RIS-aided Massive MIMO: Achieving Large Multiplexing Gains with non-Large Arrays

TL;DR

This paper demonstrates that a non-large antenna array combined with a reconfigurable intelligent surface (RIS) can achieve significant multiplexing gains, outperforming traditional massive MIMO systems without RIS.

Contribution

It introduces a novel RIS-assisted antenna structure with a specific channel estimation method and optimization of phase shifts to enhance spectral efficiency.

Findings

The proposed structure surpasses conventional massive MIMO performance.

A closed-form expression for spectral efficiency was derived.

The RIS configuration effectively reduces channel cross-correlations.

Abstract

This paper considers an antenna structure where a (non-large) array of radiating elements is placed at short distance in front of a reconfigurable intelligent surface (RIS). We propose a channel estimation procedure using different configurations of the RIS elements and derive a closed-form expression for an achievable downlink spectral efficiency by using the popular hardening lower-bound. Next, we formulate an optimization problem, with respect to the phase shifts of the RIS, aimed at minimizing the channels cross-correlations while preserving the channels individual norms. The numerical analysis shows that the proposed structure is capable of overcoming the performance of a conventional massive MIMO system without the RIS.