Achievable Rate Optimization for MIMO Systems with Reconfigurable Intelligent Surfaces

TL;DR

This paper proposes an iterative optimization algorithm for maximizing achievable rates in MIMO systems with reconfigurable intelligent surfaces, demonstrating efficiency and effectiveness through simulations especially in indoor environments.

Contribution

It introduces a joint optimization framework for MIMO and RIS elements, along with a PGM-based algorithm with guaranteed convergence and improved computational efficiency.

Findings

PGM achieves comparable rates to benchmark schemes with lower complexity.

RIS significantly enhances achievable rates in indoor environments.

A new measure, FSPL ratio, assesses RIS applicability in systems.

Abstract

Reconfigurable intelligent surfaces (RISs) represent a radical new technology that can shape the radio wave propagation in wireless communication systems and offers a great variety of possible performance and implementation gains. Motivated by this, in this paper we study the achievable rate optimization for a multi-stream multiple-input multiple-output (MIMO) system equipped with an RIS, and formulate a joint optimization problem of the covariance matrix of the transmitted signal and the RIS elements. To solve this problem, we propose an iterative optimization algorithm that is based on the projected gradient method (PGM). We derive the step size that guarantees the convergence of the proposed algorithm and we define a backtracking line search to improve its convergence rate. Furthermore, we introduce the total free space path loss (FSPL) ratio of the indirect and direct links as a first-order measure of the applicability of an RIS in the considered communication system. Simulation results show that the proposed PGM achieves the same achievable rate as a state-of-the-art benchmark scheme, but with a significantly lower computational complexity. In addition, it is demonstrated that the RIS application is particularly suitable to increase the achievable rate in an indoor environment, as in this case even a small number of RIS elements is sufficient to provide a substantial achievable rate gain.