  This paper focuses on the secure transmission of wireless-powered relay
systems with imperfect eavesdropper channel state information (ECSI). For
efficient energy transfer and information relaying, a novel two-phase protocol
is proposed, in which the relay operates in full-duplex (FD) mode to achieve
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 higher secrecy rate; 2) it enables self-energy recycling (S-ER) at
relay, in which the loopback interference (LI) generated by 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 (TSR) scheme at a
maximum WCSR gain of 80%.Results also demonstrate that the WCSR performance of
the scheme reusing idle antennas for information reception is much better than
that of schemes exploiting only one receive antenna.
