1. School of Information Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P. R. China
2. National Transportation Preparation Office, Beijing 100036, P. R. China
3. Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, P. R. China
4. School of Information Science and Technology, Shijiazhuang Tiedao University, Shijiazhuang 050043, P. R. China
5. School of Electronic and Information Engineering, Beijing Key Lab for Network-based Cooperative ATM, Beihang University, Beijing 100191, P. R. China