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International Conference on Transparent Optical Networks

Covert Communications over Turbulent Free-Space Optical Channels

Ivan B. Djordjević

Published Jul 1, 2026• 0 citations
Publisher
DOI
10.1109/icton71309.2026.11705311

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In secure communications, the primary focus is on safeguarding the content of the message. In contrast, low probability of intercept (LPI) communications prioritize the privacy of the communication process itself. The most restrictive form of LPI communication, where an adversary attempts to disrupt the communication once its presence is detected, is known as covert communication. To hide the transmitted signal in solar radiation, we propose using the Gaussian quantum states generated by the degenerate parametric down-conversion process with signal photons being modulated with the phase-shift keying (PSK). To detect the transmitted signal, we propose transmitting the unmodulated idler photons (serving as the reference) together with modulated signal photons. To establish the link, we use the spatial light modulator (SLM)-based beam steering. We successfully demonstrate dual-pump-lasers-based degenerate entanglement-assisted (EA) LPI communication at 1 Gb/s over a 1.5 km terrestrial free-space optical (FSO) link. For the same link, the number of achievable EA covert bits for EA covert FSO communication is significantly higher compared to the corresponding classical covert communication scheme.
In physical-layer security and cryptography we are concerned with the security of the transmitted data, while in low probability of intercept (LPI) communication with protecting the privacy of the end users. In our recent publications related to LPI communications and radars over free-space optical (FSO) links we proposed to hide the constant-amplitude modulated data, imposed on thermal source beam, in ambient solar radiation to protect the end users privacy and at the same time improve the reliability and security, while reducing the detectability of transmitted signal by the adversary Willie. In order to study both LPI and covert communication concepts we have developed an FSO communication testbed at the University of Arizona campus with a 1.5 km-long FSO link. Here we present results of our FSO experiments, where we conducted both LPI and covert communications at data rates ranging from 125 Mb/s to 10 Gb/s, wherein the information beam is kept completely hidden under the ambient solar radiations as random thermal noise. To improve the system reliability to atmospheric turbulence effects we make use of wavelength diversity method as a low-cost, easy to implement and far more practical alternative to conventional adaptive optics systems.
The information that is leaked to an eavesdropper during the error reconciliation phase of quantum key distribution (QKD) protocols limits the maximum bit error rate (BER) of a system. In a standard QKD protocol, parity bits are transmitted over an authenticated noiseless channel, to which Eve has access. This Letter presents the concept of using a covert classical communication channel to transmit the parity bits between Alice and Bob without Eve gaining information of the transmitted parity bits. This allows for higher secure key generation rates and operation of a QKD system in which the BER exceeds the limit of the standard protocol. This concept is then applied to a practical free-space optical system that contains multiple parallel channels, where channel loss, crosstalk, atmospheric turbulence effects, and noise are considered.