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Transmit Power Optimization of Simultaneous Transmission and Reflection RIS Assisted Full-Duplex Communications

Reconfigurable intelligent surface (RIS) has been proven to be a promising technology for improving the performance of future wireless systems. However, since conventional RIS can only reflect or transmit (i.e., refract) incident signals, it limits the applications of RISs. Fortunately, a novel simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) is proposed to improve the convenience of communication. By altering the electromagnetic properties of the STAR-RIS elements with a smart controller, it can split the incident signal into transmission and reflection signals, achieving 360° coverage. This work demonstrates the effectiveness of STAR-RIS aided full-duplex (FD) communication system, where a FD base station (BS) communicates with an uplink (UL) user and a downlink (DL) user simultaneously over the same time-frequency dimension assisted by a STAR-RIS. The objective is to minimize the total transmit power subject to the given minimal data rate requirement. We decouple the original problem into power optimization and STAR-RIS passive beamforming subproblems and adopt the alternating optimization (AO) framework to solve them iteratively. Specifically, in each iteration, we derive the closed-form expression for the optimal power design, then use the successive convex approximation (SCA) method and semidefinite program (SDP) to solve the passive beamforming optimization subproblem. Simulation results verify the convergence and effectiveness of the proposed algorithm and further reveal the performance gain compared with the half-duplex (HD) and conventional RIS.