Active Multiple Access Secure Communication Enabled by Graphene-Based Time-Modulated Metasurfaces

This article presents a technique for establishing active secure communication links over single and multiple orthogonal frequency channels via a reflective graphene-based time-modulated metasurface operating in the low terahertz frequency regime. For this purpose, the Fermi energy level of unit cells is modulated via pseudorandom radio frequency biasing signals whose dc offsets are set to steer the reflected beam toward the desired direction by imposing a spatial phase gradient profile across the surface, while their waveforms are engineered to spread the incident wave spectrum into a noise-like spectrum with a near-zero power spectral density via random modulation of the reflection phase of each element between 0 and
$\pi $
with respect to its offset phase. This allows for addressing a legitimate user in real-time who can retrieve the signal via synchronous demodulation with the pseudorandom key of the metasurface while camouflaging the signal from the adversary by lowering the probability of detection. The approach is extended to enable multichannel secure communication by dividing the metasurface into interleaved subarrays modulated with orthogonal pseudorandom keys, which provides simultaneous and independent control over multiple beams with nonoverlapping spread spectra which can be retrieved by independent legitimate users while rejecting unwarranted access by eavesdroppers.