DOA-Based Hybrid Reconfigurable Intelligent Surface Phase-Shift Design for Achievable Rate Enhancement

This study proposes a direction-of-arrival (DOA)-based phase-shift method for hybrid reconfigurable intelligent surface (RIS)-assisted communication systems. The hybrid RIS can estimate the DOA of the incident signal by utilizing its hybrid elements, which can receive a portion of the incident signal and reflect the remainder. Then, the optimal phase shifts of the hybrid RIS are calculated based on the estimated DOA without any phase-shift information feedback. Here, a tradeoff between the DOA estimation accuracy and the signal-reflecting power can be balanced by optimizing the sensing factor of the hybrid RIS, i.e., the power ratio of the received (or sensing) signal to the total incident signal at the hybrid element. To this end, the optimal sensing factor that maximizes the effective ergodic achievable rate is analytically derived. The simulation results demonstrate that the proposed hybrid RIS system with optimal sensing factor outperforms both conventional passive RIS systems and channel-estimation-based hybrid RIS systems when the number of RIS elements is large, and the line-of-sight channel is dominant.