Trust-aware buffer-aided relay selection for secure communications in cooperative wireless systems

Cooperative wireless networks have been facing security challenges due to the inherent openness of wireless channels. Previous studies on the physical layer security (PLS) of cooperative networks mainly focus on scenarios with only trusted relays. In practice, however, untrusted relays may exist in the network and such relays may refuse to cooperate or even work as a helper of an eavesdropper for packets eavesdropping/interception. To address this issue, we consider in this paper a cooperative wireless network with untrusted relays and a passive eavesdropper attempting to wiretap packets transmitted in the network. To select a secure and trusted relay, we first propose two buffer-aided relay selection schemes, one for perfect eavesdropper channel state information (CSI) case and another for no eavesdropper CSI case. We then conduct theoretical analysis to derive the models for the performance metrics of secrecy outage probability (SOP) and expected queuing delay. Finally, simulation and numerical results are provided for the validation of our theoretical models. The results indicate that our proposed schemes can outperform the typical max-ratio scheme under perfect eavesdropper CSI case and can outperform the max-link and the max-max relay selection schemes under no eavesdropper CSI case in terms of the SOP and expected queuing delay.