Secure Transmission and Self-Energy Recycling With Partial Eavesdropper CSI

This paper focuses on the secure transmission of wireless-powered relay systems with the imperfect eavesdropper channel state information. For efficient energy transfer and information relaying, a novel two-phase protocol is proposed, in which the relay operates in a full-duplex (FD) mode to achieve the simultaneous wireless power and information transmission. Compared with those existing protocols, the proposed design possesses two main advantages: 1) it fully exploits the available hardware resource (antenna element) of relay and can offer a higher secrecy rate and 2) it enables the self-energy recycling (S-ER) at relay, in which the loopback interference generated by the FD operation is harvested and reused for information relaying. To maximize the worst-case secrecy rate (WCSR) through jointly designing the source and relay beamformers coupled with the power allocation ratio, an optimization problem is formulated. This formulated problem is proved to be non-convex and the challenge to solve it is how to concurrently solve out the beamformers and the power allocation ratio. To cope with this difficulty, an alternative approach is proposed by converting the original problem into three subproblems. By solving these subproblems iteratively, the closed-form solutions of robust beamformers and power allocation ratio for the original problem are achieved. Simulations are done and results reveal that the proposed S-ER-based secure transmission scheme outperforms the traditional time-switching based relaying scheme at a maximum WCSR gain of 80%. Results also demonstrate that the WCSR performance of the scheme reusing all antennas for information reception is much better than that of schemes exploiting only one antenna.