  The paper considers physical layer security (PLS) of an untrusted unmanned
aerial vehicle (UAV) network, where a multitude of UAVs communicate in
full-duplex (FD) mode. A source-based jamming (SBJ) scheme is exploited for
secure communication without utilizing any external jammers. Firstly, the
optimal power allocation between the confidential signal and the jamming signal
is derived to maximize the secrecy rate of each link between the source and the
destination. Then, the best UAV selection scheme is proposed to maximize the
overall secrecy rate of the network. The corresponding secrecy outage
probability (SOP) and the average secrecy rate (ASR) of the network are
analyzed based on the proposed UAV selection and the optimal power allocation
schemes. Asymptotic results are also obtained to derive the achievable
diversity order. Results are validated through numerical evaluations while
providing useful network design insights such as locations and altitudes of the
UAVs.
