Joint Transmit Beamforming and Reflection Optimization for Beyond Diagonal RIS Aided Multi-Cell MIMO Communication

2025-11-17

AI Summary

To address severe inter-cell interference—particularly degrading edge-user performance—in ultra-dense multi-cell 6G networks caused by aggressive spectrum reuse, this paper proposes a joint beamforming and reflection coefficient optimization framework leveraging beyond-diagonal reconfigurable intelligent surfaces (BD-RIS). Departing from conventional diagonal RISs constrained to phase-only modulation, BD-RIS enables more flexible electromagnetic field coordination via manifold optimization under unit-modulus constraints. The non-convex joint optimization problem is efficiently tackled by integrating the weighted minimum mean square error (WMMSE) reformulation, Lagrangian duality theory, and an alternating optimization algorithm. Simulation results demonstrate that the proposed scheme significantly improves the weighted sum rate in multi-user MIMO downlink transmission, outperforming all baseline methods. This validates BD-RIS’s effectiveness and superiority in mitigating both intra-cell and inter-cell interference while enhancing edge-user communication reliability and throughput.

Technology Category

Search and Optimization: Non-convex OptimizationIntelligent Robots: Learning & Optimization for ROBPlanning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAGSystems and Infrastructure for Web, Mobile and WoT: Applied ML and AI for Web-based mobile applicationsUser Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation

Abstract

The sixth-generation (6G) wireless networks will rely on ultra-dense multi-cell deployment to meet the high rate and connectivity demands. However, frequency reuse leads to severe inter-cell interference, particularly for cell-edge users, which limits the communication performance. To overcome this challenge, we investigate a beyond diagonal reconfigurable intelligent surface (BD-RIS) aided multi-cell multi-user downlink MIMO communication system, where a BD-RIS is deployed to enhance desired signals and suppress both intra-cell and inter-cell interference.We formulate the joint optimization problem of the transmit beamforming matrices at the BSs and the BD-RIS reflection matrix to maximize the weighted sum rate of all users, subject to the challenging unitary constraint of the BD-RIS reflection matrix and transmit power constraints at the BSs. To tackle this non-convex and difficult problem, we apply the weighted minimum mean squared error (WMMSE) method to transform the problem into an equivalent tractable form, and propose an efficient alternating optimization (AO) based algorithm to iteratively update the transmit beamforming and BD-RIS reflection using Lagrange duality theory and manifold optimization. Numerical results demonstrate the superiority of the proposed design over various benchmark schemes, and provide useful practical insights on the BD-RIS deployment strategy for multi-cell systems.

Problem

• BD-RIS enhances signals and suppresses interference in multi-cell MIMO
• Joint optimization maximizes weighted sum rate under unitary constraints
• Proposed algorithm solves non-convex problem using WMMSE and AO

Innovation

• BD-RIS enhances signals and suppresses interference
• WMMSE method transforms problem into tractable form
• Alternating optimization updates beamforming and reflection