A Novel Power Allocation Scheme for Two-User GMAC with Finite Input Constellations

TL;DR

This paper introduces a Constellation Power Allocation (CPA) scheme for two-user Gaussian Multiple Access Channels that improves sum capacity and reduces decoding complexity compared to existing Constellation Rotation (CR) methods, especially at high SNR.

Contribution

The paper proposes a novel CPA scheme that varies transmit power under average power constraints, offering comparable or better sum capacity than CR with lower decoding complexity.

Findings

CPA scheme achieves sum capacity close to CR at high SNR

CPA outperforms CR under random channel phase offsets at high SNR

Maximum gain occurs when users have similar power levels

Abstract

Constellation Constrained (CC) capacity regions of two-user Gaussian Multiple Access Channels (GMAC) have been recently reported, wherein an appropriate angle of rotation between the constellations of the two users is shown to enlarge the CC capacity region. We refer to such a scheme as the Constellation Rotation (CR) scheme. In this paper, we propose a novel scheme called the Constellation Power Allocation (CPA) scheme, wherein the instantaneous transmit power of the two users are varied by maintaining their average power constraints. We show that the CPA scheme offers CC sum capacities equal (at low SNR values) or close (at high SNR values) to those offered by the CR scheme with reduced decoding complexity for QAM constellations. We study the robustness of the CPA scheme for random phase offsets in the channel and unequal average power constraints for the two users. With random phase offsets in the channel, we show that the CC sum capacity offered by the CPA scheme is more than the CR scheme at high SNR values. With unequal average power constraints, we show that the CPA scheme provides maximum gain when the power levels are close, and the advantage diminishes with the increase in the power difference.