Relay-Selection Refinement Scheme for Secrecy in Untrusted RF-EH Relay Systems with Adaptive Transmission under Outdated CSI

In this paper, we investigate physical layer security of a dual-hop untrusted multiple amplify-and-forward energy-harvesting relaying network. Two main objectives are considered: saving the relays' energies and achieving a target secrecy rate. In the presence of direct links between the source and destination nodes, for the first design objective, we propose an adaptive switching scheme between the direct and the relaying modes where the direct link is prioritized. Thus, relay nodes are required only when the direct link fails to achieve the target secrecy rate requirement. For the second design objective, to save energy while forwarding the source information securely, a greedy battery-aware relay-selection scheme is proposed. Furthermore, to reduce the impact of outdated channel state information (CSI) on the system's secrecy performance, we propose a relay-selection refinement scheme consisting of repeating the relay-selection process prior to the information forwarding phase and we derive a closed-form upper bound of the secrecy outage probability. Our derived upper bound expression shows that our proposed scheme achieves better secrecy performance compared to the direct and conventional two-phase relay transmissions. We also show that, with the relay selection refinement scheme, a significant reduction in the negative impact of outdated CSI is achieved.