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International Conference on Consumer Electronics, Communications and Networks (CECNet)

A time-saving algorithm for constructing QC-LDPC codes based on PEG algorithm

Ping Gong, Huang Liqun

Published Apr 1, 2011• 1 citation
Publisher

Abstract

We propose a new algorithm based on PEG algorithm to construct high-girth quasi-cyclic low-density parity check (QC-LDPC) codes. Before our improved algorithm in introduction, we depict the general algorithm based on PEG algorithm to construct QC - LDPC code, if necessary to eliminate the short cycles, that would bring in cycles-search. When code length of basic matrix is not too long, the algorithm is feasible, but with the code length growth, cycles-search will bring large of calculation and time-consuming sharply. Therefore we put forward an improved method by using the specific relation of basic matrix rows to replace traditional cycles-search, effectively eliminates short cycles, and largely reduces the time complexity at the same time. Through the simulation analysis, the performance can also be comparable with random constructed LDPC codes.

DOI
10.1109/cecnet.2011.5768233

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We propose a new algorithm based on PEG algorithm to construct high-girth quasi-cyclic low-density parity check (QC-LDPC) codes. Before our improved algorithm in introduction, we depict the general algorithm based on PEG algorithm to construct QC - LDPC code, if necessary to eliminate the short cycles, that would bring in cycles-search. When code length of basic matrix is not too long, the algorithm is feasible, but with the code length growth, cycles-search will bring large of calculation and time-consuming sharply. Therefore we put forward an improved method by using the specific relation of basic matrix rows to replace traditional cycles-search, effectively eliminates short cycles, and largely reduces the time complexity at the same time. Through the simulation analysis, the performance can also be comparable with random constructed LDPC codes.
Due to quasi-cyclic structure of parity-check matrices, Quasi-Cyclic Low-Density Parity-Check (QC-LDPC) codes reduce hardware requirements, and thus they have a higher practical value than random LDPC codes. This paper proposes an improved construction method of QC-LDPC based on PEG Algorithm, and the girth of the QC-LDPC codes has an increase of 2. Firstly, a base matrix is generated by the PEG algorithm, and then the base matrix is expanded using the unit circle permutation matrix, at the same time, the short loop of the base matrix is eliminated. This approach has the advantages of the PEG algorithm and the QC-LDPC codes, and simulation results show that bit error rate (BER) performance of the QC-LDPC codes presented in this paper is the same as or exceeds the one of randomly constructed LDPC codes.
Quasi-Cyclic Low-Density Parity-Check (QC-LDPC) codes have practical significance in the field of engineering, thus their construction algorithm is a hot issue in the field of LDPC codes. Based on existing construction methods, particularly the method based on Progressive Edge-Growth (PEG) algorithm, a new construction method of shift matrix is proposed. It can effectively avoid the copy of cycle structures, especially short cycles in base matrix after extension. Besides, it also relieves the time cost caused by random search, and greatly improves the length of cycles when compared with algorithms based on methods such as quadratic congruence and arithmetic sequence. Those algorithms can only eliminate short cycles with length 4, while our method can provide QC-LDPC codes without length 6 at least. Cycle analysis and simulation results show that the newly constructed QC-LDPC codes have good cycle structure and error correction performance.
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.
Based on cycle search algorithm and Progressive Edge Growth(PEG) algorithm,this paper proposes a method to construct quasi-cyclic Low-density Parity Check(LDPC) codes with big girth and linear-encoding complexity.A parity-check matrix with a form of approximate lower triangle structure is designed to achieve iterative encoding with linear-encoding complexity.PEG algorithm is used to construct the base matrix and the cycle search algorithm is used to optimize the cycle distribution.And a fast encoding method is proposed.Experimental results show that the LDPC codes constructed with the proposed method can avoid error floor and achieve better BER performance than Tanner-QC-LDPC codes and equivalent BER performance to the ones of random codes.
A modified construction method of LDPC (Low-density Parity-check) codes based on polynomial of cycles of PEG (Progressive-Edge-Growth) is proposed according to reaction on the performance of LDPC codes by polynomial of cycles based on PEG and connectivity of cycles. It can reduce the number of the shortest cycles, increase average local girth and improve connectivity of cycles while deleting short cycles by considering girth of check matrix, number of the shortest cycles and ACE (Approximate Cycle EMD), so that it can improve performance of decoding. At the same time, a construction method of QC-LDPC (quasi-cyclic-LDPC) codes based on ACE and polynomial of cycles of PEG is also proposed to deal with the complexity of implementation on hardware of unstructured codes. Simulation results show that the performance of this code is better than unstructured Mackay code and it is easier for implementation on hardware.
In this paper, we present an improvement of the PEG algorithm for constructing quasi-cyclic low-density parity-check (QC-LDPC) codes with large girth. We introduce the concept of PEG-undetectable cycles on the computation tree in the PEG algorithm for QC-LDPC codes and give a predictive method to avoid these undetected cycles. The aim is to select only the candidates that ensure the maximization of local girth for a code of girth g ≥ 10 and thus avoid a posteriori verification after the creation of a new edge i.e. keep the predictive philosophy of the PEG algorithm. The proposed method is applicable to both regular and irregular codes and also protograph type-I codes. Simulation results are presented to demonstrate the efficiency of our method in terms of minimum circulant permutation matrix pminand error performance.