Randomized beamforming with generalized selection transmission for security enhancement in MISO wiretap channels

Transmit beamforming (TBF) has been proposed as an effective approach to enhance the security performance of multiple-input single-output (MISO) wiretap channels. However, this secrecy enhancement comes at some expenses, such as power consumption and the amount of signal processing. In addition, in block fading channels, TBF fails to deliver secure communication against intelligent eavesdroppers equipped with advanced channel estimation techniques. Considering these issues associated with TBF, we propose and study randomized beamforming with generalized selection transmission (RBF/GST) to enhance physical layer security in MISO wiretap channels. With GST, Q antennas out of N antennas are selected at the transmitter to maximize the signal to noise ratio at the legitimate receiver. Moreover, RBF is responsible for delivering secure communications in the presence of advanced eavesdroppers. We characterize the physical layer secrecy of RBF/GST under passive and active eavesdropping scenarios, via our closed-form expressions for the ergodic secrecy rate and the exact and asymptotic secrecy outage probabilities. We demonstrate that RBF/GST can effectively improve communication secrecy with a reasonable amount of signal processing and power consumption.