Reconfigurable intelligent surface (RIS) is an emerging paradigm able to control the propagation environment in wireless systems. Most of the research on RIS has been dedicated to system-level optimization and, with the advent of beyond diagonal RIS (BD-RIS), to RIS architecture design. However, developing general and unified electromagnetic (EM)-compliant models for RIS-aided systems remains an open problem. In this study, we propose a universal framework for the multiport network analysis of RIS-aided systems. With our framework, we model RIS-aided systems and RIS architectures through impedance, admittance, and scattering parameter analysis. Based on these analyses, three equivalent models are derived accounting for the effects of impedance mismatching and mutual coupling. The three models are then simplified by assuming large transmission distances, perfect matching, and no mutual coupling to understand the role of the RIS in the communication model. The derived simplified models are consistent with the model used in related literature, although we show that an additional approximation is commonly considered in the literature. We discuss the benefits of each analysis in characterizing and optimizing the RIS and how to select the most suitable parameters according to the needs. Numerical results provide additional evidence of the equivalence of the three analyses.

翻译：可重构智能表面（RIS）是一种能够控制无线系统传播环境的新兴范式。目前大多数RIS研究集中在系统级优化，以及随着超越对角线RIS（BD-RIS）的出现，在RIS架构设计方面展开。然而，开发普适且统一的电磁兼容模型，以分析RIS辅助系统，仍是一个开放性问题。本研究提出了一种用于RIS辅助系统多端口网络分析的通用框架。通过该框架，我们利用阻抗、导纳和散射参数分析对RIS辅助系统及RIS架构进行建模。基于这些分析，推导出三种考虑阻抗失配和互耦效应的等效模型。随后，通过假设大传输距离、理想匹配和无互耦条件，对三种模型进行简化，以理解RIS在通信模型中的作用。所推导的简化模型与相关文献中的模型一致，但我们指出文献中通常采用了额外的近似假设。我们讨论了每种分析在表征和优化RIS方面的优势，以及如何根据需求选择最合适的参数。数值结果进一步验证了三种分析的等效性。