Secure multiple-input single-output communication - Part I: secrecy rates and switched power allocation

The use of multiple-antenna arrays has attracted much attention for the physical layer security of wireless systems, where the so-called artificial-noise solution can be applied to enhance the communication confidentiality. In the first part of this study, the authors consider secure communication over a multiple-input single-output Rayleigh-fading channel in the presence of a multiple-antenna eavesdropper - referred to as a multiple-input single-output multiple-eavesdropper (MISOME) wiretap channel. Specifically, secure beamforming with artificial noise is treated when the transmitter has access to full channel state information (CSI) of a legitimate channel but only partial CSI of an eavesdropper channel. First, the optimal power allocation between the information-bearing signal and artificial noise (or simulated interference) is derived to maximise the achievable secrecy rate in the presence of a weak or strong eavesdropper. Then, the first-order optimal power allocation strategy is developed in a switched fashion by selecting the best of weak- and strong-eavesdropper solutions for a general eavesdropping attack, and a closed-form expression is derived for the ergodic secrecy rate achieved by secure beamforming with this switched power allocation in the MISOME wiretap channel. The numerical results show that the switched power allocation is a simple but effective approach that nearly achieves the optimal secrecy rate.