Weighted Sum-Rate Maximization for Collaborative-RIS-Assisted Downlink MISO System

In this letter, a weighted sum-rate (WSR) maximization problem of cooperative reconfigurable intelligent surface (RIS) assisted downlink multiple-input-single-output (MISO) system is studied. A wireless communication framework that combines both fixed and mobile RISs is proposed for the first time. The optimization problem of maximizing the WSR is addressed by jointly devising the active beamforming and the dual-RIS passive beamforming in an alternating manner. Specifically, the active beamformer at the BS is solved by using the weighted minimum mean square error (WMMSE) method, while the passive beamforming of the RIS is achieved through the sequence phase-rotation (SPR) or the semi-definite relaxation (SDR) methods. Simulation result demonstrates that the proposed SPR method achieves the best performance, and outperforms several benchmark schemes. In addition, the WSR in the near-field performs better than that in the far-field.