Spectral–Energy Efficiency Tradeoff of Nearly-Passive RIS in MIMO URLLC Downlink: Diagonal Versus Beyond Diagonal

This letter investigates the spectral and energy efficiency (EE) tradeoff of nearly-passive (NP) reconfigurable intelligent surfaces (RIS). Locally and globally NP (GNP) RIS are analyzed, considering diagonal and beyond-diagonal (BD) implementations in multi-user multiple-input multiple-output (MU-MIMO) broadcast channels designed for ultra-reliable low-latency communication (URLLC). We demonstrate that all RIS architectures enhance the spectral efficiency with GNP BD-RIS achieving the highest gains. However, its EE is highly sensitive to the static circuit power consumption, since BD-RIS has many more circuit elements than diagonal architectures. Furthermore, we demonstrate that the benefits of BD-RIS over diagonal RIS diminish as the number of data streams per user increases due to the enhanced channel diversity in MIMO systems.