Abstract: As an emergent reconfigurable intelligent surface (RIS) architecture, simultaneous transmitting and reflecting RIS (STAR-RIS) can overcome the non-transition issue of the traditional RIS, which is expected to augment omni coverage in future sixth-generation (6G) networks. Motivated by this, a new joint beamforming optimization problem for STAR-RIS assisted multi-user multi-input single-output (MISO) secure communication systems is proposed. Specifically, considering the energy splitting protocol, we aim to jointly design the beamforming strategy at the base station (BS) and the transmission-reflection coefficients at the STAR-RIS for minimizing the total transmit power of the BS, subject to user service and secure communication requirements constraints. However, the objective function is non-convex over the optimization variables, which are highly coupled. To address this, an algorithm based on alternating optimization and semidefinite relaxation successive is proposed. Simulation results show that the proposed scheme is superior to other benchmark schemes in optimizing the power consumption while ensuring communication security.

Key words: intelligent reflecting surface, energy splitting protocol, secure communication; beamforming, alternative optimization