Performance Analysis and Enhancement through Cooperative NOMA in STAR-RIS-aided Systems

In this work, we investigate the performance of the cooperative non-orthogonal multiple access (CNOMA) downlink wireless communication system assisted by the newly emerged technology of simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs). Specifically, spectral and energy efficiency metrics (SE and EE) are used as the performance metric. The users' ergodic rates were approximated with the upper bounds defined by their derived closed-form expressions. Further, extensive Monte-Carlo simulations confirmed the remarkable tightness of the bounds. In addition, to inspect the usefulness of CNOMA in the proposed communication system, the comparison with its NOMA counterpart was incorporated into the numerical results. Thus, it was found that the cooperative link can further enhance SE and EE of the STAR-RIS-based NOMA network.