1. Institute for Silk Road Research, Xian University of Finance and Economics, Xian 710100, China
2. OPTIMAL, Northwestern Polytechnical University, Xian 710072, China
3. Beijing Etrol Technologies Co., Beijing 100095, China
4. Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A.
5. Department of Computer Science and Engineering, University of Texas at Arlington, Arlington, TX 76010, U.S.A.