China Unicom Groups, No.21 Financial Street, Xicheng District, Beijing 100140, China;
Research Institute of Information Technology(RIIT)and Tsinghua National Lab for Information Science and Technology(TNList), Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, PLA Univ.of Info.& Eng., Zhengzhou 450001, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Research Institute of Information Technology(RIIT)and Tsinghua National Lab for Information Science and Technology(TNList), Tsinghua University, Beijing 100084, China;
