An Equivalence Principle-Based Hybrid Method for Propagation Modeling in Radio Environments With Reconfigurable Intelligent Surfaces

Most existing methods for analyzing reconfigurable intelligent surface (RIS) channels are limited to free-space links and do not account for the mutual coupling of unit cells, or they fail to accurately model several types of realistic scenarios including near-field operation and interaction of RISs with rich multipath environments. Alternatively, some methods that can handle these scenarios are computationally intensive, such as pure full-wave analysis. We present an equivalence principle-based hybrid ray-tracing/full-wave method to model wave propagation in wireless communication channels enabled by RISs. This method uses ray-tracing to determine the incident waves on an RIS, then applies full-wave analysis to model the response of the RIS to these waves. Next, based on the equivalence principle, we introduce equivalent surface electric and magnetic current densities that generate the scattered fields produced by RIS. These equivalent sources are integrated with ray-tracing to derive the site-specific propagation model of the RIS channel. The proposed method readily accounts for multiple incident waves on an RIS, enabling the accurate analysis of propagation channels with RISs, with receivers in both the near and the far region of an RIS. We show that the accuracy of our method is comparable to that of full-wave analysis, through simple examples that are manageable by the finite-element method. Also, we experimentally validate the proposed technique by comparing simulation and measured data for an actual indoor radio environment with an anomalous reflection metasurface.