Circularly Optimizing Secrecy Rate in RIS-Aided Multiuser Secure Communications

This paper addresses the beamforming design in reconfigurable intelligent surface (RIS)-aided secure communications. RIS is able to manipulate the propagation of electromagnetic waves through properly adjusting the phase of each passive antenna element on the RIS. The passive and programmable nature of RIS makes it attractive to be integrated with the existing infrastructure for enhancing the secrecy. One immediate application is the multicast scenario, where one multi-antenna transmitter wishes to send private information to multiple legitimate users with the aid of an RIS. To concurrently maximize the data rate of legitimate users and minimize information leakage to eavesdroppers, the active beamformer at the sender and the passive beamformer at the RIS need to be jointly designed. We propose a suboptimal strategy to determine the active and passive beamformers in a circular manner for multi-user secure communications. The proposed strategy results in two algorithms. Their achievable secrecy rates are examined and compared with some benchmark schemes through simulations.