Practical aspects of iterative decoding

The convergence problem of iterative, block-mode, turbo decoders is discuss ed and the performance of a practical convergence criterion is presented. A fu;ed-point approach is used, whereby the saturation and stability charact eristics of the extrinsic-probability vector for each MAP decoder are deter mined by simulation and used to terminate iteration. If these vectors an sa turated and identical, or non-saturated and stable, the decoder has converg ed to a fu;ed point. The paper also examines the effect of interleaver desi gn and machine precision effects on convergence. Sometimes, finite precisio n can lead to a limit-cycle effect, and practical solutions are discussed. Once convergence has been established, it can also be used to determine wit h high confidence the effective d(free) of the decoder, even for large bloc k lengths.