Department of Computer Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Korea;Department of Computer Science and Engineering, Computer and Communication Media Research, NEC Corp., Miyazaki 4-1-1, Miyamae-ku, Kawasaki, Japan;Human-Computer Communications Laboratory, Department of Systems Engineering and Engineering Management, The Chinese University of Hong Kong, Miyazaki 4-1-1, Miyamae-ku, Hong Kong, Japan;School of Engineering, Information and Communications University, 119, Munjiro, Yuseong-gu, Daejeon, Korea