Bootstrap sequential decoding at high spectral efficiencies

Sequential decoding of large memory codes has been investigated for 8-PSK, 16-QAM, and larger constellations. The results show that high reliability can be achieved at channel signal-to-noise ratios (SNRs) where the code rate is nearly equal to R/sub 0/, yielding 1-1.5 dB coding gain over Viterbi decoding of small memory codes. Since bootstrap hybrid decoding (BHD) is known to improve the cut-off rate of a sequential decoder for binary modulation, further improvements can be expected using this method applied to TCM. We present a lower bound to the computational cut-off rate for the extension of the BHD scheme to TCM. Our analysis is based on the original derivation for the case of binary modulation proposed by Jelinek and Cocke (1971). Numerical evaluations of the expressions obtained for the cases of hard-decision 8-PSK and 8-level quantized 4AM modulation show significant improvements over the usual cut-off rate, similar to the results obtained by Jelinek et al. for BPSK modulation. These results suggest that performance close to capacity can be achieved with sufficiently powerful TCM systems and bootstrap sequential decoding.