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Asilomar Conference on Signals, Systems and Computers (ACSSC)

A class of good quasi-cyclic low-density parity check codes based on progressive edge growth graph

Zongwang Li, B. V. K. Vijaya Kumar

Published Mar 31, 2005• 143 citations
Publisher

Abstract

We present a new class of quasi-cyclic low density parity check (QC-LDPC) codes, whose quasi-cyclic nature makes them attractive for implementation. Both regular and irregular QC-LDPC codes are designed by modifying the progressive edge growth (PEG) graph with a quasi-cyclic constraint. Simulations show that these QC-LDPC codes offer hardware-friendly parity check matrices and have as good error correction performance as random LDPC codes and other good QC-LDPC codes. In addition, the proposed QC-LDPC codes offer a much more flexible set of parameters compared to the traditional designs.

DOI
10.1109/acssc.2004.1399513

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We present a new class of quasi-cyclic low density parity check (QC-LDPC) codes, whose quasi-cyclic nature makes them attractive for implementation. Both regular and irregular QC-LDPC codes are designed by modifying the progressive edge growth (PEG) graph with a quasi-cyclic constraint. Simulations show that these QC-LDPC codes offer hardware-friendly parity check matrices and have as good error correction performance as random LDPC codes and other good QC-LDPC codes. In addition, the proposed QC-LDPC codes offer a much more flexible set of parameters compared to the traditional designs.
A novel construction for quasi-cyclic (QC) regular and irregular low-density parity-check (LDPC) codes based on a modification of the QC Progressive Edge Growth (PEG) algorithm is presented. Edge placement of the PEG-based algorithm is enhanced by use of the sum-product algorithm in the design of the parity-check matrix. The proposed algorithm is highly flexible in block length and rate, in particular when compared with algebraic constructions. The codes constructed by the proposed methods are tested in the AWGN channel and performance improvements are achieved. The proposed QC-LDPC codes provide an inherent trade-off between code performance and encoding/decoding complexity.
We propose a general method for constructing regular and irregular quasi-cyclic LDPC codes based on the modified PEG (progressive edge growth) graph. The idea of permutation vector is introduced. Through permutation vectors, the QC-LDPC codes based on arbitrary circulant permutation matrices, which are not limited to identity matrix, Q-matrix or D-matrix, can be generated with flexible parameters such as block length, code rate, degree distribution. Simulation results demonstrate that QC-LDPC codes using the new PEG algorithm show an error correction performance comparable to the random LDPC codes based on the traditional PEG algorithm. In addition, the PEG QC-LDPC codes have more hardware-friendly parity check matrices than the random PEG LDPC codes and can fulfill linear-time encoding. The QC-LDPC codes based on D-matrix and Q-matrix are more suitable to be used in QC-LDPC codes than identity matrix because they outperform that of identity matrix by 0.1-0.2 dB in the new PEG algorithm.
The Progressive edge-growth (PEG) algorithm was proven to be a simple and effective approach in designing Low-density parity-check (LDPC) codes. However, the Tanner graph constructed with the PEG algorithm is non-structured, which leads to random distributions of the positions of 1's in the corresponding parity-check matrix. In this paper, an improved algorithm based on the PEG algorithm for constructing quasi-cyclic (QC) LDPC codes is proposed. The proposed algorithm can eliminate short cycles efficiently and gain low error-floors. What is more important, the proposed QC-LDPC codes construction method is hardware-friendly because of its quasi-cyclic structure. Our simulation results demonstrate that the codes constructed with the proposed algorithm has better performance than the PEG algorithm over the additive white Gaussian noise (AWGN) channel and Rayleigh flat fading channel.
Progressive Edge-Growth(PEG) algorithm is a method for constructing Low Density Parity-Check(LDPC) codes based on Tanner graph.Studies show that LDPC codes constructed with the algorithm can achieve excellent error-correcting performance.Based on the algorithm,the Quasi-Cyclic(QC) LDPC codes are proposed in this paper and simulations show that the QC-LDPC codes have almost the same excellent performance as the random codes constructed with the PEG algorithm.In addition,these codes have the property of quasi-cyclic structure,and the encoding and decoding can be implemented with simple shift registers.Besides,the code rates of the LDPC codes constructed with the proposed algorithm are more flexible to be adjusted.
Short cycles in the Tanner graph of a given LDPC code has negative effect on the performance of iterative decoding. To improve the performance, we presented an approach for constructing Quasi-Cyclic Low-Density Parity-Check (QC-LDPC) codes whose associated Tanner graph has large girth and a small amount of short cycles. Each column block of the parity check matrix was designed by adding circulant permutation sub-matrices one after another, provided the current parity check matrix has minimal number of short cycles. The condition for the proposed QC-LDPC codes to have girth at least 6 was also analyzed. Regular and irregular QC-LDPC codes with various lengths and rates can be constructed. Simulations show that the proposed QC-LDPC codes have less short cycles and better performance than existing progressive edge growth based QC-LDPC codes.
In order to implement efficient encoding,a class of nonbinary quasi-cyclic low-density parity-check(QC-LDPC) codes is proposed,which can be encoded with Richardson-Urbanke algorithm.The right part of the parity-check matrix has weight-2 columns,which allows the construction of both regular and irregular codes.Constraints on the parity-check matrix ensure a linear encoding complexity. Simulation results show that the proposed codes,when combined with higher order modulation,perform favorably with the codes constructed by the progressive-edge-growth(PEG) constructed code and close to the Shannon limits.