Exploiting Reconfigurable Intelligent Surface-Based Uplink/Downlink Wireless Systems

Abstract

This paper analyzes the performance of an uplink/downlink reconfigurable intelligent surface (RIS)-based wireless system with a multiple-antenna base station (B), where the RIS selection strategy is considered to alleviate the overhead and resources. With the goal of enhancing system performance, we consider maximal-ratio-combining (MRC) and selection-combining (SC) for the uplink transmission and maximum-ratio-transmission (MRT) along with beamforming design for the downlink transmission, where two methods of direct-path beamforming design (DBD) and reflective-path beamforming design (RBD) are proposed. We also quantify the impact of uncertain phase shift (UPS) and optimal phase shift (OPS) alignments. Accordingly, closed-form expressions for outage probability (OP) of each uplink and downlink scenario are derived. Numerical results show that, in the uplink transmission, adopting MRC at the B and OPS at RIS provides the best performance. For small antenna settings, using SC-enabled OPS provides outstanding performance when compared to employing MRC-integrated UPS. In downlink transmission, RBD achieves better performance than that of DBD. Similar to the uplink transmission, employing MRT at the B and OPS at the RIS also attains superior performance when compared to UPS.