1. Microsoft Research, Beijing, China and Beijing University of Posts and Telecommunications, Beijing, China
2. Microsoft Research, Beijing, China and Xi'an Jiaotong University, Xi'an, China
3. Microsoft Research, Beijing, China and Tsinghua University, Beijing, China
4. Microsoft Research, Beijing, China and WINLAB, Rutgers University, North Brunswick, NJ, USA
5. Microsoft Research, Beijing, China