Finite Block-Length Covert Communication in Space-Air-Ground Integrated Networks

This paper tackles the challenge of enabling covert communication in a dual-hop space-air-ground integrated network (SAGIN) comprising a satellite, an aerial decode-and-forward relay, an aerial-friendly jammer, and a terrestrial receiver. The direct satellite-to-receiver link is often compromised by deep fading, necessitating the use of relays to forward signals. To counteract the warden's detection of signals from the satellite and relay, a friendly jammer is deployed to emit artificial noise (AN) to confuse the warden's decision with two jamming schemes: uninformed jamming (UJ) and cognitive jamming (CJ). In the UJ scheme, the jammer continuously sends AN, while in the CJ scheme, the jammer adjusts its AN emission based on its detection results. Considering finite block-length communication, we derive closed-form expressions for the system's covert outage probability under both schemes, which are crucial for evaluating the warden's detection performance and the quality of service of legitimate link messages. The correctness of the results is validated through Monte Carlo simulations, and the impact of relevant parameters on covert performance is analyzed. The findings can provide new insights and methods for enhancing covert communication in SAGIN systems.