Modified Log-MAP Algorithm for Simplified Decoding of Turbo and Turbo TCM Codes

A novel method for approximating the optimal max* operator in Log-MAP decoding of turbo and turbo trellis-coded modulation (TTCM) codes is presented. The max* operation is performed on n>2 rather than on only two inputs, as it is in the conventional approach. The approximation uses a very simple max operation and a look-up table (LUT) with only eight stored values. Complexity comparison reveals a significant reduction in the number of operations required per decoding step for the presented approximation, as compared with the conventional Log-MAP decoding. Performance evaluation results are presented for both turbo and TTCM codes, showing the near-optimal performance of the novel approach.