Technique for minimizing decision feedback equalizer wordlength in the presence of a DC component

Abstract

A system for reducing the complexity of an adaptive decision feedback equalizer, for use in connection with a dual-mode QAM/VSB receiver system is disclosed. QAM and VSB symbols, which are expressed in two's compliment notation, include an extra bit required to compensate for a fixed offset term introduced by the two's compliment numbering system. A decision feedback equalizer includes a decision feedback filter section which operates on symbolic decisions represented by a wordlength which excludes the added bit representing the offset. The vestigal word is convolved with the decision feedback filter's coefficients, while a DC component, corresponding to the excluded bit, is convolved with the same coefficient values in a correction filter. The two values are summed to provide an ISI compensation signal at the input of a decision device such as a slicer. A DC component representing a pilot tone in VSB transmission systems also introduces a DC component, and additional bits, to a VSB wordlength. These additional bits are similarly excluded and the vestigal representation convolved with coefficient values in a decision feedback filter. The DC component, including the pilot tone representation, is convolved with the same coefficient values in a correction filter.

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