Low complexity turbo equalization for high data rate wireless communications

Abstract

Soft interference cancellers (SICs) have been proposed in the literature as a means for reducing the computational complexity of the so-calledturbo equalization receiver architecture. Soft-input-soft output (SISO)equalization algorithms based on linear filters have a tremendous complexity advantage over trellis-diagram-based SISO equalizers, especially forhigh-order modulations and long-delay spread frequency selective channels. In this paper, we modify the way in which the SIC incorporates softinformation. In existing literature the input to the cancellation filter isthe expectation of the symbols based solely on the apriori probabilitiescoming from the decoder, whereas here we propose to use the conditionalexpectation of those symbols, given both the apriori probabilities andthe received sequence. This modification results in performance gains atthe expense of increased computational complexity, as compared to previous SIC-based schemes. However, by introducing an approximation tothe aforementioned algorithm a linear complexity SISO equalizer can bederived. Simulation results for an 8-PSK constellation and hostile radio channels have shown the effectiveness of the proposed algorithms inmitigating the intersymbol interference (ISI).