Switched Viterbi equalisation of dynamic channels and a degenerative error mode

Abstract

The use of a linear equaliser to shorten the impulse response of a channel to a shorter desired impulse response (DIR) prior to a Viterbi algorithm (VA) detection of the data is a well-known technique. It has been shown that, for dynamic channels, it is beneficial to operate a number of equalisers in parallel, each targeted on a different DIR. Each equaliser is followed by an individual VA. A simple decision rule is used to select which of the VA outputs is presented as the actual decision. We refer to this strategy as switched DIR equalisation. A degenerate mode is occasionally observed in the operation of this system that causes the equalisers to become severely misadjusted. The mean squared error becomes small but the bit error rate becomes very large. The system can not spontaneously recover from this situation. This paper reviews the switched DIR equalisation system, explains the mechanism underlying the degeneracy and proposes a method to avoid it. Simulation results are presented which verify the efficacy of the method.