1. Institute of Information Science, Beijing Jiaotong University, Beijing 100044, P. R. China
2. Beijing Key Laboratory of Advanced Information Science and Network Technology, Beijing 100044, P. R. China
3. College of Information Engineering, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China
4. Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong