1. School of Electronic and Information Engineering, Lanzhou Jiaotong University, Lanzhou, P. R. China
2. Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, P. R. China
3. Liaocheng University, Liaocheng, P. R. China
4. School of Software Engineering, Chongqing University, Chongqing, P. R. China