Lower bounds on the lifting degree of single-edge and multiple-edge QC-LDPC codes by difference matrices

TL;DR

This paper introduces difference matrices to establish bounds and conditions for constructing optimal QC-LDPC codes with specific girths and minimal lengths, advancing code design for error correction.

Contribution

It presents new bounds on lifting degrees for QC-LDPC codes using difference matrices, including for multiple-edge codes, and provides constructions with minimal lengths and girth properties.

Findings

Lower bounds on lifting degrees for girth 6, 10, and 12 codes.

Construction of codes with shortest length for given girth and parameters.

Tighter bounds and shorter codes compared to previous literature.

Abstract

In this paper, we define two matrices named as "difference matrices", denoted by $D$ and $DD$ which significantly contribute to achieve regular single-edge QC-LDPC codes with the shortest length and the certain girth as well as regular and irregular multiple-edge QC-LDPC codes. Making use of these matrices, we obtain necessary and sufficient conditions to have single-edge $(m,n)$-regular QC-LDPC codes with girth 6, 10 and 12. Additionally, for girth 6, we achieve all non-isomorphic codes with the minimum lifting degree, $N$, for $m=4$ and $5\leq n\leq 11$, and present an exponent matrix for each minimum distance. For girth 10, we provide a lower bound on the lifting degree which is tighter than the existing bound. More important, for an exponent matrix whose first row and first column are all-zero, we demonstrate that the non-existence of 8-cycles proves the non-existence of 6-cycles related to the first row of the exponent matrix too. All non-isomorphic QC-LDPC codes with girth 10 and $n=5,6$ whose numbers are more than those presented in the literature are provided. For $n=7,8$ we decrease the lifting degrees from 159 and 219 to 145 and 211, repectively. For girth 12, a lower bound on the lifting degree is achieved. For multiple-edge category, for the first time a lower bound on the lifting degree for both regular and irregular QC-LDPC codes with girth 6 is achieved. We also demonstrate that the achieved lower bounds on multiple-edge $(4,n)$-regular QC-LDPC codes with girth 6 are tight and the resultant codes have shorter length compared to their counterparts in single-edge codes. Additionally, difference matrices help to reduce the conditions of considering 6-cycles from seven states to five states. We obtain multiple-edge $(4,n)$-regular QC-LDPC codes with girth 8 and $n=4,6,8$ with the shortest length.