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 optimization and, with the advent of beyond diagonal RIS (BD-RIS), to RIS architecture design. However, developing general and unified electromagnetic (EM)-consistent 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 typical 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研究集中于系统优化，并随着超对角RIS（BD-RIS）的出现而扩展至RIS架构设计。然而，为RIS辅助系统建立通用且统一的电磁（EM）一致性模型仍是一个悬而未决的问题。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，我们采用阻抗、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，我们推导出三种能够解释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、完美匹配和无互耦条件，将三种模型简化以理解RIS在通信模型中的作用。推导出的简化模型与相关文献中使用的典型模型具有一致性，但我们指出文献中通常还包含一个额外的近似假设。我们讨论了每种分析方法在RIS表征与优化中的优势，以及如何根据需求选择最合适的参数。数值结果为三种分析方法的等效性提供了进一步的证据。