Detection processes for V32 modems using trellis coding

Abstract

A commonly proposed receiver structure for modems using trellis coding uses an adaptive linear equaliser, to remove intersymbol interference resulting from channel distortion, and a Viterbi decoder to decode the equalised signal. Modifications are proposed to a detection process based on nonlinear (or decision feedback) equalisation, and to reduced state maximum likelihood (RSML) detectors, to allow them to detect trellis encoded signals. For channels causing moderate to high levels of distortion, it was found that detectors using nonlinear equalisation with an uncoded signal gave a better performance than detectors using either linear or nonlinear equalisation with a trellis encoded signal. However, the RSML detectors operating on trellis encoded signals, gave a useful improvement in performance over RSML detectors operating on uncoded signals, for all channels tested.