Adaptive convergent decision feedback equalizer

DETAILED DESCRIPTION The aforementioned ISI estimation divergence problem with prior adaptive decision feedback equalizers is obviated in accordance with the present invention by taking two steps to reduce the probability of error propagation.
A probability density function (PDF) for ISI error is calculated by convolving a pulse response for the expected worst case communication channel such as the American National Standard Institute (ANSI) T1.
601-1988 loop #3 with a pseudo-random data sequence.
The probability P(ISI) of decoding an error due to ISI is obtained from the PDF by taking the integral or area under the curve for voltages greater than the slicer level.
The probability of error in a communication channel having properties of a T1.
601-1988 loop #3 is 0.
67 given a slicer level H.
sub.
o for the ISDN U-interface receiver and assuming all filter coefficients are modified irrespective of the level of the data contained in the tap.
The DFE will ultimately diverge unless P(ISI) is reduced to less than 0.
5.
In the general case with a receiver using a line code having a number of levels (R), the probability of an updating error is always one half the expected P(ISI) if coefficient modifications are only performed on taps containing a received signal representing the maximum or minimum levels since these levels are not bounded by decision thresholds (slicer levels) above and below respectively.
Alternatively stated, the ISI can exceed the slicer level in these cases without registering data errors.
Accordingly, the present invention performs a coefficient modification only when the received signal in a particular filter tap is a maximum or minimum level of the selected line code.
In an ISDN U-interface having 2B1Q encoded data, coefficients are modified only on taps containing received signals representing quaternary levels +3 or -3 since no extreme bounds on the slicer level exist.
The probability of error is one-half the probability when all coefficients are modified, namely, P(ISI)=0.
335