1. Department of Electrical Engineering and the Institute of Communications Engineering, National Tsing Hua University, Hsinchu, Taiwan
2. Department of Electrical Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
3. Research Center for Information Technology Innovation, Academia Sinica, Taipei, Taiwan
4. Microsoft, Seattle, WA, USA
5. Department of Computer Science, National Yang Ming Chiao Tung University, Hsinchu, Taiwan