Multi-UAV-assisted covert communications for secure content delivery in Internet of Things

Unmanned aerial vehicles (UAVs) have found extensive applications in fast deploying Internet of Things in non-infrastructure areas due to their high maneuverability and robust networking. However, the open nature of wireless channels makes air-to-ground content delivery face serious security risks. A multi-UAV-assisted covert communication system model is proposed in this paper, which focuses on the secure content delivery from multiple UAVs to multiple legitimate ground users (LGUs) under the detection of an illegal interceptor. Our objective is to maximize the average covert rate (ACR) for all LGUs by synergistically optimizing UAV-LGU scheduling, multi-UAV transmit power, and multi-UAV trajectory, while complying with the covertness constraint. To cope with the formulated non-convex problem, we divide it into three sub-problems, and then propose an iterative algorithm to alternately solve them. Simulation results demonstrate that the proposed algorithm can significantly improve the ACR of the system by adaptively adjusting the Multi-UAV trajectory, flight speed and transmit power, while meeting the covertness constraint. Moreover, compared with the two benchmark schemes, the proposed joint optimization scheme can enhance the ACR by 5% and 31%, respectively, when the flight period is 100 s.