RIS-Aided Secure Energy-Efficiency Maximization under Uncertain CSI

Reconfigurable intelligent surface (RIS) has the revolutionary ability to customize the radio propagation environment for enhancing the secure transmission performance. However, due to the passive nature of eavesdroppers and the cascaded channel brought by the RIS, the channel state information (CSI) is imperfectly obtained at the base station, leading to uncertain CSI. Under channel uncertainty, the optimal phase-shift, power allocation, and transmission rate design for secure transmission is currently unknown due to the difficulty of handling the probabilistic constraint with coupled variables. To fill this gap, this paper investigates the energy efficient secure transmission while incorporating the probabilistic constraint. By transforming the probabilistic constraint and decoupling the variables, the secure energy-efficiency maximization problem can be solved via alternatively executing the concave-convex procedure and penalty-based method. Simulation results show that the proposed RIS-aided secure transmission scheme significantly improves the energy-efficiency compared to baseline schemes of random phase-shift, fixed phase-shift, and RIS ignoring CSI uncertainty.