Practical aspects of iterative decoding

The convergence problem of iterative, block-mode turbo decoders is discussed and the performance of a practical convergence criterion is presented. A fixed-point approach is used, whereby the saturation and stability characteristics of the extrinsic-probability vector for each MAP decoder are determined by simulation and used to terminate iteration. If these vectors are saturated and identical, or non-saturated and stable, the decoder has converged to a fixed point. The paper also examines the effect of interleaver design and machine precision effects on convergence. Sometimes, finite precision can lead to a limit-cycle effect, and practical solutions are discussed. Once convergence has been established, it can also be used to determine with high confidence the effective d_free of the decoder, even for large block lengths