Soft-decision demodulation design for COVQ over white, colored, and ISI Gaussian channels

In this work, the design of a q-bit (scalar and vector) soft-decision demod ulator for Gaussian channels with binary phase-shift keying modulation is i nvestigated. The demodulator is used in conjunction with a soft-decision ch annel-optimized vector quantization (COVQ) system. The COVQ is constructed for an expanded (q > 1) discrete channel consisting of the concatenation of the modulator, the Gaussian channel, and the demodulator. It is found that as the demodulator resolution q increases, the capacity of the expanded ch annel increases, resulting in an improvement of the COVQ performance. Conse quently, the soft-decision demodulator is designed to maximize the capacity of the expanded channel. Three Gaussian channel models are considered as f ollows: 1) additive white Gaussian noise channels; 2) additive colored Gaus sian noise channels; and 3) Gaussian channels with intersymbol interference . Comparisons are made with a) hard-decision COVQ systems, b) COVQ systems which utilize interleaving, and c) an unquantized (q = infinity) soft-decis ion decoder proposed by Skoglund and Hedelin. It is shown that substantial improvements can be achieved over COVQ systems which utilize hard-decision demodulation and/or channel interleaving. The performance of the proposed C OVQ system is comparable with the system by Skoglund and Hedelin-though its computational complexity is substantially less.