1. Beijing Advanced Innovation Center for Future Internet Technology, Beijing University of Technology, Beijing, P.R. China
2. School of Information and Communications Engineering, Beijing University of Technology, Beijing, P.R. China
3. Beijing Laboratory of Advanced Information Networks, Beijing University of Technology, Beijing, P.R. China
4. Beijing University of Posts and Telecommunications, Beijing, P.R. China
5. Electrical Engineering Department, Columbia University, New York, USA