1. School of Computer, Qinghai Normal University, Xining 810016, China
2. Qinghai Province Tibetan Information Processing Engineering Technology Research Center, Xining 810008, China
3. Key Laboratory of Tibetan Information Processing and Machine Translation of Qinghai Province, Xining 810008, China
4. State Key Laboratory of Tibetan Intelligent Information Processing and Application, Xining 810008, China
5. School of Mathematics and Statistics, Nanjing University of Information Science and Technology, Nanjing 210044, China