Implementation of a soft-decision Viterbi codec

The effects of the threshold spacing and the number of quantization levels (Q) on the bit error probability of a soft-decision decoder are investigated. An upper bound on the bit error probability is computed as a function of the threshold spacing and Es/No for Q=4 and Q=8. Threshold spacing normalized to the noise-power density, and normalized to the signal energy per code symbol are considered. Degradation of bit error probability performance is not significant even if the threshold spacings differ considerably from the optimum values. A parallel realization of the decoder with eight quantization levels is chosen. The theoretical minimum decoding data rate is at least 4 Mbs. The bit error rate performance of the decoder is measured with a polar baseband channel. The code gain of the decoder is 3.4 dB for Q=8 and 3.1 dB for Q=4.