• International Journal of Technology (IJTech)
  • Vol 10, No 5 (2019)

A New Method for Building Low-Density-Parity-Check Codes

A New Method for Building Low-Density-Parity-Check Codes

Title: A New Method for Building Low-Density-Parity-Check Codes
Tehami Mohammed Amine, Djebbari Ali

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Amine, T.M., Ali, D. 2019. A New Method for Building Low-Density-Parity-Check Codes. International Journal of Technology. Volume 10(5), pp. 953-960

Tehami Mohammed Amine Telecommunications and Digital Signal Processing Laboratory, Djillali liabes University of Sidi Bel Abbes, Algeria
Djebbari Ali Telecommunications and Digital Signal Processing Laboratory, Djillali liabes University of Sidi Bel Abbes, Algeria
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A New Method for Building Low-Density-Parity-Check Codes

This paper proposes a new method for building low-density-parity-check codes, exempt of cycle of length 4, based on a circulant permutation matrix, which needs very little memory for storage it in the encoder and a dual diagonal structure is applied to guarantee that parity check bits can be recursively computed with linear calculation complexity. The Bit Error Rate performance of the new low-density-parity-check codes was compared to the uncoded bi-phase-shift-keying over additive-white-gaussian-noise channel. This simulation shows that the proposed codes are very efficient over additive-white-gaussian-noise. The proposed codes ensure a very low encoding complexity and reduce the memory storage required for the parity-check matrix, which can be more easily built than others codes used in channel coding.

Circulant permutation matrix; Dual-diagonal matrix; Girth; Low-density-parity-check codes; Parity-check matrix


Because of their prodigious performance, low-density-parity-check (LDPC) codes are now considered optimal (Gallager, 1962). These codes are a part of linear block codes which have acquired considerable importance in error correcting performances (Yahya et al., 2009). LDPC codes can be presented by specific parity check matrix H that includes a high density of 0’-s and a low density of 1’-s (Mackay, 1999). The Tanner graph (Tanner, 1981) is a bipartite graph comprising two groups of nodes: the variable nodes and check nodes. Variable nodes depicting columns and rows are represented by check nodes and connections between these two sets are known as edges (Tanner, 1981; Juwono et al., 2013). In the Tanner graph, a cycle is defined as a path which starts and ends at the same node, if the graph contains a cycle; its minimum length is known as ‘girth’ (Tanner, 1981). Cycles especially those of length 4 decrease the bit-error-rate (BER) performance of LDPC codes, because of their impact on the independence of extrinsic information exchanged in the decoding process (Johnson & Weller, 2001). Gallager codes are classified: as regular if the weight of columns and rows (i.e. density of 1’s) is constant and as irregular if column weight and row weight are variable (Yahya et al., 2009).

The construction of these codes is of two types; the first is random construction that is flexible in design and construction (Mackay, 1999). The parity-check matrix is a superposition and/or concatenation of sub-matrices and this construction has significant drawbacks in term of the stocking and accessing a large parity-check matrix. As random building does not guarantee small cycle lengths, a second form of construction was developed; this is known as deterministic construction (Moura el al., 2004; Shin et al., 2014).

In this paper, we depict a particular category of LDPC codes, excluding cycles of length 4, which can be linearly coded by matrix H. The parity check matrix is divided into two sections: the first, which matches to the parity bits, is a dual-diagonal structure (Guolei & Dong, 2010) and the second, which matches the information bits, is a quasi-cyclic structure. For that reason, this new LDPC code is classified as irregular code.

This paper is organized as follows. Section 2 discusses the construction of the new LDPC code based on a quasi-cyclic and a dual diagonal matrix. In section 3, we propose a deterministic rule for constructing parity-check matrix with various rates. Section 4 describes the LDPC reduced-complexity encoding method, and section 5 discusses decoding complexity. Section 6 specifies the advantages of the proposed method, followed by conclusions in Section 7.


To address quality of reception and implementation constraints, LDPC code must be constructed with a low error floor, linear encoding and less complex decoding. This paper proposes a new method for constructing parity-check matrix that include girths of length 4, for different rates. Memory requirements are significantly reduced by the use of the quasi-cyclic matrices and dual- diagonal, which reduce encoding complexity.


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