VHDL implementation of a turbo decoder with log-MAP-based iterative decoding

Turbo code is one of the most significant achievements in coding theory during the last decade. By concatenating two simple convolutional codes in parallel, it has been shown that transmission systems employing turbo codes could offer near-capacity performance. More importantly, by employing a suboptimal iterative decoding structure with soft-in/soft-out (SISO) maximum a posteriori-probability (APP) decoding algorithm, the near-capacity performance is achievable at a feasible decoding complexity. Given the outstanding performance of turbo code, the challenge now is to implement it into various communication systems at affordable decoding complexity using current very large scale integration (VLSI) technologies. In this paper, we first investigated the existing four different turbo decoding algorithms. Comparisons of both their performances and implementation complexities were performed. Log-maximum a posteriori (MAP) -based turbo decoding was found to offer the best performance-complexity compromise. A register-transfer-level (RTL) 12-bit fixed-point turbo decoder based on Log-MAP algorithm was then designed and simulated using VHDL as the hardware description language. The implemented RTL model was verified by comparing its performances with those obtained from a C-language implementation of the same turbo decoder.