Coding Versus ARQ in Fading Channels: How reliable should the PHY be?

TL;DR

This paper analyzes the tradeoff between channel coding and ARQ in fading channels, deriving the optimal error probability for maximizing throughput and showing that higher reliability can reduce overall performance.

Contribution

It provides a theoretical framework for balancing coding and ARQ, revealing that larger error probabilities can optimize throughput in fading channels.

Findings

Optimal error probability decreases with SNR and diversity

High error probabilities (~10%) can maximize throughput

Very low error probabilities may significantly reduce throughput

Abstract

This paper studies the tradeoff between channel coding and ARQ (automatic repeat request) in Rayleigh block-fading channels. A heavily coded system corresponds to a low transmission rate with few ARQ re-transmissions, whereas lighter coding corresponds to a higher transmitted rate but more re-transmissions. The optimum error probability, where optimum refers to the maximization of the average successful throughput, is derived and is shown to be a decreasing function of the average signal-to-noise ratio and of the channel diversity order. A general conclusion of the work is that the optimum error probability is quite large (e.g., 10% or larger) for reasonable channel parameters, and that operating at a very small error probability can lead to a significantly reduced throughput. This conclusion holds even when a number of practical ARQ considerations, such as delay constraints and acknowledgement feedback errors, are taken into account.