1. School of Control Engineering, Chengdu University of Information Technology, Chengdu 610225, P. R. China
2. College of Information Science and Technology, Chengdu University of Technology, Chengdu 610059, P. R. China
3. School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China
4. COL Laboratory, School of Computer Science, University of Nottingham, Nottingham, UK