RIS Analysis from Communication and Electromagnetic Perspectives

The next generation of wireless communication networks will facilitate the connection of a large number of devices and a broad range of services. Serving such a large amount of user equipment (UEs) can be of very high cost if using active antenna solutions such as increasing the number of base stations (BSs using massive multiple-input multiple-output (MIMO) antennas, and deploying relays between the BSs and the UEs. In this context, a passive antenna solution, such as reconfigurable intelligent surfaces (RISs), would be more beneficial and attractive. RIS has become an emerging technology with diverse applications in future wireless networks, owing to its ability to dynamically control and optimize the propagation environment. The rapid evolution driven by escalating performance demands of coverage in blocked line-of-sight (LOS) scenarios has prompted the exploration of RIS. Motivated by the potential benefits of RIS in enhancing signal coverage and improving energy efficiency while maintaining very low costs, this dissertation studies the RIS technology in a broad range from software development to hardware design, from simulation to measurements, and from electromagnetic theory to communication theory. The dissertation consists of three major parts corresponding to RIS modeling and applications in communication and electromagnetic aspects, as well as the verification of RIS modeling methods. In the first part, we focus on RIS modeling in communication aspects and implement an RIS module in an open-access system-level simulator (SLS) integrated with a ray tracer. The RIS module involves most of the system-level challenges of RIS technology, especially large-scale fading and small-scale fading modeling. The RIS-tailored SLS is able to simulate and optimize RIS-empowered single-cell or multi-cell wireless networks in realistic environments. In addition, we develop several RIS phase shift optimization algorithms, such as maximum ratio transmission (MRT), zero-forcing (ZF), and max-min signal-to-interference-plus-noise ratio (SINR)-based approaches, to maximize the minimum received power, SINR, and rate for RIS-assisted multi-user MIMO (MU-MIMO) systems. The RIS phase quantization effects are investigated within these approaches, too. In the second part, we propose a static scatterer-based approach to improve signal propagation and coverage with passive resonate dipoles. We use theoretical models and ray tracing simulations to evaluate the contributions from such scatterers and investigate several influential factors, such as reflection material, reflection number, frequency, and the number of dipoles in indoor and outdoor scenarios. Results reveal that the contributions from such scatterers are insignificant when the direct link between the transmitting (Tx) and the receiving (Rx) antennas is not blocked.