Rate-Compatible Root-Protograph-based LDPC Codes for IR-HARQ Systems

This paper studies a new type of root-protograph-based low-density parity-check (RP-LDPC) codes, called rate-compatible RP-LDPC (RC-RP-LDPC) codes, designed for the incremental-redundancy hybrid ARQ (IR-HARQ) system over block-fading (BF) channels. We begin the study with analyzing the limitations of traditional RP-LDPC codes in the IR-HARQ system over BF channels, revealing that they fail to achieve full diversity in this scenario. To address this issue, we propose a design methodology for RC-RP-LDPC codes, which can guarantee that the resultant codes not only have both full-diversity property and outage-limit-approaching performance, but also can realize rate compatibility. The outage-boundary analyses and word-error-rate (WER) simulations demonstrate that the proposed RC-RP-LDPC codes significantly outperform prior-art protograph LDPC and RP-LDPC codes in the IR-HARQ system over BF channels.