The Explicit Coding Rate Region of Symmetric Multilevel Diversity Coding

TL;DR

This paper derives explicit, finite inequalities to characterize the coding rate region of symmetric multilevel diversity coding, enabling easier verification of rate tuple achievability and confirming the optimality of superposition coding.

Contribution

It provides a closed-form, explicit characterization of the rate region and identifies a minimal subset of inequalities needed for achievability verification.

Findings

Explicit inequalities for the rate region are derived.

A finite subset of inequalities suffices for characterization.

Superposition coding is proven optimal using the new inequalities.

Abstract

It is well known that {\em superposition coding}, namely separately encoding the independent sources, is optimal for symmetric multilevel diversity coding (SMDC) (Yeung-Zhang 1999). However, the characterization of the coding rate region therein involves uncountably many linear inequalities and the constant term (i.e., the lower bound) in each inequality is given in terms of the solution of a linear optimization problem. Thus this implicit characterization of the coding rate region does not enable the determination of the achievability of a given rate tuple. In this paper, we first obtain closed-form expressions of these uncountably many inequalities. Then we identify a finite subset of inequalities that is sufficient for characterizing the coding rate region. This gives an explicit characterization of the coding rate region. We further show by the symmetry of the problem that only a much smaller subset of this finite set of inequalities needs to be verified in determining the achievability of a given rate tuple. Yet, the cardinality of this smaller set grows at least exponentially fast with $L$. We also present a subset entropy inequality, which together with our explicit characterization of the coding rate region, is sufficient for proving the optimality of superposition coding.