STAR-RIS for Reliable Multi-User Networks: Outage and Diversity Analysis

Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is an emerging technology that enables full-space ($\mathbf{3 6 0}$ degrees) coverage on both sides of the surface. To harness the benefits of the dynamic configuration of STAR-RIS while avoiding co-channel interference, we investigate the performance of a multi-user network assisted by STARRIS. In this setup, users are divided into multiple groups, each comprising two users located on opposite sides of the STARRIS. Orthogonal time resources are allocated to each group such that the groups are served sequentially. Based on the Gamma moment matching method, we introduce a Gamma distribution to model the product of Rician, Rayleigh and mixed fading STAR-RIS channels. We then derive exact closed-form expressions for the outage probability and diversity order per user in the proposed system model. Moreover, simulation results are provided to substantiate the analytical derived expressions. Our findings highlight a reliability trade-off associated with the number of grouped users per time slot, STAR-RIS elements, and the user targeted data rates. This balance is crucial for optimizing network performance.