic intervals thereafter, the adapter  414  determines the optimum equalizer setting of the equalizer  412  during a brief training period. During the training period, it is preferred that the transmitter send a three-level pulse amplitude modulated signal to the receiver in which the three levels consist of positive and negative amplitudes of the same absolute value (e.g., +100) and an intermediate value halfway between these two (e.g., 0). In such a case, the peak detector  454  senses a negative peak value of −100 and a positive peak value of +100, this information being furnished by the peak detector to the adapter  414 . As a result, the adapter  414  defines the three valid levels of the received signal as the two peaks (i.e., +100) and the value halfway between them (i.e., 0), in the step of block  310  of FIG.  3 . The adapter defines the three regions of valid signal (sample) values corresponding to 10% deviations from each of the valid values, i.e., a top region from 90 and higher, an intermediate region from −10 to +10 and a bottom region from −90 and below. The adapter then selects the first one of the set of equalizer settings of the equalizer  412  (block  320  of FIG.  3 ), and then simultaneously calculates the whitebox metric in branch  330  of FIG.  3  and the tightness metric of branch  335  of  FIG. 3 , combines the two metrics to compute the overall metric (block  380 ) and then selects the next equalizer setting. As described above, these calculations are based upon the population of samples falling within or outside of the valid regions. The samples used in the parallel branches  330  and  335  are the digital words emanating from the output of the adder  416  (not the output of the peak detector  454 ). The foregoing calculations and equalizer setting changes are repeated again until a metric has been computed for each of the equalizer settings. Then, the equalizer setting having the best metric is selected, and the equalizer  412  is placed in the selected setting. This concludes the training period. Thereafter, actual user data is transmitted to the receiver. The user data may be contained in a 5-level pulse amplitude modulated signal rather than a 3-level signal. The slicer  452  therefore is designed to assign each processed sample value to the closest one of the five allowed signal levels. 
     The invention is applicable to adapters for various types of multi-level or multi-state signals in which the best one of a plurality of settings of a signal processor, such as an equalizer, is selected by the adapter by evaluating the goodness of each equalizer setting. While the preferred embodiment is useful with pulse amplitude modulated signals, other embodiments may be useful with other types of multi-state modulated signals such as phase modulated signals. 
     While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.