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; State Key Laboratory of Mobile Network and Mobile Multimedia Technology, Shenzhen 518055, China
3. State Key Laboratory of Mobile Network and Mobile Multimedia Technology, Shenzhen 518055, China; ZTE Corporation, Nanjing 210012, China