Cooperative beamforming for double STAR-RIS-assisted uplink NOMA system

Simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) and nonorthogonal multiple access (NOMA) are two highly promising candidate technologies for sixth-generation (6G) wireless communication systems and have garnered considerable attention from academia and industry over the past several years. In this work, a novel double-STAR-RIS-assisted uplink NOMA system is put forward, with the aim of minimizing transmit power under the constraints of phase shift matrices and minimum quality of service (QoS). To settle this optimization problem, an alternating optimization (AO) algorithm is put forward. First, the transmit power equation is obtained by deriving the formula pertaining to the QoS. Next, the transmit power minimization problem is split into several subproblems, and the optimal power allocation and passive beamforming are obtained via iteration. Last but not least, the simulation results indicate that the proposed algorithm is feasible, effective, and converges quickly, revealing that compared to conventional orthogonal multiple access (OMA) methods, the proposed scheme can decrease the total transmit power.