End-to-End Mutual Coupling Aware Communication Model for Reconfigurable Intelligent Surfaces: An Electromagnetic-Compliant Approach Based on Mutual Impedances

Abstract

Reconfigurable intelligent surfaces (RISs) are an emerging technology for application to wireless networks. We introduce a physics and electromagnetic (EM) compliant communication model for analyzing and optimizing RIS-assisted wireless systems. The proposed model has four main notable attributes: (i) it is end-to-end, i.e., it is formulated in terms of an equivalent channel that yields a one-to-one mapping between the voltages fed into the ports of a transmitter and the voltages measured at the ports of a receiver; (ii) it is EM-compliant, i.e., it accounts for the generation and propagation of the EM fields; (iii) it is mutual coupling aware, i.e., it accounts for the mutual coupling among the sub-wavelength unit cells of the RIS; and (iv) it is unit cell aware, i.e., it accounts for the intertwinement between the amplitude and phase response of the unit cells of the RIS.

Wireless Fingerprinting Localization in Smart Environments Using Reconfigurable Intelligent Surfaces

“…Importantly and unlike all other WFL publications in the literature to date, we do not use a pathloss based channel propagation model or fading model (e.g., Friis and Rayleigh) because such models cannot accurately capture the EM coupling effects caused by the RIS elements (e.g., (beamform, scatter, null) that can completely change the radio map spatial power distribution. Instead, we use a recently proposed end-to-end model [24] based on impedance coupling of thin wire antennas which allows us to accurately test and benchmark our proposed solutions while also gaining realistic engineering insights into the design capabilities and challenges related to RISenhanced WFL methods.…”

“…2 illustrates four representative example of radio maps corresponding to four different configurations of the RIS in an indoor space generated using our simulation approach described in Sec. IV [24]. Thus, by changing the configuration of the RIS, i.e., the load impedance of the dipoles thus applying a phase shift to the reflected EM waves, one can get a much more diverse set of radio maps, i.e., a high dimensional fingerprint by using just one AP and one RIS.…”

“…We will use the impedance based model developed in [24] to obtain simulated RSSI values at the MU. While the full power of this model is not required for our simplified WFL setting (see Fig.…”

“…3 shows the configuration modeled by ( 5), the single antenna transmitter T models the AP, the single antenna receiver R models the MU, and the multi-element surface S models the RIS. A detailed discussion in [24,Corollary 1. ] shows that the channel transfer function between AP and MU through the RIS can be found by an order reduction procedure, which yields…”

“…In this section we will construct two heuristic state selection (HSS) estimators to help compute a good RIS configuration set of size M Q without the need for a training database while only using the radio simulation tools developed in Sec. IV and [24]. We refer to the first estimator as the naive estimator (HSS-1) since it simply attempts to choose RIS combinations which produce maximally different radio maps.…”