1. International School of Information Science and Engineering, Dalian University of Technology, and the Key Laboratory for Ubiquitous Network and Service Software of Liaoning Province, P. R. China
2. School of Software, Dalian University of Technology, and the Key Laboratory for Ubiquitous Network and Service Software of Liaoning Province, P. R. China
3. Department of Computer Science, City University of Hong Kong, P. R. China