Secrecy-Aware Relay and Antenna Selection for MIMO Wiretap Spectrum-Sharing Network

To address secrecy performance in a multi-relay underlay wiretap spectrum sharing network, we present a secrecy rate-optimal relay and antenna selection scheme, which serves as a fundamental benchmark. It jointly selects a transmit antenna at a secondary source, a receive antenna at its destination, and a relay between them to maximize the secrecy rate at the destination under the malicious attempt of an eavesdropper. We derive exact and asymptotic expressions for the secrecy outage probability, which accurately reveal the secrecy diversity order and the coding gain. Under a proportional interference constraint, full secrecy diversity order is achieved only when the main link is stronger than the wiretap link, otherwise the diversity gain is lost. We then consider a practical scenario where the secondary users have only the mean channel power gains of the wiretap links. Under such channel state information (CSI), we propose a statistical CSI-based relay and antenna selection scheme and numerically show that this can be used as a better performance/complexity tradeoff. Both the schemes substantially outperform the conventional relay and antenna selection scheme, which requires no CSI about the wiretap links.