Simultaneous Secure and Covert Communications With Reconfigurable Intelligent Surface and Artificial Noise: An Energy-Efficient Design

In this letter, we investigate a reconfigurable intelligent surface (RIS) assisted secure and covert communication system, consisting of a transmitter (Alice), a receiver (Bob), a RIS, a warden (Willie) and an eavesdropper (Eve). Specifically, Willie attempts to detect the existence of the signals transmitted from Alice and Eve attempts to decode the confidential information. To enhance the security of confidential transmissions between Alice and Bob, we propose a RIS and artificial noise (AN) assisted secrecy energy efficiency (SEE) maximization scheme, namely RIS-AN-SEEM, in which Alice transmits confidential messages to Bob in an opportunistic manner while continuously transmits AN to confuse Willie and Eve. With the covertness constraint that the detection accuracy at the warden is less than a given tolerable level, a SEE maximization problem is formulated to jointly optimize the signal power, AN power, and RIS phase shifts. Since the formulated problem is non-convex, an alternating optimization algorithm is developed with the assistance of successive convex approximation and Dinkelbach methods. Simulation results reveal that the proposed RIS-AN-SEEM scheme outperforms RIS and AN assisted SEE maximization scheme based on power allocation factor optimization (RIS-AN-SEEM-PA), RIS assisted SEE maximization (RIS-SEEM) scheme and AN assisted SEE maximization (AN-SEEM) scheme in terms of the SEE.