Three-dimensional trajectory optimization for secure UAV-enabled cognitive communications

Unmanned aerial vehicles (UAVs) are envisioned as a promising means of providing wireless services for various complex terrains and emergency situations. In this paper, we consider a wireless UAV-enabled cognitive communication network, where a rotary-wing UAV transmits confidential information to a ground cognitive user over the spectrum assigned to primary users (PUs), while eavesdroppers attempt to wiretap the legitimate transmission. In order to enhance the secrecy performance of wireless communications, the secrecy rate (SR) of the UAV-enabled cognitive communication system is maximized through optimizing UAV three-dimensional (3D) flying trajectory while satisfying the requirements of UAV's initial and final locations and guaranteeing the constraint of maximum speed of UAV and the interference threshold of each PU. However, the formulated SR maximization (SRM) problem is non-convex. For the purpose of dealing with this intractable problem, we employ the difference of two-convex functions approximation approach to convert the non-convex optimization problem into a convex one, which is then solved through applying standard convex optimization techniques. Moreover, an iterative 3D trajectory optimization algorithm for SRM scheme is proposed to achieve the near-optimal 3D trajectory. Simulation results show that our proposed 3D trajectory optimization based SRM algorithm has good convergence, and the proposed SRM scheme outperforms the benchmark approach in terms of the SR performance.