1. College of Air Traffic Management, Civil Aviation University of China, Tianjin 300000, China.
2. Department of Computer Science, Middlesex University, London NW4 4BE, U.K. (e-mail: x.cheng@mdx.ac.uk)
3. College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China (e-mail: xdhuang@nuaa.edu.cn)
4. College of Engineering, Taif University, Taif 21944, Saudi Arabia.