On the placement of RF energy harvesting node in wireless networks with secrecy considerations

The potential dual use of radio-frequency (RF) signals for carrying energy and information brings an exciting opportunity for energy harvesting (EH) from ambient RF signals in wireless communication networks. To maximize the efficiency of harvesting wireless energy, it is desirable to have the EH node located close to the transmitter. However, when the transmitted information is confidential, the EH node should be regarded as a potential eavesdropper, and hence, it is preferred to have the EH node located far from the transmitter. Therefore, the placement of EH nodes in wireless networks becomes an interesting problem when secrecy is an important consideration. In this paper, we investigate the optimal placement of the EH node with physical-layer security considerations by formulating and solving two optimization problems. The first problem maximizes the average EH power subject to a secrecy outage constraint, while the second problem minimizes the secrecy outage probability subject to an EH constraint. Our results also demonstrate the tradeoff between secrecy and EH performances caused by the placement of the EH node.