On the Optimal Broadcast Rate of the Two-Sender Unicast Index Coding Problem with Fully-Participated Interactions

TL;DR

This paper determines the optimal broadcast rates for the two-sender unicast index coding problem with fully-participated interactions, especially when interaction digraphs contain cycles, by relating them to single-sender problems and using graph coloring techniques.

Contribution

It provides the first complete characterization of optimal broadcast rates for all cases with cycles in interaction digraphs in the two-sender unicast index coding problem.

Findings

Optimal broadcast rates are derived for all unsolved cases with cycles.

Achievable rates are provided for finite message sizes using graph coloring.

The results relate two-sender problem rates to single-sender problem rates.

Abstract

The problem of two-sender unicast index coding consists of two senders and a set of receivers. Each receiver demands a unique message and possesses some of the messages demanded by other receivers as its side-information. Every demanded message is present with at least one of the senders. Senders avail the knowledge of the side-information at the receivers to reduce the number of broadcast transmissions. Solution to this problem consists of finding the optimal number of coded transmissions from the two senders. One important class of the two-sender problem consists of the messages at the senders and the side-information at the receivers satisfying \emph{fully-participated interactions}. This paper provides the optimal broadcast rates, for all the unsolved cases of the two-sender problem with fully-participated interactions when the associated \emph{interaction digraphs} contain cycles. The optimal broadcast rates are provided in terms of those of the three independent single-sender problems associated with the two-sender problem. This paper also provides an achievable broadcast rate with $t$-bit messages for any finite $t$ and any two-sender problem with fully-participated interactions belonging to $(i)$ any one of the six instances (classes) of the two-sender problem when the associated interaction digraph does not contain any cycle, and $(ii)$ one of the classes of the two-sender problem when the associated interaction digraph contains cycles. The achievable broadcast rates are obtained by exploiting the symmetries of the confusion graph to color the same according to the two-sender graph coloring.