1. State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing 100044, China; Beijing Engineering Research Center of High-speed Railway Broadband Mobile Communications, Beijing 100044, China
2. CEMSE Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955--6900, Saudi Arabia
3. Moving Wireless Network Research Section, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Republic of Korea