1. Guangzhou Institute of Technology, Xidian University, Guangzhou, China
2. State Key Laboratory of Integrated Services Networks, School of Telecommunication Engineering, Xidian University, Xi’an, Shaanxi, China
3. State Key Laboratory of Integrated Services Networks, Xidian University, Xi’an, China
4. Department of Informatics, University of Oslo, Oslo, Norway
5. School of Electrical Engineering and Computer Science, Royal Institute of Technology, Stockholm, Sweden