Secure Selection in Untrusted Decode-and-Forward Relay Networks with Direct Links

This paper studies user selection for an untrusted decode-and-forward relay channel, in which one base station is selected for secure transmission to the destination node in the presence of direct links. To enhance the secrecy performance, three selection criteria are investigated based on the the direct and the relaying links. The close-form analytical secrecy outage probability (SOP) as well as the asymptotic expressions are derived to reveal the impact of the direct and the relaying link on the system secrecy performance. From the asymptotic expressions, it shows that the proposed sub-optimal criterion, which substantially reduces the system complexity, achieves the same diversity order with the optimal criterion.