Secure Energy-Efficient Multi-RIS-Aided SWIPT Networks

This paper considers a SWIPT network in which an access point (AP) serves multiple information decoding receivers (IDRs) and energy harvesting receivers (EHRs) assisted by several dynamically controlled reconfigurable intelligent surfaces (RIS). An energy efficiency (EE) maximization problem is formulated by the joint optimization of the transmit beamforming, artificial noise (AN) covariance, the passive beamforming at the RISs, and the RIS on/off control mechanism. Due to the non-convexity of the problem, semi-definite relaxation (SDR) is utilized to simplify the problem. A practical solution based on alternation optimization is proposed to obtain the suboptimal solution. Furthermore, a low-complexity greedy search method is proposed for the RIS on/off control. Simulation results show that the EE is significantly enhanced by employing dynamic control of multiple RIS with AN. In addition, the effect of increasing the circuit power and the number of RISs can be harmful to the system-wide EE.