Rate Maximization for Half-Duplex Multiple Access with Cooperating Transmitters

TL;DR

This paper derives optimal resource allocation strategies for a half-duplex Gaussian multiple access channel with transmitter cooperation, demonstrating how cooperation levels adapt with inter-user link strength to approach capacity.

Contribution

It introduces a practical scheme combining rate splitting and superposition coding, providing algorithms for optimal resource allocation and analyzing the transition from no cooperation to full cooperation.

Findings

Optimal resource allocation schemes are derived for different channel conditions.

The scheme approaches capacity when inter-user links are strong.

Full cooperation with decode-forward relaying is rate-optimal in strong inter-user link scenarios.

Abstract

We derive the optimal resource allocation of a practical half-duplex scheme for the Gaussian multiple access channel with transmitter cooperation (MAC-TC). Based on rate splitting and superposition coding, two users transmit information to a destination over 3 phases, such that the users partially exchange their information during the first 2 phases and cooperatively transmit to the destination during the last one. This scheme is near capacity-achieving when the inter-user links are stronger than each user-destination link; it also includes partial decode-forward relaying as a special case. We propose efficient algorithms to find the optimal resource allocation for maximizing either the individual or the sum rate and identify the corresponding optimal scheme for each channel configuration. For fixed phase durations, the power allocation problem is convex and can be solved analytically based on the KKT conditions. The optimal phase durations can then be obtained numerically using simple search methods. Results show that as the interuser link qualities increase, the optimal scheme moves from no cooperation to partial then to full cooperation, in which the users fully exchange their information and cooperatively send it to the destination. Therefore, in practical systems with strong inter-user links, simple decode-forward relaying at both users is rate-optimal.