Rate-Splitting Multiple Access for Downlink MIMO: A Generalized Power Iteration Approach

TL;DR

This paper introduces a novel precoder design framework for rate-splitting multiple access in downlink MIMO systems, utilizing a generalized power iteration approach to optimize spectral efficiency.

Contribution

It proposes a new optimization method that approximates and reformulates the sum spectral efficiency problem, enabling eigenvector-based solutions for RSMA precoding.

Findings

Significant improvement in sum spectral efficiency over existing methods

Effective approximation of non-smooth functions using LogSumExp

Eigenvector-based algorithm converges to a local optimum

Abstract

Rate-splitting multiple access (RSMA) is a general multiple access scheme for downlink multi-antenna systems embracing both classical spatial division multiple access and more recent non-orthogonal multiple access. Finding a linear precoding strategy that maximizes the sum spectral efficiency of RSMA is a challenging yet significant problem. In this paper, we put forth a novel precoder design framework that jointly finds the linear precoders for the common and private messages for RSMA. Our approach is first to approximate the non-smooth minimum function part in the sum spectral efficiency of RSMA using a LogSumExp technique. Then, we reformulate the sum spectral efficiency maximization problem as a form of the log-sum of Rayleigh quotients to convert it into a tractable form. By interpreting the first-order optimality condition of the reformulated problem as an eigenvector-dependent nonlinear eigenvalue problem, we reveal that the leading eigenvector of the derived optimality condition is a local optimal solution. To find the leading eigenvector, we propose an algorithm inspired by a power iteration. Simulation results show that the proposed RSMA transmission strategy provides significant improvement in the sum spectral efficiency compared to the state-of-the-art RSMA transmission methods.