Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface Aided RSMA Communications: Outage Probability Analysis

This paper investigates the use of simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) to assist the communication with multiple devices distributed according to a Poisson point process. First, using rate-splitting multiple access (RSMA) transmission scheme, the downlink STAR-RIS multi-user system is studied, and its fundamental statistics are provided. Then, by leveraging stochastic geometry tools, and adopting an approach based on the cumulative distribution function, the outage probability is obtained for the two types of devices, i.e., the ones in the transmission zone and those in the reflection zone. For such, the distributions of the channel gains and the devices’ distances are also characterized. The closed-form expressions with Meijer-G functions for the outage probability are provided in two cases for the STAR-RIS operation, energy splitting and mode switching. Simulation results and comparisons are provided, and the impact of various system parameters on the outage performance is analyzed. In particular, it is shown that as the RIS size, the RSMA power splitting factor, and the devices’ density, increase, the STAR-RIS aided RSMA helps achieve enhanced performance in both operation cases, i.e., energy splitting and mode switching.