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）作为一种通过多层堆叠可重构智能表面（RIS）实现波束成形的技术已崭露头角。目前，SIM仅采用对角RIS（D-RIS）实现，而基于超越对角RIS（BD-RIS）的SIM尚未得到探索。此外，目前尚缺乏考虑互耦效应的SIM模型。为填补这些空白，我们推导了SIM辅助系统的物理一致性信道模型，并阐明了相关工作中获得简化模型所需的假设条件。利用该模型，我们证明采用BD-RIS实现的单层SIM能够以有限复杂度达到性能上限。