SLM steering-based covert communication over strong atmospheric turbulence channels

In secure communications, the primary goal is to protect the content of the transmitted message. In contrast, low probability of intercept (LPI) communications prioritize the privacy of the communication process itself. The most restrictive form of LPI communications, where an adversary disables communication upon detecting the presence of a communication attempt, is referred to as covert communication. To hide the transmitted signal within solar radiation, we propose using an incoherent Gaussian source modulated with phase-shift keying (PSK). To facilitate the detection process, we suggest transmitting a portion of the unmodulated carrier. For establishing the communication link, we utilize spatial light modulator (SLM)-based beam steering. To explore these concepts, we have developed a 1.5 km free-space optical link at the University of Arizona campus. Our experimental results demonstrate the record number of covert bits transmitted in both strong and saturation turbulence regimes.