Adaptive convergent decision feedback equalizer

An adaptive convergent decision feedback equalizer for cancelling intersymbol interference in a pulse amplitude modulated data communication system, comprising:

• (a) sampling means, synchronous to a system clock having a baud period, for extracting sign, magnitude and error information from a received signal at discrete intervals of time within said baud period;
• (b) delay line means coupled to said sampling means and having a plurality of tap outputs, for providing a history cache of said sign and magnitude information from previous baud periods;
• (c) means, coupled to said tap outputs, for modifying a select tap output when information corresponding therein, contains a maximum and minimum level of said received signal, said tap output modified by adding thereto a product of said error information and a modification step size .DELTA.;
• and
• (d) means, coupled to said delay means and said means for modifying, for multiplying said sign and magnitude information stored in said tap by said tap output.

An adaptive convergent decision feedback equalizer circuit for reducing intersymbol interference in a received signal, comprising:

• (a) a plurality N of taps;
• (b) means, coupled to said taps, for calculating a tap coefficient (HDFE.sub.i) for an ith tap of said N taps so that:
• HDFE.sub.i (T.sub.+1)=HDFE.sub.i (T)+M.sub.i
• wherein T is a baud period, T.sub.+1 is a subsequent baud period to T and M.sub.i is an incremental change;
• and
• (c) means for generating an estimate (ISI.sub.est) of said intersymbol interference so that:
• ##EQU2## wherein H.sub.o is a compensating reference signal for slicer level threshold variation.

A method for cancelling intersymbol interference on a received signal in a data communication system, comprising the steps of:

• (a) caching said received signal in a N-tap transversal filter and generating an intersymbol interference estimate therefrom;
• (b) processing a tap coefficient output HDFE for a tap in said N-tap transversal filter in which said tap represents a maximum level;
• (c) processing a tap coefficient output HDFE for a tap in said N-tap transversal filter in which said tap represents a minimum level;
• and
• (d) enabling a number P of said taps at a startup time and then enabling a remaining number N-P of said taps at a time t.sub.min after said startup time, wherein said time t.sub.min is a minimum time and said number P is a minimum number of said taps that allows said transversal filter to generate said estimate of said intersymbol interference without diverging.

A method of cancelling intersymbol interference in a pulse amplitude modulated data communication system, comprising of the steps of:

• (a) extracting sign, magnitude and error information from a received signal at discrete intervals of time within a baud period (T);
• (b) caching said sign and magnitude information from previous baud periods in at least one tap;
• (c) modifying a tap output when said tap contains a maximum and minimum level of said received signal, said tap output modified by adding thereto a product of said error information and a modification step size (.DELTA.);
• and
• (d) multiplying said sign and magnitude information stored in said tap by said tap output.

A method of preventing an adaptive decision feedback equalizer having an N-tap transversal filter from diverging, comprising the steps of:

• (a) updating a tap output when said output contains a signal representing a maximum or minimum level;
• (b) enabling a minimum number (P) of said N taps on startup when a sum of uncancelled intersymbol interference in said signal and a voltage error of an incorrect intersymbol interference estimation in said signal yields less than 0.5 probability of an error;
• and
• (c) enabling a remaining (N-P) number of said N taps at a time (t.sub.min) for which convergence occurs after enabling said minimum number P of taps.

A method of reducing intersymbol interference in a received signal using an adaptive convergent decision feedback equalizer circuit comprising a plurality N of taps, the method comprising:

• (a) calculating a tap coefficient (HDFE.sub.i) for an ith tap of said N taps so that:
• HDFE.sub.i (T.sub.+1)=HDFE.sub.i (T)+M.sub.i
• wherein T is a baud period, T.sub.+1 is a subsequent baud period to T and M.sub.i is an incremental change;
• and
• (b) generating an estimate (ISI.sub.est) of said intersymbol interference so that:
• ##EQU3## wherein H.sub.o is a compensating reference signal for slicer level threshold variation.