Abstract
This work investigates simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) in multiple-access (MA) bidirectional network architecture. The proposed optimization framework incorporates two practical operational protocols of STAR-RIS to optimize the channel gains for users and obviate the necessity for uplink power control while adhering to specified quality of service (QoS) constraints. The proposed approach undergoes thorough evaluation under key optimization problems: QoS feasible region, and energy efficiency. The closed-form solutions are validated through simulations, showcasing the notable advantages that STAR-RIS can provide to the considered multiple-access networks. Simulation findings revealed that in the context of the proposed system model, the mode switching approach can attain a superior QoS threshold rate compared to the energy splitting mode. The proposed framework has demonstrated its ability to meet bidirectional communication needs with a high degree of accuracy and precision.