Dilki Wijekoon, Amine Mezghani, George Alexandropoulos, Ekram Hossain
Mutual Coupling (MC) emerges as an inherent feature in Reconfigurable Intelligent Surface (RIS) structures, particularly when they are fabricated with sub-wavelength inter-element spacing. Hence, their realistic modeling and efficient optimization need to accurately incorporate MC-induced effects. In addition, the design of electromagnetics-compliant transmit/receive radiation patterns constitutes another critical factor for efficient RIS operation. These radiation patterns together with MC naturally lead to the emergence of non-local RIS structures, whose operation can be effectively described via non-diagonal phase configuration matrices. In this paper, we present a physically-consistent joint optimization framework for the MC and the radiation patterns of non-local RIS structures for the case of RIS-assisted multi-user Multiple-Input Single-Output (MISO) communication systems. Both conventional reflective as well as transmissive RIS setups are considered. Assuming the availability of statistical properties of the wireless environment for the targeted RIS deployment, we particularly devise a novel offline optimization approach for the static scattering S-parameters of the RIS, which is followed by a dynamic, per-channel-realization optimization of the metasurface’s response-tunable elements and the transmitter’s active precoder. Our extensive simulation results, using both parametric and geometric channel models, showcase the validity of the proposed two-step optimization framework over benchmark schemes, indicating that improved performance can be achievable without the need for optimizing the MC and the radiation patterns of the RIS on the fly, which can be rather cumbersome.