1. Chongqing Key Lab of Computer Network and Communication Chongqing University of Posts and Telecommunications, Chongqing 400065, P.R. China
2. School of Information Science and Technology, Southwest Jiaotong University Chengdu 600031, P.R. China. E-mail: gongzzzz@126.com; wanggy@ieee.org
3. Institute of Computer Science and Technology Chongqing University of Posts and Telecommunications, Chongqing 400065, P.R. China