A catastrophic error mode in adaptive predictive DIR equalisation of dynamic channels

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. Also OFDM systems often require channel truncation. It has previously 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 catastrophic error mode is observed in the operation of adaptive DIR systems in slowly varying channels. This error mode causes the equaliser to become severely maladjusted. The system cannot spontaneously recover from this situation. This paper reviews the switched DIR equalisation system and explains the mechanism underlying the error mode in the adaptive DIR system. Simulation results are presented that indicate the efficacy of the dual switched DIR MLSE-VA to operate successfully in comparison to adaptive DIR MLSE-VA systems in time varying frequency selective Rayleigh fading channels.