Stacked intelligent metasurface (SIM) has emerged as a technology enabling wave domain beamforming through multiple stacked reconfigurable intelligent surfaces (RISs). SIM has been implemented so far with diagonal RIS (D-RIS), while SIM implemented with beyond diagonal RIS (BD-RIS) remains unexplored. Furthermore, a model of SIM accounting for mutual coupling is not yet available. To fill these gaps, we derive a physically consistent channel model for SIM-aided systems and clarify the assumptions needed to obtain the simplified model used in related works. Using this model, we show that 1-layer SIM implemented with BD-RIS achieves the performance upper bound with limited complexity.

翻译：堆叠智能超表面（SIM）是一种通过多个堆叠可重构智能表面（RIS）实现波域波束赋形的新兴技术。目前SIM均采用对角线RIS（D-RIS）实现，而基于超越对角线RIS（BD-RIS）的SIM实现方案尚未被探索。此外，现有研究尚未建立考虑互耦效应的SIM模型。为填补这些空白，本文推导了SIM辅助系统的物理一致性信道模型，并阐明了相关工作中简化模型所需的前提假设。基于该模型，我们证明采用BD-RIS实现的单层SIM能以有限复杂度达到性能上界。