Are Stacked Intelligent Metasurfaces (SIMs) Better than Single-layer Reconfigurable Intelligent Surfaces (RISs) for Wideband Multi-user MIMO Communication Systems?

2026-02-26

AI Summary

This work addresses the limitations of single-layer reconfigurable intelligent surfaces (RIS) in broadband multi-user MIMO systems, where frequency selectivity and mutual coupling impair efficient and stable beamforming. To overcome these challenges, the authors propose a coupling-aware, broadband multi-subband circuit-level modeling framework that jointly optimizes base station precoding and phase configurations of multi-layer smart intelligent metasurfaces (SIMs). A partially reconfigurable SIM architecture is introduced to reduce implementation complexity while supporting carrier aggregation and ensuring consistent phase responses across frequency bands. Simulation results demonstrate that, in broadband high-SNR multi-user scenarios, the proposed approach significantly outperforms conventional single-layer RIS by effectively mitigating mutual coupling distortions, with the partially reconfigurable design achieving performance close to the global optimum.

Technology Category

• Intelligent Robots: Multi-Robot Systems
• Search and Optimization: Sampling/Simulation-based Search
• Cognitive Modeling & Cognitive Systems: Agent Architectures

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Web applications in cross-disciplinary domains and verticals such as mixed reality, smart cities, and digital health
• User Modeling, Personalization and Recommendation: User modeling and simulation for interactive and conversational systems
• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG

Abstract

Cascaded or stacked intelligent metasurfaces (SIMs) have emerged as a promising technology to overcome the physical limitations of single-layer reconfigurable intelligent surfaces (RISs) in wideband wireless communication. By intelligently manipulating electromagnetic waves, SIMs enhance signal propagation in complex environments and offer additional degrees of freedom for beamforming. This paper proposes a coupling-aware, wideband, circuit-based framework that captures frequency-dependent mutual coupling and wideband channel responses over multiple subbands. Based on this model, we formulate a joint active and passive beamforming design that optimizes the base-station precoder to enable carrier aggregation across frequency-selective subbands, together with metasurface phase shifts, to maximize spectral efficiency. Simulation results reveal the importance of accounting for coupling and wideband effects, and show that performance depends strongly on operating conditions. Single-layer RIS configurations can be favorable in narrowband and/or low-SNR regimes, whereas SIMs can significantly outperform under wideband multi-user conditions by mitigating coupling-induced distortion and maintaining a more consistent phase response across frequencies. The results provide physical insights into design trade-offs between structural simplicity and wideband adaptability, highlighting SIMs as a scalable solution for future-generation wideband multi-user MIMO systems. We further show that partially reconfigurable SIM architectures achieve near-optimal performance with reduced complexity.

Problem

• Reconfigurable Intelligent Surfaces
• Stacked Intelligent Metasurfaces
• Wideband Communication
• Multi-user MIMO
• Mutual Coupling

Innovation

• Stacked Intelligent Metasurfaces
• Wideband MIMO
• Mutual Coupling
• Joint Beamforming
• Carrier Aggregation