Analysis and Optimization of Reconfigurable Intelligent Surfaces Based on S-Parameters Multiport Network Theory

In this paper, we consider a reconfigurable intel-ligent surface (RIS) and model it by using multiport network theory. We first compare the representation of RIS by using the impedance and scattering parameters, by proving their equivalence and discussing their distinct features. Then, we develop an algorithm for optimizing the RIS configuration in the presence of electromagnetic mutual coupling. We show that the proposed algorithm based on optimizing the scattering parame-ters results in better performance than existing algorithms based on optimizing the impedance parameters. This is attributed to the fact that small perturbations of the step size of the proposed algorithm result in larger variations of the scattering parameters, hence increasing the convergence speed of the algorithm.