  In this paper, we study the resource allocation and trajectory design for
secure unmanned aerial vehicle (UAV)-enabled communication systems, where
multiple multi-purpose UAV base stations are dispatched to provide secure
communications to multiple legitimate ground users (GUs) in the existence of
multiple eavesdroppers (Eves). Specifically, by leveraging orthogonal frequency
division multiple access (OFDMA), active UAV base stations can communicate to
their desired ground users via the assigned subcarriers while idle UAV base
stations can serve as jammer simultaneously for communication security
provisioning. To achieve fairness in secure communication, we maximize the
average minimum secrecy rate per user by jointly optimizing the
communication/jamming subcarrier allocation policy and the trajectory of UAVs,
while taking into account the constraints on the minimum safety distance among
multiple UAVs, the maximum cruising speed, the initial/final locations, and the
existence of cylindrical no-fly zones (NFZs). The design is formulated as a
mixed integer non-convex optimization problem which is generally intractable.
Subsequently, a computationally-efficient iterative algorithm is proposed to
obtain a suboptimal solution. Simulation results illustrate that the
performance of the proposed iterative algorithm can significantly improve the
average minimum secrecy rate compared to various baseline schemes.
