1. State Key Laboratory of Rail Traffic Control and Safety, Beijing Engineering Research Center of High-speed Railway Broadband Mobile Communications, and the School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing, China
2. Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA
3. School of Information Engineering, Zhengzhou University, Zhengzhou, China
4. School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA