Secrecy outage performance of MISO wiretap channels with outdated CSI

In this paper, the secrecy outage performance of a multiple-input single-output (MISO) wiretap channel is investigated. Different from previous works, we assume that the legitimate transmitter employs maximal-ratio transmission (MRT) beamforming scheme with outdated channel state information (CSI) of the main channel. A closed-form expression for the secrecy outage probability is derived, from which an asymptotic secrecy analysis is carried out. Furthermore, a closed-form expression for the probability of non-zero secrecy rate is derived. Our results are corroborated by Monte Carlo simulations and show that full diversity gain cannot be realized in the presence of outdated CSI. In addition, the MRT scheme is analytically compared with a transmit antenna selection scheme and the secrecy outage improvement is quantified.