Performance Bounds for Bi-Directional Coded Cooperation Protocols

TL;DR

This paper derives and compares performance bounds for three bi-directional coded cooperation protocols over shared channels, providing insights into their efficiency and potential advantages in cellular systems.

Contribution

It introduces tight bounds for a two-phase protocol and analyzes three protocols, including a hybrid, with numerical evaluations and insights into their relative performance.

Findings

Two-phase protocol bounds are tight and achievable.

The hybrid four-phase protocol can outperform others in some cases.

Bounds are close in certain regimes, indicating efficiency of proposed protocols.

Abstract

In coded bi-directional cooperation, two nodes wish to exchange messages over a shared half-duplex channel with the help of a relay. In this paper, we derive performance bounds for this problem for each of three protocols. The first protocol is a two phase protocol were both users simultaneously transmit during the first phase and the relay alone transmits during the second. In this protocol, our bounds are tight and a multiple-access channel transmission from the two users to the relay followed by a coded broadcast-type transmission from the relay to the users achieves all points in the two-phase capacity region. The second protocol considers sequential transmissions from the two users followed by a transmission from the relay while the third protocol is a hybrid of the first two protocols and has four phases. In the latter two protocols the inner and outer bounds are not identical, and differ in a manner similar to the inner and outer bounds of Cover's relay channel. Numerical evaluation shows that at least in some cases of interest our bounds do not differ significantly. Finally, in the Gaussian case with path loss, we derive achievable rates and compare the relative merits of each protocol in various regimes. This case is of interest in cellular systems. Surprisingly, we find that in some cases, the achievable rate region of the four phase protocol sometimes contains points that are outside the outer bounds of the other protocols.