Transactions Papers Convergence Behavior of Iteratively Decoded Parallel Concatenated Codes

Abstract—Mutual information transfer characteristics of soft in/soft out decoders are proposed as a tool to better understand the convergence behavior of iterative decoding schemes. The exchange of extrinsic information is visualized as a decoding trajectory in the extrinsic information transfer chart (EXIT chart). This allows the prediction of turbo cliff position and bit error rate after an ar-bitrary number of iterations. The influence of code memory, code polynomials as well as different constituent codes on the conver-gence behavior is studied for parallel concatenated codes. A code search based on the EXIT chart technique has been performed yielding new recursive systematic convolutional constituent codes exhibiting turbo cliffs at lower signal-to-noise ratios than attain-able by previously known constituent codes. Index Terms—Convergence, iterative decoding, mutual informa-tion, turbo codes. I