Overlapping look-up-and-add echo canceller requiring a smaller memory

Abstract

An echo canceller which incorporates an overlapping look-up-and-add structure is suitable for cancelling both the linear and nonlinear components of an echo. In the well-known overlapping look-up-and-add structure, the cancellation provided is the sum of a plurality of memory outputs wherein each memory is addressed by an associated transmitted symbol in a sequence of symbols along with one symbol immediately preceding and/or one symbol immediately succeeding each associated symbol. In accordance with the present invention, this structure is modified so that each memory is addressed by the plurality of bits representative of an associated symbol in the sequence of symbols along with less than all of the bits representative of one or more symbols adjacent to the associated symbol in the sequence. This modification, which may be incorporated in a fixed or adaptive echo canceller, substantially reduces the required memory size and, in the case of an adaptive canceller, provides a corresponding decrease in the convergence time. In the preferred embodiment, each memory is addressed by its associated symbol and only the sign bit of immediately preceding and/or immediately succeeding symbols.