1. College of Computer Science and Electronic Engineering, Hunan University, Changsha, China
2. College of Electrical and Information Engineering, Hunan University, Changsha, China
3. School of Physics and Electronics, Hunan Normal University, Changsha, China
4. Department of Electrical and Computer Engineering, Duke University, Durham, NC, USA
5. School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, China