  Unmanned aerial vehicles (UAVs) can be utilized as aerial base stations to
provide communication service for remote mobile users due to their high
mobility and flexible deployment. However, the line-of-sight (LoS) wireless
links are vulnerable to be intercepted by the eavesdropper (Eve), which
presents a major challenge for UAV-aided communications. In this paper, we
propose a latency-minimized transmission scheme for satisfying legitimate
users' (LUs') content requests securely against Eve. By leveraging
physical-layer security (PLS) techniques, we formulate a transmission latency
minimization problem by jointly optimizing the UAV trajectory and user
association. The resulting problem is a mixed-integer nonlinear program
(MINLP), which is known to be NP hard. Furthermore, the dimension of
optimization variables is indeterminate, which again makes our problem very
challenging. To efficiently address this, we utilize bisection to search for
the minimum transmission delay and introduce a variational penalty method to
address the associated subproblem via an inexact block coordinate descent
approach. Moreover, we present a characterization for the optimal solution.
Simulation results are provided to demonstrate the superior performance of the
proposed design.
