1. School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, China.
2. School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, China (e-mail: yhzhou@bjtu.edu.cn)
3. Institute of Transportation Engineering, Tsinghua University, Beijing 100084, China.
4. Faculty of Software and Information Science, Iwate Prefectural University, Takizawa 020-0193, Japan.