On the Alternative Relaying Diamond Channel with Conferencing Links

TL;DR

This paper analyzes the half-duplex alternative relaying diamond channel with rate-limited conferencing links, deriving achievable rates for decode-and-forward and amplify-and-forward schemes, and showing conditions where simple conferencing schemes are optimal.

Contribution

It introduces and compares two conferencing strategies in the diamond relay channel, deriving closed-form rates and establishing when simpler schemes are sufficient.

Findings

Two-side conferencing achieves optimal rates under general conditions.

One-side conferencing is as effective as two-side in many scenarios.

DF scheme can be capacity-achieving with one-side conferencing under certain conditions.

Abstract

In this paper, the diamond relay channel is considered, which consists of one source-destination pair and two relay nodes connected with rate-limited out-of-band conferencing links. In particular, we focus on the half-duplex alternative relaying strategy, in which the two relays operate alternatively over time. With different amounts of delay, two conferencing strategies are proposed, each of which can be implemented by either a general two-side conferencing scheme (for which both of the two conferencing links are used) or a special-case one-side conferencing scheme (for which only one of the two conferencing links is used). Based on the most general two-side conferencing scheme, we derive the achievable rates by using the decode-and-forward (DF) and amplify-and-forward (AF) relaying schemes, and show that these rate maximization problems are convex. By further exploiting the properties of the optimal solutions, the simpler one-side conferencing is shown to be equally good as the two-side conferencing in term of the achievable rates under arbitrary channel conditions. Based on this, the DF rate in closed-form is obtained, and the principle to use which one of the two conferencing links for one-side conferencing is also established. Moreover, the DF scheme is shown to be capacity-achieving under certain conditions with even one-side conferencing. For the AF relaying scheme, one-side conferencing is shown to be sub-optimal in general. Finally, numerical results are provided to validate our analysis.