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Chinese Journal of Electronics

Construction of Type‐II QC LDPC Codes Based on Perfect Cyclic Difference Set

Lijun Zhang, Bing Li, L. L. Cheng

Published Jan 1, 2015• 10 citations
Publisher

Abstract

Quasi-cyclic (QC) Low-density paritycheck (LDPC) codes are constructed from combination of weight-0 (null matrix) and Weight-2 (W2) Circulant matrix (CM), which can be seen as a special case of the general type-II QC LDPC codes. The shift matrix of the codes is built on the basis of one integer sequence, called perfect Cyclic difference set (CDS), which guarantees the girth of the code at least six. Simulation results show that the codes can perform well in comparison with a variety of other LDPC codes. They have excellent error floor and decoding convergence characteristics.

DOI
10.1049/cje.2015.01.024

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Quasi-cyclic (QC) Low-density paritycheck (LDPC) codes are constructed from combination of weight-0 (null matrix) and Weight-2 (W2) Circulant matrix (CM), which can be seen as a special case of the general type-II QC LDPC codes. The shift matrix of the codes is built on the basis of one integer sequence, called perfect Cyclic difference set (CDS), which guarantees the girth of the code at least six. Simulation results show that the codes can perform well in comparison with a variety of other LDPC codes. They have excellent error floor and decoding convergence characteristics.
Type-II quasi-cyclic (QC) LDPC codes are constructed from combinations of weight-0, weight-1 and weight-2 circulant matrices. The structure of cycles of length 2nare investigated, and necessary and sufficient conditions for a type-II QC LDPC parity check matrix H to have girth at least 2(n+ 1) are given. An explicit construction of type-II codes which guarantees girth at least 6 is presented. A necessary and sufficient condition for a QC matrix with one or more rows of circulants, to be fullrank is derived.
In order to solve the problems of high coding complexity and inflexible code length selection in quasi-cyclic low-density Parity-check (QC-LDPC) codes, a deterministic structure is proposed based on the perfect cyclic difference sets. The number of shifts of base matrix can be obtained by a simple addition and subtraction. Because of the special structure of the base matrix and the perfect cyclic difference sets, it saves the storage space and reduces the complexity of the hardware implementation. The girth is at least six and the code-length and code-rate can be selected flexibly. Simulation results demonstrate that the Net Coding Gain of the irregular PCDS-QC-LDPC(2680, 1340) code based on perfect cyclic difference sets has respectively improved 0.13dB and 0.32dB than the code based on PEG-QC-LDPC(2680,1340) and array dispersion and masking AD-MASK-QC-LDPC(2680,1340) with the rate of 0.5 and the bit error rate of 10 -6 by using the sum-product algorithm iterative decoding in the additive white Gaussian channel.
From Sidon sequences, two explicit methods for constructing (2 J , 2 L )‐regular type‐II quasi‐cyclic (QC) low‐density parity‐check (LDPC) codes are proposed without four cycles. The advantage of the new methods is two‐fold: (i) compared with the existing lower bound of circulant permutation matrix (CPM) sizes above which type‐II QC‐LDPC codes exist without four cycles, the novel constructions enable much tighter lower bounds; and (ii) the codes constructed by the first method perform almost as well as type‐II QC‐LDPC codes from perfect cyclic difference set, while possessing a much more flexible CPM sizes.
Combining the perfect cyclic difference sets with the quasi-cyclic codes,a new algorithm for constructing QC-LDPC codes based on cyclic difference sets is proposed.First,the author obtains a small initial matrix that meet the requirements of the weights based on cyclic difference sets,and then the zeros elements,non-zero elements of the initial matrix are replaced by all-zero matrices and cyclic shift matrices.For the purpose of determining the shift coefficient,the cycle length of six is eliminated by computer searching.Simulations show that,under the AWGN channel and the BP decoding algorithm,our QC-LDPC codes are better than the random LDPC codes in the bit error rate and they are more proper for hardware implementation.
In this correspondence we present a special class of quasi-cyclic low-density parity-check (QC-LDPC) codes, called block-type LDPC (B-LDPC) codes, which have an efficient encoding algorithm due to the simple structure of their parity-check matrices. Since the parity-check matrix of a QC-LDPC code consists of circulant permutation matrices or the zero matrix, the required memory for storing it can be significantly reduced, as compared with randomly constructed LDPC codes. We show that the girth of a QC-LDPC code is upper-bounded by a certain number which is determined by the positions of circulant permutation matrices. The B-LDPC codes are constructed as irregular QC-LDPC codes with parity-check matrices of an almost lower triangular form so that they have an efficient encoding algorithm, good noise threshold, and low error floor. Their encoding complexity is linearly scaled regardless of the size of circulant permutation matrices.