Distance-Adaptive Absorption-Peak Hopping (DA-APH) Modulation for Terahertz Covert Communications

Covert communication, aiming at concealing the existence of data transmission from an eavesdropper, is attracting increasing concerns for communication security. With the trend of moving to higher carrier frequencies, Terahertz (THz) band communication, i.e., wavelength at 0.03-3mm, is envisioned as a promising technique to support bandwidth-hungry applications, as well as improve physical layer security due to its naturally strong directivity and high path loss. Narrow-beam transmission widely adopted in THz communications can effectively prevent from eavesdropping outside the beam sector. However, the challenge still remains when an eavesdropper resides inside the beam sector. To ensure the covertness under such condition, in this paper, a distance-adaptive absorption peak hopping (DA-APH) modulation scheme is proposed for THz covert communications, which takes advantages of distance- and frequency-selective spectral windows, and the frequency-hopping mechanism over the THz spectrum. In particular, the pulse waveform model with polarization is developed to guarantee the reliability of transmission and covertness from eavesdropping. Furthermore, a distance-adaptive carrier frequency selection scheme is designed to choose optimal hopping frequencies at molecular absorption peaks in the THz band.