Beamforming design for max-min SINR in RIS-based hybrid relaying

Reconfigurable intelligent surface (RIS) technology is regarded as one of the important technologies for future 6G mobile communication networks. However, RIS can only passively reflect signals, rather than actively process received signals like the traditional relay. Therefore, a hybrid decode-and-forward (DF) relay and RIS-assisted multiple-input single-output (MISO) multiuser downlink communication system is considered to combine the advantages of both. Then the beamforming at the base station (BS), the DF relay, and the phase vector of the RIS elements is jointly designed, aiming at achieving max-min signal-to-interference-plus-noise ratio (SINR). Due to the non-convexity caused by coupling between the beamforming vectors, the optimization problem is split into three semidefinite relaxation (SDR) sub-problems, and then the alternate optimization method is adopted to solve the non-convex problem. The numerical results verify the effectiveness and superiority of the proposed optimization algorithm in improving system performance compared with other benchmark schemes.