1. School of Software and the Key Laboratory for Ubiquitous Network and Service Software of Liaoning Province, Dalian University of Technology, Dalian, P. R. China
2. Department of Computer Science, City University of Hong Kong, Hong Kong, P. R. China
3. Department of Computer Network and Communication, Sichuan University, Chengdu, Sichuan, P. R. China
4. School of Cyber Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, China
5. Physical Sciences and Engineering College, Cardiff University, Cardiff, U.K.