1. College of Electronic Information Engineering, Chongqing University of Science and Technology, Chongqing 400050, China. pengjun70@126.com
2. Department of Computer Science, California State University, Sacramento, CA 95819, USA. zhangd@ecs.csus.edu
3. Department of Computer Science and Engineering, Chongqing University, Chongqing 400044, China. xfliao@cqu.edu.cn