College of Computer Science, Beijing University of Technology, Chaoyang District, Beijing, China. jjz01@bjut.edu.cn.|College of Computer Science, Beijing University of Technology, Chaoyang District, Beijing, China. jjz01@bjut.edu.cn.|College of Computer Science, Beijing University of Technology, Chaoyang District, Beijing, China. jjz01@bjut.edu.cn.|Department of Electronic and Computer Engineering, Brunel University, Uxbridge, Middlesex UB8 3PH, UK. asoke.nandi@brunel.ac.uk.