1. School of Intelligence Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450 046, China
2. GuangDong Overseas Chinese Vocational School, and Guangdong Communication Polytechnic, Guangzhou 510 520, China
3. School of Computers, Guangdong University of Technology, Guangzhou 510 006, China
4. Department of Computer Science, University of California, Davis, CA 95616, USA