1. Department of Computer Science and Engineering, Chongqing University, Chongqing 400044, P. R. China
2. Department of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China
3. Department of Automation and Computer-Aided Engineering, The Chinese University of Hong Kong, China
4. Department of Mathematics, Southeast University, Nanjing 210096, P. R. China