Scattering and Eavesdropping in Terahertz Wireless Link by Wavy Surfaces

Wireless communication at terahertz (THz) frequencies is viewed as one of the potentials for future 6G wireless systems. However, the path of secure data transmission by THz links is facing many complex challenges. Of that, scattering-induced eavesdropping threat remains one of the most critical but less explored. With the updating of carrier frequency into the THz range, the wavelength becomes comparable to the surface roughness of typical objects in our daily life. Surface scattering, which is negligible in current wireless networks, becomes serious and would lead to desperate signal eavesdropping. For a non-line-of-sight (NLOS) link achieved by surface reflection, how does the scattering affect link security at the physical layer? Such direct information is crucial for the inspection of security threats and proposing of possible solutions. Using metallic wavy surfaces, we characterize possibilities of signal eavesdropping caused by bistatic scattering, by several unmodulated channels at different frequencies. We also demonstrate a real-time data transmission for this eavesdropping configuration, with a data rate of 4 gigabits/s by 16 quadrature amplitude modulation. We observe that successful signal interception from an NLOS link happens, even when transceiver designs incorporating backscatter measures are considered. This is likely to be a common security threat for any THz communication systems in which NLOS channels are employed to achieve wider user coverage.