Physically-Consistent Modeling and Optimization of Non-local RIS-Assisted Multi-User MIMO Communication Systems

TL;DR

This paper introduces a physically-consistent optimization framework for non-local RIS-assisted multi-user MIMO systems, jointly optimizing mutual coupling, radiation patterns, and beamforming to enhance performance.

Contribution

It presents a novel offline optimization approach for jointly designing mutual coupling, radiation patterns, and beamforming in non-local RIS systems, considering both reflective and transmissive configurations.

Findings

Optimization improves system performance over benchmarks.

Offline design reduces real-time computational complexity.

Validated with parametric and geometric channel models.

Abstract

Mutual Coupling (MC) emerges as an inherent feature in Reconfigurable Intelligent Surfaces (RISs), particularly, when they are fabricated with sub-wavelength inter-element spacing. Hence, any physically-consistent model of the RIS operation needs to accurately describe MC-induced effects. In addition, the design of the ElectroMagnetic (EM) transmit/receive radiation patterns constitutes another critical factor for efficient RIS operation. The latter two factors lead naturally to the emergence of non-local RIS structures, whose operation can be effectively described via non-diagonal phase shift matrices. In this paper, we focus on jointly optimizing MC and the radiation patterns in multi-user MIMO communication systems assisted by non-local RISs, which are modeled via the scattering parameters. We particularly present a novel problem formulation for the joint optimization of MC, radiation patterns, and the active and passive beamforming in a physically-consistent manner, considering either reflective or transmissive RIS setups. Differently from the current approaches that design the former two parameters on the fly, we present an offline optimization method which is solved for both considered RIS functionalities. Our extensive simulation results, using both parametric and geometric channel models, showcase the validity of the proposed optimization framework over benchmark schemes, indicating that improved performance is achievable without the need for optimizing MC and the radiation patterns of the RIS on the fly, which can be rather cumbersome.