DSD (Double Soft Decision) Concatenated FEC Scheme in Mobile Satellite Communication Systems

In order to realize a higher-code-gain forward error correction scheme in mobile satellite communication systems, a novel concatenated coding scheme employing soft decision decoding for not only inner codes but also outer codes (DSD (Double Soft Decision) concatenated forward error correction scheme) is proposed. Soft-decision outer decoding can improve the bit error probability of inner decoded data, which traditionally cannot be sufficiently corrected in fading channels. In this scheme, likelihood information from an inner Viterbi decoder is used in the decoding of outer codes. A technique using the path memory circuit status 1.0 ratio for likelihood information is newly proposed, and it is shown that this method is the most reliable even though it requires the simplest hardware among the alternative likelihood information extracting methods. A computer simulation clarifies that the proposed DSD scheme improves Pe performance to one-third that of the conventional hard-decision outer decoding. More-over, to reduce the interleaving delay time in fading channel or inner decoded data of concatenated codes, a parallel forward error correction scheme is proposed.