Northern Telecom Limited

Babak Sayar

Sami A Aly

A receiver for a digital data transmission system comprises a sampling device for sampling digital data signals received from a subscriber loop. A precursor equalizer connected to the output of the sampling device equalizes, in effect filters, the sampled digital data signal. A timing recovery circuit connected to the output of the precursor equalizer controls the sampling device in dependence upon the filtered digital data signal. The precursor equalizer has a difference equation such that the equalized digital data signal has at least one zero-crossing occurring substantially one baud before the main cursor of each pulse. For twisted wire pair subscriber loops, the precursor equalizer may have a different equation: ##EQU1## which produces a single precursor zero-crossing. Alternatively, the precursor equalizer may have a difference equation: ##EQU2## which produces two precursor zero crossings, y(n) being the input to the filter, w(n) its output, h.sub.0 the main cursor height before equalization and h.sub.-1 the precursor height before equalization. The latter precursor equalizer may be modified by multiplying each of the coefficients (h.sub.-1 /h.sub.0).sup.2 and h.sub.-1 /h.sub.0 by a factor ##EQU3## for more precise determination of the zero crossings. A (1-Z.sup.-1) filter may be provided ahead of the precursor equalizer.

Equalizer for ISDN-U interface

In a receiver for a loop interface of a digital data transmission system, a received four-level substantially randomized digital signal is sampled and a multilevel slicer compares the samples selectively with a plurality of thresholds to extract the symbols. A timing recovery circuit determines the sampling instant as a function of a precursor sample and a main pulse sample one baud apart, according to the timing function:

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Ottawa, Canada

Babak Sayar