Benefits of multiple transmit antennas in secure communication: A secrecy outage viewpoint

This paper investigates secure multi-antenna transmission in slow fading channels without the eavesdropper's channel state information. The use of multiple transmit antennas enables the transmitter to strengthen the signal reception at the intended receiver while simultaneously confusing the eavesdropper by delivering artificial noise. A recently developed secrecy outage formulation, which can separately measure the quality of service and the level of security, is used to characterize the security performance. We show that an arbitrarily low secrecy outage probability cannot be achieved by adding more transmit antennas alone without optimizing other system parameters. To facilitate the practical system design, we present an on-off transmission scheme with optimal artificial noise power allocation, which minimizes the secrecy outage probability whilst guaranteeing a minimum required quality of service.