1. State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China
2. State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China, and also with the National Mobile Communications Research Laboratory, Southeast University, Nanjing 211189, China
3. Electrical and Computer Engineering Department, Auburn University, Auburn, AL 36849-5201, USA
4. School of Information Engineering, Zhengzhou University, Zhengzhou 450001, China