Energy-efficient design for hybrid RIS transmitter enabled multi-user communications

A novel downlink hybrid reconfigurable intelligent surface (RIS) transmitter enabled multi-user communication framework is studied. Specifically, elements on the RIS can flexibly switch between active and passive modes to deliver information to multiple users. The system energy efficiency is maximized by jointly optimizing RIS element mode scheduling, transmission beamforming vector, and power allocation coefficients, subject to the user's individual rate requirement and the maximum RIS amplification power constraint. We first exploit the Dinkelbach approach to transform the original mixed-integer nonlinear programming problem into a nonfractional optimization problem. Then, an alternating optimization based algorithm is developed to address this problem. In particular, the optimal RIS element operating mode is determined by the exhaustive search method. Then, the RIS beamforming and power allocation coefficients are alternately designed. Finally, numerical results show that a significant performance improvement can be reaped by the proposed scheme compared to the baseline schemes employing full-active RIS or full-passive RIS.