Spectral Efficiency Maximization for Active RIS-aided Cell-Free Massive MIMO Systems with Imperfect CSI

TL;DR

This paper investigates spectral efficiency maximization in active RIS-assisted cell-free massive MIMO systems with imperfect CSI, proposing a SAC-based phase shift design and deriving closed-form expressions for channel estimation and spectral efficiency.

Contribution

It introduces a SAC algorithm for phase shift optimization and derives closed-form expressions for channel gain and spectral efficiency under imperfect CSI.

Findings

SAC algorithm effectively maximizes uplink spectral efficiency.

Closed-form expressions relate active RIS parameters to spectral efficiency.

Proposed methods are robust under imperfect CSI conditions.

Abstract

A cell-free network merged with active reconfigurable reflecting surfaces (RIS) is investigated in this paper. Based on the imperfect channel state information (CSI), the aggregated channel from the user to the access point (AP) is initially estimated using the linear minimum mean square error (LMMSE) technique. The central processing unit (CPU) then detects uplink data from individual users through the utilization of the maximum ratio combining (MRC) approach, relying on the estimated channel. Then, a closed-form expression for uplink spectral efficiency (SE) is derived which demonstrates its reliance on statistical CSI (S-CSI) alone. The amplitude gain of each active RIS element is derived in a closed-form expression as a function of the number of active RIS elements, the number of users, and the size of each reflecting element. A soft actor-critic (SAC) algorithm is utilized to design the phase shift of the active RIS to maximize the uplink SE. Simulation results emphasize the robustness of the proposed SAC algorithm, showcasing its effectiveness in cell-free networks under the influence of imperfect CSI.