1. School of Space Information, Space Engineering University, Beijing, China
2. School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo, China
3. College of Electronic Engineering, National University of Defense Technology, Hefei, China
4. Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, China
5. Department of Informatics and Telecommunications, University of Thessaly, Lamia, Greece
6. Department of Information Systems, University of Maryland, Baltimore County, Baltimore, MD, USA