This paper investigates unmanned aerial vehicle (UAV) enabled secure transmission systems, where a source UAV (SUAV) intends to send confidential signals to the legitimate user in the presence of a potential eavesdropper. To improve the system's secrecy rate, a jamming UAV (JUAV) is leveraged to cooperatively send interference signals to the potential eavesdropper. By taking into account the limited energy budget of UAV, our goal is to maximize the system secrecy energy efficiency (SEE), namely the achievable secrecy rate per energy consumption unit, by jointly optimizing UAV trajectory and transmit power under the constraints of UAV mobility as well as the maximum transmit power. The formulated problem is shown to be a non-convex fractional optimization problem, which is challenging to solve. To this end, we decompose the original problem into two sub-problems, and then an efficient iterative algorithm is proposed by leveraging block coordinate descent and Dinkelbach method in combination with successive convex approximation techniques. Simulation results show that the proposed scheme outperforms other benchmarks significantly in terms of the system SEE.
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