1. School of Information and Electrical Engineering, Hebei University of Engineering, Handan, China
2. State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications, Beijing, China
3. School of Computer and Communication Engineering, University of Science and Technology Beijing, Beijing, China
4. School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Northern Ireland, U.K.
5. Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA