1. Jiangsu Key Laboratory of Meteorological Observation and Information Processing, Nanjing University of Information Science and Technology, Nanjing 210044, P. R. China
2. School of Electronic and Information Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, P. R. China
3. Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA
4. School of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China