Adaptive optics-based covert communications in strong atmospheric turbulence regime

In this invited paper we will review our recent research activities related to the low probability of intercept (LPI) and covert communications over atmospheric turbulence channels. Compared to secure communications, in which we are concerned with protecting the message content, in LPI communications we are concerned with the privacy of communicating parties. Covert communication represents the most restrictive version of LPI communications, in which the adversary shuts down the communication link once the presence of a communication attempt is detected. We recently proposed to use the incoherent sources with Gaussian distribution and modulate the incoherent optical beam by imposing the phase-shift keying (PSK). By ensuring that the transmitted power satisfies the covert communication threshold we ensure that the communication is either LPI or covert. Given that distribution of the transmitted signal matches the distribution of the background solar radiation, the transmitted signal is extremely difficult to detect. To detect the transmitted signal, we use the local oscillator laser and select a particular wavelength for information recovery. To improve tolerance to turbulence effects, adaptive optics is used. To study the proposed concepts, we have developed a 1.5 km free-space optical link at the University of Arizona campus. We have demonstrated the record number of covert bits transmitted over strong turbulence channels with or without adaptive optics.