1. University of Electronic Science and Technology of China, School of Computer Science and Engineering, Chengdu, Sichuan, China, 611731 (e-mail: ZY_LoYe@126.com)
2. School of Computer Science and Engineering, UESTC, Chengdu, Sichuan, China, (e-mail: chxxu@uestc.edu.cn)
3. School of Telecommunication Engineering, Xidian University, 47905 Xian, Shaanxi, China, (e-mail: dr.nan.cheng@ieee.org)
4. Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada, (e-mail: sshen@uwaterloo.ca)