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.

翻译：级联或堆叠智能超表面(SIMs)已成为克服单层可重构智能表面(RISs)在宽带无线通信中物理局限性的有前景技术。通过智能调控电磁波，SIMs增强了复杂环境中的信号传播，并为波束成形提供了额外的自由度。本文提出一种耦合感知的宽带电路建模框架，该框架能刻画多子带间频率相关的互耦效应与宽带信道响应。基于此模型，我们构建了联合有源与无源波束成形设计方案，通过优化基站预编码器以实现跨频率选择性子带的载波聚合，同时结合超表面相位调控，以最大化频谱效率。仿真结果揭示了考虑耦合效应与宽带特性的重要性，并表明性能表现强烈依赖于工作条件。单层RIS配置在窄带和/或低信噪比场景下可能更具优势，而SIMs在宽带多用户条件下通过抑制耦合引起的失真并维持更稳定的跨频段相位响应，能够显著超越单层RIS。这些结果为结构简洁性与宽带适应性之间的设计权衡提供了物理层面的见解，凸显了SIMs作为面向新一代宽带多用户MIMO系统的可扩展解决方案的潜力。我们进一步证明，部分可重构的SIM架构能以较低复杂度实现接近最优的性能。