RIS and Cell-Free Massive MIMO: A Marriage For Harsh Propagation Environments

TL;DR

This paper explores how integrating Reconfigurable Intelligent Surfaces (RIS) with Cell-Free Massive MIMO can significantly improve performance in challenging environments, providing new analysis, estimation methods, and control schemes.

Contribution

It introduces an aggregated channel estimation method and a RIS configuration control scheme for enhanced Cell-Free Massive MIMO performance.

Findings

Asymptotic analysis shows performance gains with large APs and RIS elements.

Derived a lower bound for channel capacity with finite APs and RIS elements.

Numerical results confirm the effectiveness of RIS in improving system throughput.

Abstract

This paper considers Cell-Free Massive Multiple Input Multiple Output (MIMO) systems with the assistance of an RIS for enhancing the system performance. Distributed maximum-ratio combining (MRC) is considered at the access points (APs). We introduce an aggregated channel estimation method that provides sufficient information for data processing. The considered system is studied by using asymptotic analysis which lets the number of APs and/or the number of RIS elements grow large. A lower bound for the channel capacity is obtained for a finite number of APs and engineered scattering elements of the RIS, and closed-form expression for the uplink ergodic net throughput is formulated. In addition, a simple scheme for controlling the configuration of the RIS scattering elements is proposed. Numerical results verify the effectiveness of the proposed system design and the benefits of using RISs in Cell-Free Massive MIMO systems are quantified.