Secure beamforming and self-energy recycling with full-duplex wireless-powered relay

In this paper, we investigate a two-hop full-duplex wireless-powered relaying system consists of a source, a relay, and a destination in the presence of a passive eavesdropper. The relay assists the transmission of confidential information from the source to the destination, while simultaneously harvesting the energy with time switching scheme by the radio-frequency (RF) signals. Our goal is to maximize the physical-layer security under harvested energy constraints by designing the full-duplex wireless-powered relay, whose two relaying strategies are considered, namely amplify-and-forward (AF) and decode-and-forward (DF). The relay beamforming vector design is proposed for AF protocol, and is jointly optimized with the time ratio parameter in the case of DF protocol to maximize the physical-layer security under harvested energy constraints. Moreover, for the simultaneous energy and secure message transfer at the relay, a two-phase method is provided, which enables the relay to avoid the self-interference caused by full-duplex operation, and also to harvest the energy from the self-interference. The proposed algorithmic solutions leverage the rank relaxation, Majorization-Minimization (MM) programming, and line search method. Numerical results show that the proposed full-duplex relaying system outperforms the half-duplex relaying system in energy harvesting. Moreover, the trade-off between AF and DF protocols according to the occurrence probability of non-zero secrecy rate can be observed in terms of physical-layer security.