Unequal Message Protection: Asymptotic and Non-Asymptotic Tradeoffs

TL;DR

This paper investigates unequal message protection (UMP) in coding, analyzing the tradeoffs between message class rates and error protection levels, revealing performance losses compared to classical codes and proposing practical construction methods.

Contribution

It generalizes finite block length bounds for UMP codes, evaluates their performance on key channels, and explores practical code constructions like coset codes.

Findings

UMP codes have performance loss compared to homogeneous codes.

Header construction is suboptimal at finite block lengths.

Coset codes offer a practical approach to UMP code implementation.

Abstract

We study a form of unequal error protection that we term "unequal message protection" (UMP). The message set of a UMP code is a union of $m$ disjoint message classes. Each class has its own error protection requirement, with some classes needing better error protection than others. We analyze the tradeoff between rates of message classes and the levels of error protection of these codes. We demonstrate that there is a clear performance loss compared to homogeneous (classical) codes with equivalent parameters. This is in sharp contrast to previous literature that considers UMP codes. To obtain our results we generalize finite block length achievability and converse bounds due to Polyanskiy-Poor-Verd\'{u}. We evaluate our bounds for the binary symmetric and binary erasure channels, and analyze the asymptotic characteristic of the bounds in the fixed error and moderate deviations regimes. In addition, we consider two questions related to the practical construction of UMP codes. First, we study a "header" construction that prefixes the message class into a header followed by data protection using a standard homogeneous code. We show that, in general, this construction is not optimal at finite block lengths. We further demonstrate that our main UMP achievability bound can be obtained using coset codes, which suggests a path to implementation of tractable UMP codes.