Beamforming Design for Beyond Diagonal RIS-Aided Cell-Free Massive MIMO Systems

2025-03-10

AI Summary

This work addresses the joint optimization of active beamforming at base stations and passive beamforming at beyond-diagonal reconfigurable intelligent surfaces (BD-RISs) in cell-free massive MIMO systems, subject to transmit power constraints and the unitary matrix constraint on BD-RIS coefficients, with the objective of maximizing sum spectral efficiency (SE). Method: We introduce BD-RIS—capable of simultaneously reflecting and transmitting incident signals—into the cell-free architecture for the first time, and propose a unified beamforming framework. To handle the unitary constraint, we develop a Riemannian manifold-based optimization algorithm integrating alternating optimization and fractional programming. Contribution/Results: Compared to conventional diagonal RISs, the proposed BD-RIS-aided design achieves approximately 40% higher sum SE, significantly enhancing both coverage and capacity performance in cell-free massive MIMO networks.

Technology Category

• Search and Optimization: Mixed Discrete/Continuous Search
• Constraint Satisfaction and Optimization: Mixed Discrete/Continuous Optimization
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Search and Retrieval-Augmented AI: Personalized, context-aware and across-device search

Abstract

Reconfigurable intelligent surface (RIS)-aided cell-free (CF) massive multiple-input multiple-output (mMIMO) is a promising architecture for further improving spectral efficiency (SE) with low cost and power consumption. However, conventional RIS has inevitable limitations due to its capability of only reflecting signals. In contrast, beyond-diagonal RIS (BD-RIS), with its ability to both reflect and transmit signals, has gained great attention. This correspondence focuses on using BD-RIS to improve the sum SE of CF mMIMO systems. This requires completing the beamforming design under the transmit power constraints and unitary constraints of the BD-RIS, by optimizing active and passive beamformer simultaneously. To tackle this issue, we introduce an alternating optimization algorithm that decomposes it using fractional programming and solves the subproblems alternatively. Moreover, to address the challenge introduced by the unitary constraint on the beamforming matrix of the BD-RIS, a manifold optimization algorithm is proposed to solve the problem optimally. Simulation results show that BD-RISs outperform RISs comprehensively, especially in the case of the full connected architecture which achieves the best performance, enhancing the sum SE by around 40% compared to ideal RISs.

Problem

• Optimize beamforming for BD-RIS in CF mMIMO systems.
• Enhance spectral efficiency with low cost and power.
• Address unitary constraints on BD-RIS beamforming matrix.

Innovation

• BD-RIS enhances signal reflection and transmission.
• Alternating optimization for beamforming design.
• Manifold optimization addresses unitary constraints.