Constrained SOVA decoding in concatenated codes
In practical communication systems, error detection and correction codes such as cyclic redundancy check (CRC) codes and turbo codes (TCs) are almost always concatenated. While TCs are used to remove errors, CRC codes may be applied to applications such as ARQ, early stop of turbo decoding. Considering the scenario that a large data frame for TC consists of multiple short packets, each with CRC parity bits, we propose a constrained-SOVA (C-SOVA) algorithm to fully take advantage of the interim CRC detection results to redesign the TC decoding algorithm. One more interleaver is added at the encoder so that bits in a correctly decoded packet are permuted to the entire range of frame. At the decoder, the C-SOVA has been designed that includes constraining trellis, updating the extrinsic information, and reducing the number of tracing back. As a result, not only a the decoding complexity is significantly reduced, but a high coding gain has been preserved due to elimination of certain error patterns as well. Simulations have been presented to demonstrate the excellent features of the proposed scheme. Although SOVA algorithm and concatenated codes are considered in this paper, the proposed idea using constraints is applicable to any trellis-based decoding algorithms including maximum a posterior probability (MAP) algorithm, and to any scenarios that partial bits are known correct in the course of turbo decoding.