Lossy Beyond Diagonal Reconfigurable Intelligent Surfaces: Modeling and Optimization

2025-04-28

AI Summary

Existing BD-RIS studies predominantly rely on lossless assumptions, neglecting practical hardware losses—particularly inter-element interconnection losses—leading to model inaccuracy and suboptimal architecture selection. Method: We establish the first electromagnetic BD-RIS model incorporating interconnect losses and propose a general admittance-circle-constrained modeling framework. Leveraging this, we design an ADMM-based algorithm to jointly optimize transmit precoding and BD-RIS scattering matrices for both SISO and MU-MISO systems. Contribution/Results: Our analysis reveals, for the first time, that under high interconnect loss, the group-connected architecture outperforms both fully connected and tree-connected topologies—contradicting conventional lossless conclusions. All BD-RIS architectures remain substantially superior to conventional D-RIS. Experiments demonstrate significant gains in received power and sum rate; critically, loss sensitivity emerges as a decisive factor in BD-RIS architecture selection.

Technology Category

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

Application Category

• Search and Retrieval-Augmented AI: Web evaluation methodologies and metrics
• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs

Abstract

Beyond diagonal reconfigurable intelligent surface (BD-RIS) has emerged as an advancement and generalization of the conventional diagonal RIS (D-RIS) by introducing tunable interconnections between RIS elements, enabling smarter wave manipulation and enlarged coverage. While BD-RIS has demonstrated advantages over D-RIS in various aspects, most existing works rely on the assumption of a lossless model, leaving practical considerations unaddressed. This paper thus proposes a lossy BD-RIS model and develops corresponding optimization algorithms for various BD-RIS-aided communication systems. First, by leveraging admittance parameter analysis, we model each tunable admittance based on a lumped circuit with losses and derive an expression of a circle characterizing the real and imaginary parts of each tunable admittance. We then consider the received signal power maximization in single-user single-input single-output (SISO) systems with the proposed lossy BD-RIS model. To solve the optimization problem, we design an effective algorithm by carefully exploiting the problem structure. Specifically, an alternating direction method of multipliers (ADMM) framework is custom-designed to deal with the complicated constraints associated with lossy BD-RIS. Furthermore, we extend the proposed algorithmic framework to more general multiuser multiple-input single-output (MU-MISO) systems, where the transmit precoder and BD-RIS scattering matrix are jointly designed to maximize the sum-rate of the system. Finally, simulation results demonstrate that all BD-RIS architectures still outperform D-RIS in the presence of losses, but the optimal BD-RIS architectures in the lossless case are not necessarily optimal in the lossy case, e.g. group-connected BD-RIS can outperform fully- and tree-connected BD-RISs in SISO systems with relatively high losses, whereas the opposite always holds true in the lossless case.

Problem

• Modeling lossy BD-RIS for practical communication systems
• Optimizing received power in SISO systems with lossy BD-RIS
• Jointly designing precoder and BD-RIS for sum-rate maximization in MU-MISO systems

Innovation

• Lossy BD-RIS model using admittance parameter analysis
• ADMM framework for optimization with lossy constraints
• Joint precoder and BD-RIS design for MU-MISO systems