Secrecy Energy Efficiency Optimization in AN-Aided Distributed Antenna Systems With Energy Harvesting

In this paper, the artificial-noise (AN)-aided downlink distributed antenna system with simultaneous wireless information and power transfer is considered, in which the legitimate user uses the power-splitting (PS) scheme to simultaneously decode information and harvest energy. To prevent eavesdroppers from wiretapping the legitimate user, AN is transmitted with the confidential signal at the transmitter. The secrecy energy efficiency maximization (SEEM) problem is studied under the constraints of the minimum secrecy rate requirement, minimum required harvested energy, total radio frequency transmission power, and range of the PS ratio. Because of the coupling of variables, the formulated SEEM problem is non-convex and intractable. Then, a double-layer iterative optimization algorithm is proposed. The outer layer is a 1-D search algorithm for the introduced tight relaxation variable, whereas the inner one is based on the Dinkelbach method to address the fractional optimization problem. The closed-form expressions for the power of the transmitted confidential signal and AN power are further derived. The simulation results reveal the efficiency of the proposed algorithm and the advantage of AN in improving the system secrecy energy efficiency.