On the Secrecy Capacity of a Full-Duplex Wirelessly Powered Communication System

In this paper, we investigate the secrecy capacity of a wirelesly powered communication system comprised of an energy transmitter (ET), an energy harvesting user (EHU), and a passive eavesdropper (EVE). The ET transmits radio-frequency energy that is used for powering the EHU as well as for generating interference at the EVE. The EHU uses the energy harvested from the ET to transmit confidential messages back to the ET. Both the ET and the EHU operate in the full-duplex (FD) mode. As a result, both the EHU and the ET are subjected to self-interference, which has different effects at the two nodes. In particular, the self-interference impairs the decoding of the received message at the ET, whilst it serves as an additional energy source at the EHU. For this system model, we derive an upper and a lower bound on the secrecy capacity. For the lower bound, we propose a simple achievability scheme. Our numerical results show significant improvements in terms of achievable secrecy rate when the proposed communication scheme is employed compared to its half-duplex counterpart.