1. Corporate R&D Center, SK Telecom Co., Ltd., 65 Eulji-ro, Jung-gu, Seoul 04539, Korea ygjin@sk.com
2. School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, 123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Korea jwshin@gist.ac.kr
3. School of Electrical and Computer Engineering and Institute of New Media and Communications, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea nkim@snu.ac.kr