Uplink Performance of RIS-aided Cell-Free Massive MIMO System with Electromagnetic Interference

TL;DR

This paper analyzes the uplink spectral efficiency of RIS-aided cell-free massive MIMO systems considering electromagnetic interference, proposing power control algorithms and revealing the impact of EMI, channel correlations, and RIS size on performance.

Contribution

It introduces a practical system model with EMI at RIS, derives closed-form spectral efficiency expressions, and proposes power control methods to mitigate EMI effects.

Findings

EMI significantly degrades spectral efficiency, especially for poorly conditioned UEs.

Increasing RIS elements improves SE with diminishing returns beyond a point.

Spatial correlations among RIS elements worsen performance under EMI.

Abstract

Cell-free (CF) massive multiple-input multiple-output (MIMO) and reconfigurable intelligent surface (RIS) are two promising technologies for realizing future beyond-fifth generation (B5G) networks. In this paper, we consider a practical spatially correlated RIS-aided CF massive MIMO system with multi-antenna access points (APs) over spatially correlated fading channels. Different from previous work, the electromagnetic interference (EMI) at RIS is considered to further characterize the system performance of the actual environment. Then, we derive the closed-form expression for the system spectral efficiency (SE) with the maximum ratio (MR) combining at the APs and the large-scale fading decoding (LSFD) at the central processing unit (CPU). Moreover, to counteract the near-far effect and EMI, we propose practical fractional power control (FPC) and max-min power control algorithms to further improve the system performance. We unveil the impact of EMI, channel correlations, and different signal processing methods on the uplink SE of user equipments (UEs). The accuracy of our derived analytical results is verified by extensive Monte-Carlo simulations. Our results show that the EMI can substantially degrade the SE, especially for those UEs with unsatisfactory channel conditions. Besides, increasing the number of RIS elements is always beneficial in terms of the SE, but with diminishing returns when the number of RIS elements is sufficiently large. Furthermore, the existence of spatial correlations among RIS elements can deteriorate the system performance when RIS is impaired by EMI.