Turbo-equalization for bandwidth-efficient digital communications over frequency-selective channels

This thesis deals with the problem of combining equalization with decoding for high data ratebandwidth-efficient transmissions over frequency-selective channels. Specifically, we focus on Turbo-Equalization which establishes an exchange of soft information between the equalizer and the decoderin an iterative manner. We first consider MAP turbo-equalizers which rely on trellis-based equalizersoptimum in the sense of minimizing the symbol error probability. We show that these receivers achievesignificant performance gains over the conventional approach where equalization and decoding areperformed separately. However, the complexity of MAP turbo-equalizers becomes rapidly prohibitivefor transmissions with high-order modulations over long delay spread channels. Hence, we then inves-tigate reduced-complexity turbo-equalizers relying on filtering-based equalizers optimized accordingto the MMSE criterion. These equalizers share the common property of accounting explicitly forthe presence of a priori information in the computation of the optimum filters coefficients and thusoutperform classical MMSE equalizers. Our analysis shows that MMSE turbo-equalizers offer an at-tractive alternative to MAP turbo-equalizers for multilevel coded transmissions over long delay spreadfrequency-selective channels. We finally describe the implementation of an MMSE turbo-equalizeron the low-cost fixed-point TMS320VC5509 DSP device which is typically targeted at portable ter-minals. Using only C programming, a promising data rate of 42 Kbits/s has been achieved with 5iterations. This result demonstrates the possibility of realizing such receivers in practice with currenthardware capabilities.