Modelling of Wireless Links with Reconfigurable Intelligent Surfaces Using Multiport Network Analysis

Reconfigurable intelligent surfaces (RISs) are an emerging technology for engineering the channels of future wireless communication systems. The vast majority of research publications on RIS are focusing on system-level optimization and are using very simple models. Most of these models are based on the idea, that the reactive lossless terminations of the RIS antenna elements will only change the phase of the signals, that are received and propagated forward to the final destination, without changing the amplitude. There are only few publications with a focus on physically modeling using multiport network analysis. Some are based on impedance parameters and some on scattering parameters. In the scattering parameter approach the idea of changing only the phases is still carried on, while the impedance parameter approach shows a different result. We will show that even with a very simple model based on isotropic radiators, ignoring any mismatch, mutual coupling, and losses this idea of changing the phases only cannot be true. We will demonstrate how this misconception can be resolved by correctly mapping the impedance parameters to scattering parameters and vice versa. A careful interpretation of the different parameters shows the equivalence of the different models. With some simple scenarios based on deterministic LOS channels as well as on Rayleigh fading channels from RIS to the user, the importance of the careful use of the described models becomes apparent.