Trellis coded modulation based on time-varying mapping and encoders for utilizing a channel intersymbol interference

Abstract

This paper proposes and investigates a trellis coded modulation using time-varying mapping and encoders which is very useful to increase minimum Euclidean free distance(d _free ) and obtain large coding gain by intentionally utilizing intersymbol interference(ISI) in a channel. If M-ary modulation signals are transmitted in a channel with ISI, the received signals will be mapped into more than M signal points due to ISI,i.e. a channel memory. In the presence of ISI, optimum allocation of distorted signal points due to ISI is derived so as to maximize Euclidean distance in the signal points at every instant considering the trellis diagram. In order to maintain the allocation in the state transition, a method to utilize time-varying mapping and encoders is described. In particular, the coded 4PSK using the time-varying mapping and encoders with a code rate 1/2 in the presence of ISI, 1+1/2D can achieve much larger d _free and lower BER than the Ungerboeck's coded 4PSK using the time-invariant mapping and encoder in a channel without ISI, providing that their decoding complexity is equivalent. The improvement can increase as a constraint length of the encoder.