Secure communication via an energy-harvesting untrusted relay with imperfect CSI

Abstract

This paper studies the secure communication of an energy-harvesting system in which a source communicates with a destination via an amplify-and-forward (AF) untrusted relay. The relay uses the power-splitting policy to harvest energy from wireless signals. The source is equipped with multiple antennas and uses transmit antenna selection (TAS) and maximum ratio transmission (MRT) to enhance the harvested energy at the relay; for performance comparison, random antenna selection (RAS) is examined. The relay and destination are single-antenna nodes. To create a positive secrecy capacity, destination-assisted jamming is deployed. Because the use of multiple antennas can cause the imperfect channel state information (CSI), the channel between the source and the relay is examined in two cases: perfect CSI and imperfect CSI. To evaluate the secrecy performance, analytical expressions for the secrecy outage probability (SOP) and the average secrecy capacity (ASC) for the TAS, MRT, and RAS schemes are derived. Moreover, a high-power approximation for the SOP is presented. The accuracy of the analytical results is verified by Monte Carlo simulations. The results show the benefit of using multiple antennas in improving the secrecy performance. Specifically, MRT performs better than TAS, and both of them outperform RAS. Moreover, the results provide valuable insight into the effects of various system parameters, such as the channel correlation coefficient, energy-harvesting efficiency, secrecy rate threshold, power-splitting ratio, transmit powers, and locations of the relay, on the secrecy performance.