Effects of Outdated CSI on the Secrecy Performance of MISO Wiretap Channels with Transmit Antenna Selection

Multiple antennas at the transmitter enhance the diversity performance of multiple-input single-output (MISO) wiretap channels by selecting a single antenna that maximizes the instantaneous signal-to-noise ratio (SNR) at the intended receiver. However, the expected diversity performance may not be fully realized when channel state information (CSI) is outdated at the transmitter. Owing to this fact, in this letter we analyze the effects of outdated CSI on the secrecy outage performance of MISO wiretap channels with transmit antenna selection. More specifically, assuming Nakagami-m fading, a closed-form expression for the exact secrecy outage probability is derived, from which an asymptotic analysis is carried out. Our asymptotic results reveal that the expected diversity gain cannot be realized when CSI is outdated during the antenna selection process. The validity of our formulations is corroborated by Monte Carlo simulations.