Concatenated convolutional codes for digital communications

Three systems using a concatenation of two short convolutional codes in place of a single long convolutional code are proposed. Specifically, the systems use a R=2/3 with K=3 code, and a R=3/4 with K=2 code. While the first two systems employ a soft decision followed by a hard decision Viterbi decoder, the third system employs two soft decision Viterbi decoders. The performances of the proposed systems are better than the baseline system with a long convolutional code, R=1/2 with K=7, by 0.6 db to 1.5 db with the same bandwidth requirement. However, the most attractive feature of these systems is the reduction in the complexity of the decoders, and hence the entire system, at the expense of increased latency from interleaving and de-interleaving. The system complexities vary from 25% to 50% that of the baseline system. An additional Reed-Solomon outer code corrects the burst errors of the above systems. In addition, using OQPSK instead of QPSK reduces the susceptibility of the signal to spectral regrowth caused by amplifier nonlinearity