On Multiple-Antenna Techniques for Physical-Layer Range Security in the Terahertz Band

Terahertz (THz) communications have naturally promising physical layer security (PLS) performance since narrow beams created by antenna arrays can effectively prevent eavesdroppers outside the beam from overhearing legitimate messages. However, eavesdroppers residing in the THz beam sector are still threatening even with the equipment of traditional multi-antenna techniques. This security challenge is referred to range security, since the range of the legitimate receiver and the eavesdropper differs. To address this problem, in this paper, a fundamental theoretical analysis on multiple-antenna techniques to provide range security is studied. Based on this, the frequency diverse array, as a candidate multiple-antenna technique, is proven ineffective in addressing the range security problem. Moreover, a widely-spaced array and beamforming design for THz range security is demonstrated. A non-constrained optimum approaching (NCOA) algorithm is proposed to achieve sub-optimal performance based on a THz-specific multiple-antenna array structure. Simulation results illustrate that our proposed widely-spaced antenna communication scheme can ensure a 6 bps/Hz secrecy rate when the transmit power is 10 dBm and the propagation distance is 10 m.