  Terahertz (THz) communications have naturally promising physical layer
security (PLS) performance in the angular domain due to the high directivity
feature brought by the ultra-massive multiple-antenna techniques. However,
traditional multiple-antenna techniques fail to combat eavesdroppers residing
in the THz beam sector, even when the communication distances of legitimate
users and eavesdroppers are different. This THz range security challenge
motivates us to study new multiple-antenna techniques to provide THz range and
angular security. In this paper, we first conduct a theoretical analysis of the
secrecy capacity of the multiple antenna channel under the range security
scenario. Based on this, the frequency diverse array, as a candidate
multiple-antenna technique, is proven ineffective in addressing the range
security problem. Then, motivated by the theoretical analysis, a novel
widely-spaced array and beamforming design for THz range security are proposed,
which realize communications in the near-field regions. A non-constrained
optimum approaching (NCOA) algorithm is developed to achieve the optimal
secrecy rate. Simulation results illustrate that under the range security
scenario where the eavesdropper is inside the beam sector, 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.
