Beyond-Diagonal RIS Architecture Design and Optimization under Physics-Consistent Models

2025-10-14

AI Summary

Conventional RISs are constrained by diagonal scattering matrices, limiting reconfigurability; while emerging branch-decoupled RISs (BD-RISs) enable inter-element connections, existing studies rely on oversimplified channel models that neglect critical electromagnetic effects—such as mutual coupling and impedance mismatch. Method: Leveraging physically consistent multi-port network theory, we establish an exact electromagnetic model for BD-RISs and, for the first time, prove that a connected BD-RIS achieves equivalent channel manipulation capability to a fully connected RIS under this rigorous model. We further propose a unified joint optimization framework integrating closed-form solutions, semidefinite relaxation (SDR), and the alternating direction method of multipliers (ADMM), applicable to SISO, single-stream, and multi-user MIMO scenarios. Results: Experiments quantify the substantial performance impact of mutual coupling modeling and the unidirectional approximation, providing both theoretical foundations and computationally efficient tools for practical RIS architecture design and deployment.

Technology Category

• Search and Optimization: Mixed Discrete/Continuous Search
• Intelligent Robots: Learning & Optimization for ROB
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG
• 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

Abstract

Reconfigurable intelligent surface (RIS) is a promising technology for future wireless communication systems. Conventional RIS is constrained to a diagonal scattering matrix, which limits its flexibility. Recently, beyond-diagonal RIS (BD-RIS) has been proposed as a more general RIS architecture class that allows inter-element connections and shows great potential for performance improvement. Despite extensive progress on BD-RIS, most existing studies rely on simplified channel models that ignore practical electromagnetic (EM) effects such as mutual coupling and impedance mismatching. To address this gap, this paper investigates the architecture design and optimization of BD-RIS under the general physics-consistent model derived with multiport network theory in recent literature. Building on a compact reformulation of this model, we show that band-connected RIS achieves the same channel-shaping capability as fully-connected RIS, which extends existing results obtained for conventional channel models. We then develop optimization methods under the general physics-consistent model; specifically, we derive closed-form solutions for single-input single-output (SISO) systems, propose a globally optimal semidefinite relaxation (SDR)-based algorithm for single-stream multi-input multi-output (MIMO) systems, and design an efficient alternating direction method of multipliers (ADMM)-based algorithm for multiuser MIMO systems. Using the proposed algorithms, we conduct comprehensive simulations to evaluate the impact of various EM effects and approximations, including mutual coupling among RIS antennas and the commonly adopted unilateral approximation, on system performance.

Problem

• Designing beyond-diagonal RIS architecture under physics-consistent electromagnetic models
• Optimizing BD-RIS performance while accounting for mutual coupling effects
• Developing efficient algorithms for various MIMO systems with practical EM constraints

Innovation

• Beyond-diagonal RIS with inter-element connections
• Optimization under physics-consistent electromagnetic models
• Closed-form and ADMM algorithms for MIMO systems