1. School of Science, P. O. Box 122, Beijing University of Posts and Telecommunications, Beijing 100876, China
2. State Key Laboratory of Software Development Environment, Beijing University of Aeronautics and Astronautics, Beijing 100083, China
3. Key Laboratory of Optical Communication and Lightwave Technologies, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China
4. Department of Mathematics and LMIB, Beijing University of Aeronautics and Astronautics, Beijing 100083, China