WASABI: Widely-Spaced Array and Beamforming Design for Terahertz Range-Angle Secure Communications

Terahertz (THz) communications have naturally promising physical layer security (PLS) performance in the angular domain due to the high directivity feature. However, if eavesdroppers reside in the beam sector, the directivity fails to work effectively to handle this range-domain security problem. More critically, with an eavesdropper inside the beam sector and nearer to the transmitter than the legitimate receiver, i.e., in close proximity, secure communication is jeopardized. This open challenge motivates this work to study PLS techniques to enhance THz range-angle security. In this paper, a novel widely-spaced array and beamforming (WASABI) design for THz range-angle secure communication is proposed, based on the uniform planar array and hybrid beamforming. Specifically, the WASABI design is theoretically proved to achieve the optimal secrecy rate powered by the non-constrained optimum approaching (NCOA) algorithm with more than one RF chain, i.e., with the hybrid beamforming scheme. Moreover, with a low-complexity and sub-optimal analog beamforming, the WASABI scheme can achieve sub-optimal performance with less than 5% secrecy rate degradation. Simulation results illustrate that our proposed widely-spaced antenna communication scheme can ensure a 6bps/Hz secrecy rate when the transmit power is 10dBm. Finally, a frequency diverse array, as an advocated range security candidate in the literature, is proven to be ineffective to enhance range security.