Secrecy capacity analysis of untrusted relaying energy-harvesting systems with hardware impairments

In this paper, we study the impact of hardware impairments, which can act as one of the factors that cause degradation in the performance of communication systems, on the secrecy capacity of an untrusted relaying wireless energy-harvesting (WEH) system. In the system, the energy-constrained relay is an untrusted node which can overhear the source's confidential signal while assisting the source-destination communication. The relay operates in the amplify-and-forward (AF) mode and uses the power-splitting (PS) protocol for harvesting energy. The destination sends an artificial noise (AN) signal during the source-relay communication. The AN signal acts as an additional energy source and an interference source at the relay. In our study, we derive an approximation of the average secrecy capacity (ASC) for the high-power-regime approximation in order to evaluate the secrecy performance of the proposed system, which is also the upper bound for the ASC. The analytical results are confirmed via Monte Carlo simulations. The numerical results provide valuable insights into the effect of the various system parameters, such as the power-splitting ratio, the relay's location, the trade-off between the source's power and the destination's power, and the level of hardware impairments, on the secrecy performance.