Secure Beamforming Design for Reconfigurable Intelligent Surface Assisted Downlink Transmission

Reconfigurable intelligent surface (RIS) is a promising technology to reconfigure the wireless channel, which can be used for signal modulation with low hardware cost. In this paper, the problem of resource allocation is investigated for an RIS assisted wireless communication system with one eavesdropper, where the RIS acts as a transmitter. The secure energy efficiency of the considered system is maximized via jointly optimizing the transmit power and phase beamforming of the RIS. To solve this problem, an alternating algorithm is proposed with optimizing transmit power and phase beamforming iteratively. With given transmit power, we apply the Schmidt orthogonalization to the channel and use the Dinkelbach method to iteratively derive the optimal phase beamformer by maximizing the secure energy efficiency. After solving the phase beamformer, we substitute the derived phase shifter to the previous optimization problem. By using the Dinkelbach method and analysing the solutions of the quadratic equation, we derive the optimal transmit power in closed form. Simulation results show that our proposed algorithm yields higher secure energy efficiency compared to the conventional schemes.