WMMSE-Based Joint Transceiver Design for Multi-RIS Assisted Cell-free Networks Using Hybrid CSI

TL;DR

This paper proposes a WMMSE-based joint transceiver design for multi-RIS assisted cell-free networks using hybrid CSI, optimizing beamforming, combining, and phase shifts to enhance uplink communication efficiency.

Contribution

It introduces an asymptotic AO algorithm based on free probability theory that reduces overhead by relying on long-term channel statistics.

Findings

Achieves high communication rates in simulations.

Reduces interaction overhead compared to traditional methods.

Effectively utilizes multiple RISs to improve propagation environment.

Abstract

In this paper, we consider cell-free communication systems with several access points (APs) serving terrestrial users (UEs) simultaneously. To enhance the uplink multi-user multiple-input multiple-output communications, we adopt a hybrid-CSI-based two-layer distributed multi-user detection scheme comprising the local minimum mean-squared error (MMSE) detection at APs and the one-shot weighted combining at the central processing unit (CPU). Furthermore, to improve the propagation environment, we introduce multiple reconfigurable intelligent surfaces (RISs) to assist the transmissions from UEs to APs. Aiming to maximize the weighted sum rate, we formulate the weighted sum-MMSE (WMMSE) problem, where the UEs' beamforming matrices, the CPU's weighted combining matrix, and the RISs' phase-shifting matrices are alternately optimized. Considering the limited fronthaul capacity constraint in cell-free networks, we resort to the operator-valued free probability theory to derive the asymptotic alternating optimization (AO) algorithm to solve the WMMSE problem, which only depends on long-term channel statistics and thus reduces the interaction overhead. Numerical results demonstrate that the asymptotic AO algorithm can achieve a high communication rate as well as reduce the interaction overhead.