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This paper considers secure communications for an underlay cognitive radionetwork (CRN) in the presence of an external eavesdropper (Eve). The secrecyperformance of CRNs is usually limited by the primary receiver's interferencepower constraint. To overcome this issue, we propose to use an unmanned aerialvehicle (UAV) as a friendly jammer to interfere with Eve in decoding theconfidential message from the secondary transmitter (ST). Our goal is tojointly optimize the transmit power and UAV's trajectory in thethree-dimensional (3D) space to maximize the average achievable secrecy rate ofthe secondary system. The formulated optimization problem is nonconvex due tothe nonconvexity of the objective and nonconvexity of constraints, which isvery challenging to solve. To obtain a suboptimal but efficient solution to theproblem, we first transform the original problem into a more tractable form anddevelop an iterative algorithm for its solution by leveraging the innerapproximation framework. We further extend the proposed algorithm to the caseof imperfect location information of Eve, where the average worst-case secrecyrate is considered as the objective function. Extensive numerical results areprovided to demonstrate the merits of the proposed algorithms over existingapproaches.

UAV-Assisted Secure Communications in Terrestrial Cognitive Radio Networks: Joint Power Control and 3D Trajectory Optimization