On Two-Pair Two-Way Relay Channel with an Intermittently Available Relay

TL;DR

This paper characterizes the capacity region of a two-pair two-way relay channel with an intermittently available relay, highlighting the importance of relay state information quality in cooperative communication.

Contribution

It provides the first capacity region characterization for a multi-user two-way relay network with intermittent relay availability, considering both delayed and instantaneous state information.

Findings

Capacity region characterized within a bounded gap.

Relay state information quality is the key bottleneck.

First analysis of shared intermittent relay in multi-user setting.

Abstract

When multiple users share the same resource for physical layer cooperation such as relay terminals in their vicinities, this shared resource may not be always available for every user, and it is critical for transmitting terminals to know whether other users have access to that common resource in order to better utilize it. Failing to learn this critical piece of information may cause severe issues in the design of such cooperative systems. In this paper, we address this problem by investigating a two-pair two-way relay channel with an intermittently available relay. In the model, each pair of users need to exchange their messages within their own pair via the shared relay. The shared relay, however, is only intermittently available for the users to access. The accessing activities of different pairs of users are governed by independent Bernoulli random processes. Our main contribution is the characterization of the capacity region to within a bounded gap in a symmetric setting, for both delayed and instantaneous state information at transmitters. An interesting observation is that the bottleneck for information flow is the quality of state information (delayed or instantaneous) available at the relay, not those at the end users. To the best of our knowledge, our work is the first result regarding how the shared intermittent relay should cooperate with multiple pairs of users in such a two-way cooperative network.