1. State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications, Beijing 100876, China
2. School of Computer Sciences and Technology, Communication University of China, Beijing 100024, China
3. Department of Information and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan
4. Anyang Normal University, Anyang, Henan 455002, China