Due to the tiny wavelength of millimeter waves (mmWave), tens of antennas can be packed into a small area in mmWave transceivers. However, implementing a radio-frequency (RF) chain for each antenna is impractical due to the high cost and power of mixed-signal devices. To reduce the cost and get benefit from the antennas, an analog RF beamformer is implemented using variable gain amplifiers and analog phase shifters. In this paper, we design the RF precoder for physical layer security. We consider two degrees of channel knowledge at the transmitter. For full channel knowledge at the transmitter, the RF precoder is optimized to maximize the secrecy rate. Two solution algorithms, with different computational complexity, are proposed based on semidefinite relaxation and gradient ascent. For partial channel knowledge at the transmitter, the RF precoder is designed to maximize the secrecy rate under secrecy outage constraint. The problem is safely approximated, and solved by a gradient ascent algorithm within bisection search. Numerical results show that the proposed algorithms converge quickly, and outperform the conventional secrecy schemes.
['Yahia R. Ramadan', 'Ahmed S. Ibrahim', 'Mohamed M. Khairy']
RF beamforming for secrecy millimeter wave MISO-OFDM systems