Secure Transmission via Lens Antenna Array in Millimeter Wave MISO Systems

Millimeter wave (mmWave) communication is considered as a promising technology for future wireless communication systems. Since a radio-frequency (RF) chain has high cost and high power consumption, it is impractical to implement fully digital beamforming in the mmWave communication systems. In this paper, we propose a secure transmission scheme with limited RF chains by implementing a lens antenna array in a mmWave multiple-input single-output multiple-eavesdropper (MISOME) wiretap channel. Due to the energy-focusing property of a lens antenna array, the proposed scheme requires a few RF chains for secure communication, not just the number of all antennas. We adopt artificial noise (AN) to degrade the wiretap channel in both cases of available channel state information of the eavesdropper (CSIE) at the transmitter and unavailable that. For the CSIE available case, a simple antennas selection scheme is proposed. We propose an algorithm for selecting antennas and designing analog phase shifters in the CSIE unavailable case. Numerical results demonstrate a remarkable secrecy performance enhancement compared with other hybrid beamforming strategies exploiting a conventional uniform planar array (UPA)