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IEEE Military Communications Conference (MILCOM)

Short Low-Error-Floor Tanner Codes with Hamming Nodes

Gianluigi Liva, W.E. Ryan

Published Mar 22, 2006• 29 citations
Publisher

Abstract

While it is fairly easy to design good low-density parity-check codes with medium or large block lengths, and with code rates 1/2 or greater, much more effort is required for short lengths and low rates. We propose a code structure which incorporates Hamming codes into a code's graph, leading to a type of a Tanner code. The incorporation of Hamming codes into the graph, which we call "Hamming code doping", tends to lead to larger minimum distances and hence low error-rate floors. We present an iterative decoding algorithm tailored to graphs possessing "Hamming nodes". Further, a density evolution analysis based on the Gaussian approximation is presented, which is a useful tool for finding good codes. Finally, we present numerical results of some Hamming-doped Tanner codes simulated on the AWGN channel. The simulated codes exhibit remarkably low error floors, while simultaneously displaying good decoding thresholds

DOI
10.1109/milcom.2005.1605687

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In this paper, a novel methodology for designing structured generalized LDPC (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call LDPC code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel.

IEEE Transactions on Communications•Jan 1, 2008•136 citations
While it is fairly easy to design good low-density parity-check codes with medium or large block lengths, and with code rates 1/2 or greater, much more effort is required for short lengths and low rates. We propose a code structure which incorporates Hamming codes into a code's graph, leading to a type of a Tanner code. The incorporation of Hamming codes into the graph, which we call "Hamming code doping", tends to lead to larger minimum distances and hence low error-rate floors. We present an iterative decoding algorithm tailored to graphs possessing "Hamming nodes". Further, a density evolution analysis based on the Gaussian approximation is presented, which is a useful tool for finding good codes. Finally, we present numerical results of some Hamming-doped Tanner codes simulated on the AWGN channel. The simulated codes exhibit remarkably low error floors, while simultaneously displaying good decoding thresholds
We study the design of structured Tanner codes with low error-rate floors on the AWGN channel. The design technique involves the “doping” of standard LDPC (proto-)graphs, by which we mean Hamming or recursive systematic convolutional (RSC) code constraints are used together with single-parity-check (SPC) constraints to construct a code’s protograph. We show that the doping of a “good” graph with Hamming or RSC codes is a pragmatic approach that frequently results in a code with a good threshold and very low error-rate floor. We focus on low-rate Tanner codes, in part because the design of low-rate, low-floor LDPC codes is particularly difficult. Lastly, we perform a simple complexity analysis of our Tanner codes and examine the performance of lower-complexity, suboptimal Hamming-node decoders.
We study the design of structured Tanner codes with low error-rate floors on the binary additive white Gaussian noise channel (BAWGNC) and the binary erasure channel (BEC). The design technique involves the “doping of standard LDPC protographs, meaning Hamming or recursive systematic convolutional (RSC) code constraints are used together with singleparity-check (SPC) constraints to construct a code’s protograph. We show that the doping of a “good graph with Hamming or RSC codes is a pragmatic approach that frequently results in a code with a good threshold and very low error-rate floor. We focus on low-rate Tanner codes, in part because the design of low-rate, low-floor LDPC codes is particularly difficult.
Gallager proposed and developed low density parity check (LDPC) codes in the early 1960s. LDPC codes were rediscovered in the early 1990s and shown to be capacity approaching over the additive white Gaussian noise (AWGN) channel. Subsequently, density evolution (DE) optimized symbol node degree distributions were used to significantly improve the decoding performance of short to medium length irregular LDPC codes. Currently, the short to medium length LDPC codes with the lowest error floor are DE optimized irregular LDPC codes constructed using progressive edge growth (PEG) algorithm modifications which are designed to increase the approximate cycle extrinsic message degrees (ACE) in the LDPC code graphs constructed. The aim of the present work is to find efficient means to improve on the error floor performance published for short to medium length irregular LDPC codes over AWGN channels in the literature. An efficient algorithm for determining the girth and ACE distributions in short to medium length LDPC code Tanner graphs has been proposed. A cyclic PEG (CPEG) algorithm which uses an edge connections sequence that results in LDPC codes with improved girth and ACE distributions is presented. LDPC codes with DE optimized/’good’ degree distributions which have larger minimum distances and stopping distances than previously published for LDPC codes of similar length and rate have been found. It is shown that increasing the minimum distance of LDPC codes lowers their error floor performance over AWGN channels; however, there are threshold minimum distances values above which there is no further lowering of the error floor performance. A minimum local girth (edge skipping) (MLG (ES)) PEG algorithm is presented; the algorithm controls the minimum local girth (global girth) connected in the Tanner graphs of LDPC codes constructed by forfeiting some edge connections. A technique for constructing optimal low correlated edge density (OED) LDPC codes based on modified DE optimized symbol node degree distributions and the MLG (ES) PEG algorithm modification is presented. OED rate-½ (n, k)=(512, 256) LDPC codes have been shown to have lower error floor over the AWGN channel than previously published for LDPC codes of similar length and rate. Similarly, consequent to an improved symbol node degree distribution, rate ½ (n, k)=(1024, 512) LDPC codes have been shown to have lower error floor over the AWGN channel than previously published for LDPC codes of similar length and rate. An improved BP/SPA (IBP/SPA) decoder, obtained by making two simple modifications to the standard BP/SPA decoder, has been shown to result in an unprecedented generalized improvement in the performance of short to medium length irregular LDPC codes under iterative message passing decoding. The superiority of the Slepian Wolf distributed source coding model over other distributed source coding models based on LDPC codes has been shown.
In this paper, a novel methodology for designing structured generalized low-density parity-check (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call low-density parity-check (LDPC) code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel.
Tanner codes represent a broad class of graph-based coding schemes, including low-density parity-check (LDPC) and turbo codes. Whereas many different classes of LDPC and turbo codes have been proposed and studied in the past decade, very little work has been performed on the broader class of Tanner codes. In this paper we propose a design technique which leads to efficiently encodable quasi-cyclic Tanner codes based on both Hamming and single parity check (SPC) nodes. These codes are characterized by fast message-passing decoders and can be encoded using shift-register-based circuits. The resulting schemes exhibit excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel. 1
In this paper, a novel methodology for designing structured generalized LDPC (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call LDPC code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel.