Decision feedback equalizer employing fixed ratio postcursor taps for minimizing noise and intersymbol interference in signals conveyed over high speed data service loop
First Claim
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1. A decision feedback equalizer comprising:
- a linear transversal filter section to which successive received symbols of an input symbol sequence are sequentially introduced, said linear transversal filter section including a feedforward multistage delay line comprised of a series of 1st-Mth symbol value storage stages, M being an integer greater than 1, and a feedforward weighting and summing stage operative to multiply respective ones of a plurality of 1st-Mth weighting coefficients W1 -WM by respective ones of a plurality of 1st-Mth received symbols stored in said feedforward multistage delay line, and thereby obtain a plurality of 1st-Mth products, said 1st-Mth products being summed to provide a weighted and summed feedforward symbol value output;
a symbol decision unit which is coupled to receive a differential symbol value associated with a difference between said weighted and summed feedforward symbol value output and a weighted and summed feedback symbol value output and being operative to generate a received symbol value estimate in accordance with said difference; and
a decision feedback transversal filter section to which successive symbol value estimates generated by said symbol decision unit are sequentially applied, said decision feedback transversal filter section including a feedback multistage delay line comprised of a series of 1st through Nth symbol value storage stages, N being an integer greater than 1, and a feedback weighting and summing stage operative to multiply respective ones of a plurality of 1st through Nth weighting coefficients by respective ones of a plurality of 1st through Nth symbol decision outputs stored in said feedback multistage delay line, and thereby obtain a plurality of 1st through Nth products, said 1st through Nth products being summed to provide a weighted and summed feedback output, said weighted and summed feedback output being differentially combined with said weighted and summed feedforward symbol value output of said linear transversal filter section and the resulting differential signal being applied to said symbol decision unit which outputs received symbol value estimates; and
whereinsaid feedforward weighting and summing stage of said linear transversal filter section has its largest valued, cursor weighting coefficient WC associated with an M-Kth symbol stage of said feedforward multistage delay line, where K is an integer equal to or greater than 1, and K respective postcursor weighting coefficients WM-K to WM associated with the last K symbol stages of said feedforward multistage delay line having fractional weighting coefficients of said cursor weighting coefficients WC.
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Abstract
The feedforward filter section of a decision feedback equalizer is modified to include one or more postcursor taps, that are sequentially weighted at decreasing binary fractions of the cursor tap. Such a modified feedforward filter section, combined with the placement of a simple anti-aliasing filter upstream of the sampling point, results in an optimum feedforward filter configuration that is not anticausal, and offers a substantially improved performance over conventional DFB equalizer structures. Optimum performance is achieved when such a postcursor filter structure is augmented with an adaptive noise canceler coupled in the DFB path.
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Citations
22 Claims
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1. A decision feedback equalizer comprising:
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a linear transversal filter section to which successive received symbols of an input symbol sequence are sequentially introduced, said linear transversal filter section including a feedforward multistage delay line comprised of a series of 1st-Mth symbol value storage stages, M being an integer greater than 1, and a feedforward weighting and summing stage operative to multiply respective ones of a plurality of 1st-Mth weighting coefficients W1 -WM by respective ones of a plurality of 1st-Mth received symbols stored in said feedforward multistage delay line, and thereby obtain a plurality of 1st-Mth products, said 1st-Mth products being summed to provide a weighted and summed feedforward symbol value output; a symbol decision unit which is coupled to receive a differential symbol value associated with a difference between said weighted and summed feedforward symbol value output and a weighted and summed feedback symbol value output and being operative to generate a received symbol value estimate in accordance with said difference; and a decision feedback transversal filter section to which successive symbol value estimates generated by said symbol decision unit are sequentially applied, said decision feedback transversal filter section including a feedback multistage delay line comprised of a series of 1st through Nth symbol value storage stages, N being an integer greater than 1, and a feedback weighting and summing stage operative to multiply respective ones of a plurality of 1st through Nth weighting coefficients by respective ones of a plurality of 1st through Nth symbol decision outputs stored in said feedback multistage delay line, and thereby obtain a plurality of 1st through Nth products, said 1st through Nth products being summed to provide a weighted and summed feedback output, said weighted and summed feedback output being differentially combined with said weighted and summed feedforward symbol value output of said linear transversal filter section and the resulting differential signal being applied to said symbol decision unit which outputs received symbol value estimates; and
whereinsaid feedforward weighting and summing stage of said linear transversal filter section has its largest valued, cursor weighting coefficient WC associated with an M-Kth symbol stage of said feedforward multistage delay line, where K is an integer equal to or greater than 1, and K respective postcursor weighting coefficients WM-K to WM associated with the last K symbol stages of said feedforward multistage delay line having fractional weighting coefficients of said cursor weighting coefficients WC. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method of processing successive received symbols of a symbol sequence received over a dispersive communication channel comprising the steps of:
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(a) sampling said received symbol sequence to produce a sequence of symbol samples; (b) coupling said sequence of symbol samples to a linear transversal filter section, said linear transversal filter section including a feedforward multistage delay line comprised of a series of 1st-Mth symbol value storage stages, M being an integer greater than 1, and a feedforward weighting and summing stage operative to multiply respective ones of a plurality of 1st-Mth weighting coefficients W1 -WM by respective ones of a plurality of 1st-Mth received symbol samples stored in said feedforward multistage delay line, and thereby obtain a plurality of 1st-Mth products, said 1st-Mth products being summed together to provide a feedforward weighted and summed symbol value output; (c) providing a symbol decision unit which is operative to receive a differential symbol value associated with a difference between said feedforward weighted and summed symbol value output and a weighted and summed feedback symbol value output, and generating a received symbol value estimate in accordance with said difference; (d) coupling respective received symbol value estimates generated by said symbol decision unit in step (c) to a decision feedback transversal filter section, said decision feedback transversal filter section including a feedback multistage delay line comprised of a series of 1st through Nth symbol value storage stages, and a feedback weighting and summing stage operative to multiply respective ones of a plurality of 1st through Nth weighting coefficients by respective ones of a plurality of 1st through Nth symbol decision outputs stored in said feedback multistage delay line, N being an integer greater than 1, and thereby obtain a plurality of 1st through Nth products, said 1st through Nth products being summed to provide a feedback weighted and summed output; (e) differentially combining the feedback weighted and summed output provided in step (d) with said feedforward weighted and summed symbol value output provided by said linear transversal filter section in step (b) and applying the resulting differential signal being to said symbol decision unit provided in step (b) so that said symbol decision unit outputs received symbol value estimates; and
whereinstep (b) comprises establishing the weighting coefficients of said feedforward weighting and summing stage of said linear transversal filter section such that the largest valued, cursor weighting coefficient WC is associated with an M-Kth symbol stage of said feedforward multistage delay line, where K is an integer equal to or greater than 1, and K respective postcursor weighting coefficients WM-K to WM associated with the last K symbol stages of said feedforward multistage delay line having fractional weighting coefficients of said cursor weighting coefficients WC. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
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Specification