  Reconfigurable Intelligent Surfaces (RISs) have attracted major attention in
the last few years, thanks to their useful characteristics. An RIS is a nearly
passive thin surface that can dynamically change the reradiated field, and can
therefore realize anomalous reflection, refraction, focalization, or other wave
transformations for engineering the radio propagation environment or realizing
novel surface-type antennas. Evaluating the performance and optimizing the
deployment of RISs in wireless networks need physically consistent frameworks
that account for the electromagnetic characteristics of dynamic metasurfaces.
In this paper, we introduce a general macroscopic model for evaluating the
scattering from an RIS. The proposed method decomposes the wave reradiated from
an RIS into multiple scattering contributions and is aimed at being embedded
into ray-based models. Since state-of-the-art ray-based models can already
efficiently simulate specular wave reflection, diffraction, and diffuse
scattering, but not anomalous reradiation, we enhance them with an approach
based on Huygens' principle and propose two possible implementations for it.
Multiple reradiation modes can be modeled through the proposed approach, using
the power conservation principle. We validate the accuracy of the proposed
model by benchmarking it against several case studies available in the
literature, which are based on analytical models, full-wave simulations, and
measurements.
