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IEEE International Symposium on Information Theory (ISIT)

Quasicyclic low density parity check codes

M.P.C. Fossorier

Published Jan 1, 2003• 17 citations
Publisher

Abstract

In this work, the construction of low density parity check codes (LDPCs) from circulant permutation matrices is investigated. It is shown that such codes can not have a Tanner graph representation with girth larger than 12, and a relatively loose necessary and sufficient condition for the code to have a girth of 6, 8, 10 or 12 is derived. These results suggest that families of LDPC codes with such girth values are relatively easy to obtain and consequently, additional parameters such as the minimum distance or the number of redundant check sums should be considered.

DOI
10.1109/isit.2003.1228164

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In this work, the construction of low density parity check codes (LDPCs) from circulant permutation matrices is investigated. It is shown that such codes can not have a Tanner graph representation with girth larger than 12, and a relatively loose necessary and sufficient condition for the code to have a girth of 6, 8, 10 or 12 is derived. These results suggest that families of LDPC codes with such girth values are relatively easy to obtain and consequently, additional parameters such as the minimum distance or the number of redundant check sums should be considered.
In this correspondence, the construction of low-density parity-check (LDPC) codes from circulant permutation matrices is investigated. It is shown that such codes cannot have a Tanner graph representation with girth larger than 12, and a relatively mild necessary and sufficient condition for the code to have a girth of 6, 8,10, or 12 is derived. These results suggest that families of LDPC codes with such girth values are relatively easy to obtain and, consequently, additional parameters such as the minimum distance or the number of redundant check sums should be considered. To this end, a necessary condition for the codes investigated to reach their maximum possible minimum Hamming distance is proposed.
In this correspondence, the construction of low-density parity-check (LDPC) codes from circulant permutation matrices is investigated. It is shown that such codes cannot have a Tanner graph representation with girth larger than , and a relatively mild necessary and sufficient condition for the code to have a girth of or is derived. These results suggest that families of LDPC codes with such girth values are relatively easy to obtain and, consequently, additional parameters such as the minimum distance or the number of redundant check sums should be considered. To this end, a necessary condition for the codes investigated to reach their maximum possible minimum Hamming distance is proposed.
To solve the problem of the girth 4 which will lead to a serious decrease in the performance of quasi-cyclic low-density parity-check(QC LDPC) codes,this paper presents the necessary and sufficient conditions of Tanner codes without girth 4 in the form of several theorems.These theorems can be used as the constraint conditions for the construction of Tanner codes.To construct the Tanner codes without girth 4 needs to adjust the dimension and the shift factors of the circulant matrices of the given sparse parity-check matrices according to the proposed theorems.Examples are provided to verify the theorems,and computer simulation results show that the Tanner codes obtained by proposed constructions achieve good BER performance compared with that of randomly constructed LDPC codes.
This paper gives necessary and sufficient conditions for the Tanner graph of a quasi-cyclic (QC) low-density parity-check (LDPC) code based on the all-one protograph to have girth 6, 8, 10, and 12, respectively, in the case of parity-check matrices with column weight 4. These results are a natural extension of the girth results of the already-studied cases of column weight 2 and 3, and it is based on the connection between the girth of a Tanner graph given by a parity-check matrix and the properties of powers of the product between the matrix and its transpose. The girth conditions can be easily incorporated into fast algorithms that construct codes of desired girth between 6 and 12; our own algorithms are presented for each girth, together with constructions obtained from them and corresponding computer simulations. More importantly, this paper emphasizes how the girth conditions of the Tanner graph corresponding to a parity-check matrix composed of circulants relate to the matrix obtained by adding (over the integers) the circulant columns of the parity-check matrix. In particular, we show that imposing girth conditions on a parity-check matrix is equivalent to imposing conditions on a square circulant submatrix obtained from it.
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.
Low-density parity-check (LDPC) codes based on affine permutation matrices, APM-LDPC codes, have been attracted recently, because of some advantages rather than QC-LDPC codes in minimum-distance, cycle distribution and error-rate performance. In this letter, circulant and anti-circulant permutation matrices are used to define a class of LDPC codes, called AQC-LDPC codes, which can be considered as an special case of APM-LDPC codes. In fact, each AQC-LDPC code can be verified by a sign matrix and a slope matrix which are helpful to show each cycle in the Tanner graph by a modular linear equation. For the normal sign matrix A , if -1 ϵ, A, it is shown that the corresponding AQC-LDPC code has maximum-girth 8. Finally, two explicit constructions for AQC-LDPC codes with girths 6, 8 are presented which have some benefits rather than the explicitly constructed QC and APM LDPC codes in minimum-distance, cycle distributions and bit-error-rate performances.