Iterative Decoding Convergence and Termination of Serially Concatenated Codes

In this paper, we demonstrate that the previously proposed arrangements of serially concatenated codes have extrinsic-information-transfer (EXIT) functions that intersect each other at points that are near but not at the (1, 1) point in the top-right-hand corner of the EXIT chart, which is typically associated with elevated error floors. We propose a novel arrangement having EXIT functions that do not intersect before the (1, 1) point, which is typically associated with approaching the maximum-likelihood (ML) bit-error-ratio (BER) performance. Our method employs an inner recursive code that is terminated using specifically designed termination sequences, which have a minimum Hamming distance of at least two between each other. Additionally, we provide optimal termination sequences for a range of inner code designs. Finally, we demonstrate that our novel approach can facilitate useful BER reductions in the challenging application scenario when employing short frame lengths on the order of 100 bits, which are typical in wireless sensor networks, for example.