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Proceedings of the IEICE Engineering Sciences Society/NOLTA Society Conference

A Study on Removing Four-Cycle in Tanner Graph for Convolutional Codes(2)

Maekawa Naoki, Toshiyuki Shohon

Published Aug 25, 2015• 0 citations
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In this paper, time-invariant spatially coupled low-density parity-check convolutional codes (SC-LDPC-CCs) are considered, and the connections existing between their low-weight codewords and cycles in their Tanner graphs are studied. Using the polynomial representation of these codes, we show that parity-check matrices having columns with weight ≥2 can be analyzed considering a certain number of parity-check sub-matrices having regular columns with weight 2. These sub-matrices are associated to cycles in the code Tanner graph and define as many codes we denote as component codes. Based on this observation, we find that codewords of the main code can be expressed as combinations of codewords of the component codes. The design of codes free of codewords up to a certain weight is also addressed. We show that low-weight codewords in the main code can be avoided by removing some types of cycles in its Tanner graph. Our design approach is applied to some well-known ensembles of SC-LDPC-CCs to prove its effectiveness.
This article proposes two methods to improve the sum-product soft-in/soft-out decoding performance of convolutional codes. The first method is to transform a parity check equation in such a way as to remove cycles of length four in a Tanner graph of a convolutional code, and performs sum-product algorithm (SPA) with the transformed parity check equation. This method improves the performance of (7,5)8convolutional code (CC1). However, for (45,73)8convolutional code (CC2), the method does not effect. The second proposed method is to use a higher order parity check equation in comparison with a normal parity check equation for SPA decoding. This method improves the performance for both convolutional codes (CC1, CC2). The performance is close to that by BCJR algorithm and it is less complex than BCJR algorithm.