Hierarchical and High-Girth QC LDPC Codes

TL;DR

This paper introduces hierarchical quasi-cyclic LDPC codes with high girth, designed through a novel girth-maximizing algorithm, enhancing hardware friendliness and approaching channel capacity.

Contribution

It proposes a new hierarchical code structure, a girth-maximizing optimization method, and a technique to eliminate short cycles, advancing LDPC code design.

Findings

Designed girth-10 QC LDPC codes from protographs.

Hierarchical structure generalizes QC LDPC codes.

Girth maximization improves code performance.

Abstract

We present a general approach to designing capacity-approaching high-girth low-density parity-check (LDPC) codes that are friendly to hardware implementation. Our methodology starts by defining a new class of "hierarchical" quasi-cyclic (HQC) LDPC codes that generalizes the structure of quasi-cyclic (QC) LDPC codes. Whereas the parity check matrices of QC LDPC codes are composed of circulant sub-matrices, those of HQC LDPC codes are composed of a hierarchy of circulant sub-matrices that are in turn constructed from circulant sub-matrices, and so on, through some number of levels. We show how to map any class of codes defined using a protograph into a family of HQC LDPC codes. Next, we present a girth-maximizing algorithm that optimizes the degrees of freedom within the family of codes to yield a high-girth HQC LDPC code. Finally, we discuss how certain characteristics of a code protograph will lead to inevitable short cycles, and show that these short cycles can be eliminated using a "squashing" procedure that results in a high-girth QC LDPC code, although not a hierarchical one. We illustrate our approach with designed examples of girth-10 QC LDPC codes obtained from protographs of one-sided spatially-coupled codes.