Robust Performance of Secure Transmission in Multi-Antenna Untrusted Relay Networks

In this work, we investigate the secrecy performance in multi-antenna untrusted relay networks, where destination-aided jamming is incorporated to help reduce the amount of confidential information leaked to an untrusted relay. We conduct a robust secrecy performance analysis for the worst-case scenario, where the antenna selection technique is performed at the untrusted relay for data reception and retransmission, and the selected antenna is the one with the highest probability of interrupting the confidential information. In addition, maximal ratio transmission is used at the source and destination for sending information and jamming signals. Under this transmission scheme, we derive the novel exact and asymptotic closed-form expressions for the secrecy outage probability (SOP) to evaluate the secrecy performance with arbitrary number of antennas. Numerical and simulation results indicate that there exists an optimal number of transmit antennas at the source to achieve the tradeoff between reliability and security.