1. School of Computer and Information Technology, Beijing Jiaotong University, Beijing, 100044, P.R. China
2. State Key Laboratory of Rail Traffic Control and Safety Beijing Jiaotong University, Beijing, 100044, P.R. China and National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China
3. Shanghai Telecommunications Technologies Research Institute of China Telecom, Shanghai 201315, China