Mutual Coupling Impedance Effect on RIS-Assisted Terahertz Communication

Terahertz (THz) communication based on reconfigurable intelligent surfaces (RIS) is a promising technology in future wireless networks, with the capabilities of ultra-high data rates and energy-efficient beam communicating. Nevertheless, the mutual coupling impedance of the closely spaced RIS reveals the inefficacy in existing channel estimation schemes, while it is usually ignored in the literature. In this paper, we fill this gap by developing an EM-compliant channel model that integrates mutual coupling explicitly for RIS-assisted THz systems. We evaluate the performance of mutual impedance-induced influence to the accuracy of the root mean square error (RMSE), which is considered as a pivotal metric and investigate the impact of mutual coupling on the accuracy of channel estimation. By extensive simulations, we show that the ignorance of mutual coupling results in large estimation errors (e.g. RMSE increases by up to 40% with the transmit power of 50 dBm and the element spacing of 0.02 λ), especially in the dense RIS arrays with the sub-wavelength element spacing. We also study fundamental design trade-offs between RIS complexity (e.g. element density) and the estimation accuracy. The above-findings underscore the significance of considering the mutual coupling effects in channel estimation models when targeting reliable and robust RIS-assisted THz systems.