HARQ and AMC: Friends or Foes?

TL;DR

This paper compares the performance of adaptive modulation and coding (AMC), hybrid ARQ (HARQ), and their combination in different fading channel conditions, revealing when HARQ enhances or hinders throughput.

Contribution

It provides a comprehensive analysis of AMC and HARQ interactions, identifying conditions where their combination is beneficial or counterproductive, and suggests improvements.

Findings

HARQ may be counterproductive at high SNR in fast fading channels

HARQ offers moderate gains in slow fading channels

Combining AMC and HARQ requires careful optimization for different channel conditions

Abstract

To ensure reliable communication in randomly varying and error-prone channels, wireless systems use adaptive modulation and coding (AMC) as well as hybrid ARQ (HARQ). In order to elucidate their compatibility and interaction, we compare the throughput provided by AMC, HARQ, and their combination (AMC-HARQ) under two operational conditions: in slow- and fast block-fading channels. Considering both, incremental redundancy HARQ (HARQ-IR) and repetition redundancy HARQ (HARQ-RR) we optimize the rate-decision regions for AMC/HARQ and compare them in terms of attainable throughput. Under a fairly general model of the channel variation and the decoding functions, we conclude that i) adding HARQ on top of AMC may be counterproductive in the high average signal- to-noise ratio regime for fast fading channels, and ii) HARQ is useful for slow fading channels, but it provides moderate throughput gains. We provide explanations for these results which allow us to propose paths to improve AMC-HARQ systems.