Secure transmission in wireless powered cooperative communication network with finite energy storage

This paper investigates secure transmission in a wireless powered cooperative communication network (WPCCN), where multiple intermediate energy harvesting (EH) nodes are deployed to assist the secure transmission between a source and a destination in the presence of an eavesdropper. Each intermediate EH node has a finite energy storage (battery) to accumulate the energy harvested from the source's radio frequency (RF) signal. In order to exploit the intermediate nodes for secure improvement, a relay-and-jammer selection scheme called energy threshold based best-relay and bestjammer (ETBB) is proposed according to the energy storage status and channel gains. Specifically, one node is selected as relay to forward the source's confidential signal, and another node is selected to jam the eavesdropper. Moreover, for evaluating the secrecy performance, the evolution of the EH nodes' energy storage has to be tracked firstly since the residual energy at each EH node is a critical factor that affects the transmit power. Using finite-state Markov Chain (FSMC), we study the long-term energy evolution at the EH nodes. Based on this, we derive the analytical expression for the hybrid outage probability (HOP) of the considered WPCCN. Finally, simulation results reveal that the ETBB scheme outperforms the existing bR-bJ scheme due to the consideration of the energy accumulation issue and the energy storage status at the EH nodes. And, there is a suitable value of the energy threshold for the ETBB scheme to achieve the optimal secrecy performance.