Superiority of TDMA in a Class of Gaussian Multiple-Access Channels with a MIMO-AF-Relay

TL;DR

This paper demonstrates that in a Gaussian MAC with an AF relay and multiple antennas, a TDMA scheme outperforms joint access in achievable sum rates, challenging the necessity of joint access for capacity.

Contribution

The paper introduces a TDMA-based approach for Gaussian MACs with AF relays, showing it achieves higher sum rates than joint access methods.

Findings

TDMA scheme yields higher sum rates than joint access.

TDMA simplifies optimization in multi-antenna relay channels.

Results question the need for joint access to reach capacity.

Abstract

We consider a Gaussian multiple-access channel (MAC) with an amplify-and-forward (AF) relay, where all nodes except the receiver have multiple antennas and the direct links between transmitters and receivers are neglected. Thus, spatial processing can be applied both at the transmitters and at the relay, which is subject to optimization for increasing the data rates. In general, this optimization problem is non-convex and hard to solve. While in prior work on this problem, it is assumed that all transmitters access the channel jointly, we propose a solution where each transmitter accesses the channel exclusively, using a time-division multiple-access (TDMA) scheme. It is shown that this scheme provides higher achievable sum rates, which raises the question of the need for TDMA to achieve the general capacity region of MACs with AF relay.