Viterbi receivers in the presence of severe intersymbol interference

A method of reducing the number of states in the Viterbi receiver, called PDFE (parallel decision feedback estimation), is described. Four different ISI (intersymbol interference) removal systems and two different TCM (trellis-coded modulation) systems using PDFE are presented and simulated. The simulated channel is a telephone cable operating at high bit rates and causing severe ISI. Gains of between 0.6 and 2.3 dB have been achieved in this severe ISI environment using Viterbi receivers with various proportions of four or 16 states devoted to either ISI mitigation or TCM decoding. With no ISI, the gain of four-state TCM is 3.1 dB and the gain of 16-state TCM is 4.0 dB. The gains of TCM received using PDFE with the severe ISI HDSL (high-rate digital subscriber line) channel are about one half of the gains with no ISI