Function Computation through a Bidirectional Relay

TL;DR

This paper studies the problem of function computation in a three-node wireless network with correlated sources, establishing rate regions for zero-error and epsilon-error scenarios using graph coloring and information-theoretic methods.

Contribution

It provides a rate characterization for zero-error function computation in relay networks, including a graph-theoretic approach and inner bounds for both zero-error and epsilon-error cases.

Findings

Zero-error and epsilon-error rate regions are equivalent for certain functions.

Graph coloring techniques characterize zero-error rate regions.

Inner bounds are derived for both error scenarios.

Abstract

We consider a function computation problem in a three node wireless network. Nodes A and B observe two correlated sources $X$ and $Y$ respectively, and want to compute a function $f(X,Y)$. To achieve this, nodes A and B send messages to a relay node C at rates $R_A$ and $R_B$ respectively. The relay C then broadcasts a message to A and B at rate $R_C$. We allow block coding, and study the achievable region of rate triples under both zero-error and $\epsilon$-error. As a preparation, we first consider a broadcast network from the relay to A and B. A and B have side information $X$ and $Y$ respectively. The relay node C observes both $X$ and $Y$ and broadcasts an encoded message to A and B. We want to obtain the optimal broadcast rate such that A and B can recover the function $f(X,Y)$ from the received message and their individual side information $X$ and $Y$ respectively. For this problem, we show equivalence between $\epsilon$-error and zero-error computations-- this gives a rate characterization for zero-error computation. As a corollary, this also gives a rate characterization for the relay network under zero-error for a class of functions called {\em component-wise one-to-one functions} when the support set of $p_{XY}$ is full. For the relay network, the zero-error rate region for arbitrary functions is characterized in terms of graph coloring of some suitably defined probabilistic graphs. We then give a single-letter inner bound to this rate region. Further, we extend the graph theoretic ideas to address the $\epsilon$-error problem and obtain a single-letter inner bound.