| Summary: | Communication assisted by unmanned aerial vehicles (UAVs) has been regarded as an effective technique for reliability improvement in both military and civilian domains, whereas it also makes the information vulnerable to passive eavesdropping due to its wide broadcast. In this paper, we investigate the secure millimeter wave (mmWave) communication assisted by multiple UAV-enabled relays and jammers, where exist multiple randomly distributed eavesdroppers on the ground. Leveraging the models of 3D-antenna gain and stochastic geometry, new closed-form expressions of secrecy outage probability are derived on the basis of the opportunistic relay selection scheme involving the characteristics of air-to-ground channel, and the secrecy improvement is demonstrated when the relay density increases. In addition, a cooperative jamming scheme, where a part of UAVs transmit the jamming signals, is designed to degrade the qualities of eavesdropping channels and further enhance physical layer security. The simulation results show the impacts of different system parameters on secrecy outage probability and verify our analysis. It's also revealed that there exist the optimal attitude of UAVs, jamming power and density of jammers for achieving the best secrecy performance.
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