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Symposium on ICT and Energy Efficiency and Workshop on Information Theory and Security (CIICT)

Thresholds of precoded (3,6) LDPC codes under iterative and maximum-likelihood decodings

Huijuan Zheng, Sheng Tong

Published Jan 1, 2012• 0 citations

Abstract

The technique of precoding is recently applied to low-density parity-check (LDPC) codes by Divsalar et al., resulting in precoded LDPC codes. It is shown that precoding can improve not only the iterative decoding thresholds of regular LDPC codes but also their maximum-likelihood (ML) decoding thresholds. In this paper, we consider thresholds of precoded (3,6) LDPC codes under both iterative and ML decodings. For iterative decoding, the tool of EXIT charts is employed. With the EXIT chart, thresholds of precoded (3,6) LDPC codes with different precoding ratios are obtained. It is shown that the iterative decoding threshold is reduced with the increase of the precoding ratio until the precoding ratio reaches at 0.25. When the precoding ratio is greater than 0.25, the increase of precoding ratio may impair the iterative decoding threshold. Thus, the optimal precoding ratio under iterative decoding is revealed to be 0.25, i.e., half of the information bits are to be precoded. For ML decoding analysis, we first drive the asymptotic ensemble distance spectrum of precoded (3,6) LDPC codes. Using the derived asymptotic ensemble distance spectrum along with Divsalar's simple bound, we calculate the thresholds of precoded (3,6) LDPC codes with different precoding ratios under ML decoding. It is shown that the increase of precoding ratio always leads to an improved ML decoding threshold and thus the optimal ML decoding threshold is achieved at a precoding ratio of 0.5, i.e., all information bits are to be precoded.

DOI: 10.1049/cp.2012.1888

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The performance of maximum-likelihood (ML) decoding on the binary erasure channel for finite-length low-density parity-check (LDPC) codes from two random ensembles is studied. A tightened union-type upper bound on the ML decoding error probability based on the precise coefficients of the average weight spectrum is presented. For LDPC codes from the Gallager ensemble and the Richardson-Urbanke ensemble, new upper bounds on the ML decoding performance based on computing the rank of submatrices of the code parity-check matrix are derived. A new lower bound on the ML decoding threshold followed from the latter error probability bound is obtained. An improved lower bound on the error probability for codes with a known estimate on the minimum distance is presented as well. A new low-complexity near-ML decoding algorithm for quasi-cyclic LDPC codes is proposed and simulated. Its performance is compared to the simulated belief propagation and ML decoding performance and simulated performance of the best known improved iterative decoding techniques, as well as, with the derived upper bounds on the ML decoding performance and with decoding thresholds obtained by the density evolution technique.

In this paper, we derive an upper bound for the average block error probability of a standard irregular low-density parity-check (LDPC) code ensemble under the maximum-likelihood (ML) decoding. Moreover, we show that the upper bound asymptotically decreases polynomially with the code length. Furthermore, when we consider several regular LDPC code ensembles as special cases of standard irregular ones over an additive white Gaussian noise channel, we numerically show that the signal-to-noise ratio (SNR) thresholds at which the proposed bound converges to zero as the code length tends to infinity are smaller than those for a bound provided by Miller et al. We also give an example of a standard irregular LDPC code ensemble which has a lower SNR threshold than a given regular LDPC code ensemble.

Maximum-likelihood LDPC decoder analysis. In Chapter 8, the performance of LDPC codes under ML decoding is analyzed. ML decoding is intended here either as the block-wise or the symbol-wise decoding criterion (see Section 2.2). More specifically, the asymptotic analysis on the ML decoding threshold addresses the performance in terms of symbol-wise ML decoding, whereas finite-length bounds are provided for the block error probability under block-wise ML decoding. While the focus is on unstructured LDPC code ensembles, the results in this chapter can be considered to a large extent valid for other LDPC code ensembles.

This paper investigates decoding of low-density parity-check (LDPC) codes over the binary erasure channel (BEC). We study the iterative and maximum-likelihood (ML) decoding of LDPC codes on this channel. We derive bounds on the ML decoding of LDPC codes on the BEC. We then present an improved decoding algorithm. The proposed algorithm has almost the same complexity as the standard iterative decoding. However, it has better performance. Simulations show that we can decrease the error rate by several orders of magnitude using the proposed algorithm. We also provide some graph-theoretic properties of different decoding algorithms of LDPC codes over the BEC which we think are useful to better understand the LDPC decoding methods, in particular, for finite-length codes.

In modern communication systems Low-Density Parity Check (LDPC) codes are widely used. It is because of their performance under iterative decoding, hardware-friendly structure and well-known construction methods. For one to construct good LDPC code Density Evolution (DE) can be used, it estimates the decoding threshold, that is it estimates performance of the code. LDPC codes can be further extended to Generalized LDPC (GLDPC) codes, where simple SPC code is replaced with some component code. With more complex structure common DE is no longer correct. This letter presents the upper bound for decoding error and, therefore, an upper bound of a iterative threshold (in dB). It will be shown that the Gaussian approximation gives an upper bound for a decoding error. As one of the results it means that EXIT-charts analysis also gives an upper bound for an iterative threshold for a GLDPC code.

The Gallager ensembles of binary regular LDPC codes and binary images of nonbinary regular LDPC codes are studied. Recurrent procedure for computing average spectra for these two ensembles is presented. By using the existing bounding techniques, estimates on the error probability of the maximum-likelihood (ML) decoding over an AWGN channel with BPSK signaling for short codes from different ensembles of LDPC codes are obtained. The numerical results show performance of the ML decoding for different code ensembles. Conclusions drawn based on the average code spectra are then verified by near-ML decoding simulations for both randomly selected and the best known short codes. The asymptotic ML decoding thresholds for AWGN and BSC channels are calculated. As expected, codes with the ML decoding performance superior to that of the average code in the ensemble, are easy to find. However, comparison of the the presented results with simulation results for belief propagation (BP) decoding shows that the ML decoding performance should not be used as a target for searching for good iteratively decodable codes.