Energy Efficiency of Non-Diagonal RIS-Aided Wireless Communication Systems

Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising technology for enhancing wireless communication by dynamically controlling the propagation environment. Recently, a non-diagonal RIS architecture has been proposed, enabling more advanced signal manipulation by allowing signals impinging on one element to be reflected from another element after appropriate phase-shift adjustment. This paper analyzes the energy efficiency of non-diagonal RIS-assisted wireless communication systems in high- and low-signal-to-noise-ratio (SNR) regime. We derive closed form expressions of the spectral and energy efficiency for both the non-diagonal and its diagonal counterpart, which is used as a benchmark for comparison. Simulation results reveal that non-diagonal RIS systems are the preferred choice for communication systems that prioritize spectral efficiency. Interestingly, for energy efficiency, the selection between diagonal and non-diagonal RIS architectures depends on the received SNR conditions, with diagonal RIS systems excelling at high SNR and non-diagonal RIS systems performing better at low SNR scenarios.