Bounds on the Minimum Distance of Punctured Quasi-Cyclic LDPC Codes

TL;DR

This paper derives bounds on the minimum distance of punctured quasi-cyclic LDPC codes, evaluates these bounds for AR4JA codes, and discusses implications for code design and performance.

Contribution

It generalizes existing bounds to punctured QC codes and provides tighter bounds for specific classes, including AR4JA codes used in space communications.

Findings

Upper bounds are below ensemble lower bounds for large block lengths.

Bounds are tightened for certain classes of punctured QC codes.

Evaluation on AR4JA codes shows significant gap at large block lengths.

Abstract

Recent work by Divsalar et al. has shown that properly designed protograph-based low-density parity-check (LDPC) codes typically have minimum (Hamming) distance linearly increasing with block length. This fact rests on ensemble arguments over all possible expansions of the base protograph. However, when implementation complexity is considered, the expansions are frequently selected from a smaller class of structured expansions. For example, protograph expansion by cyclically shifting connections generates a quasi-cyclic (QC) code. Other recent work by Smarandache and Vontobel has provided upper bounds on the minimum distance of QC codes. In this paper, we generalize these bounds to punctured QC codes and then show how to tighten these for certain classes of codes. We then evaluate these upper bounds for the family of protograph codes known as AR4JA codes that have been recommended for use in deep space communications in a standard established by the Consultative Committee for Space Data Systems (CCSDS). At block lengths larger than 4400 bits, these upper bounds fall well below the ensemble lower bounds.