Role of a Relay in Bursty Multiple Access Channels

TL;DR

This paper characterizes the degrees of freedom region in bursty MIMO Gaussian MACs with a relay, showing relays can provide significant DoF gains and help achieve collision-free performance, especially with correlated user traffic.

Contribution

It extends noisy network coding to bursty channels, establishes conditions for collision-free DoF, and analyzes how relay gains scale with additional antennas and correlated traffic.

Findings

Relays can provide DoF gains in bursty channels unlike in non-bursty channels.

The relaying gain can scale with additional antennas at the relay.

Relays help achieve collision-free performance under certain conditions.

Abstract

We investigate the role of a relay in multiple access channels (MACs) with bursty user traffic, where intermittent data traffic restricts the users to bursty transmissions. As our main result, we characterize the degrees of freedom (DoF) region of a $K$-user bursty multi-input multi-output (MIMO) Gaussian MAC with a relay, where Bernoulli random states are introduced to govern bursty user transmissions. To that end, we extend the noisy network coding scheme to achieve the cut-set bound. Our main contribution is in exploring the role of a relay from various perspectives. First, we show that a relay can provide a DoF gain in bursty channels, unlike in conventional non-bursty channels. Interestingly, we find that the relaying gain can scale with additional antennas at the relay to some extent. Moreover, observing that a relay can help achieve collision-free performances, we establish the necessary and sufficient condition for attaining collision-free DoF. Lastly, we consider scenarios in which some physical perturbation shared around the users may generate data traffic simultaneously, causing transmission patterns across them to be correlated. We demonstrate that for most cases in such scenarios, the relaying gain is greater when the users' transmission patterns are more correlated, hence when more severe collisions take place. Our results have practical implications in various scenarios of wireless networks such as device-to-device systems and random media access control protocols.