1. School of Information and Electrical Engineering, Hebei University of Engineering, Handan, China
2. School of Electronics, Peking University, Beijing, China
3. Chongqing Key Laboratory of Mobile Communications Technology, Chongqing University of Posts and Telecommunications, Chongqing, China
4. School of Computer Science and Engineering, Nanyang Technological University, Singapore
5. Department of Electrical and Computer Engineering, University of Houston, Houston, TX, USA