1. School of Information Science and Engineering, Shandong Normal University, Jinan 250358, China.
2. School of Information Science and Engineering, Shandong Normal University, Jinan 250358, China (e-mail: jiandesun@hotmail.com)
3. National Engineering Laboratory for Integrated Aerospace-Ground-Ocean Big Data Application Technology and the School of Computer Science, Northwestern Polytechnical University, Xi'an 710072, China.
4. School of Information Science and Engineering, Shandong Normal University, Jinan 250358, China, and also with the Department of Information Engineering and Computer Science, University of Trento, 38123 Trento, Italy (e-mail: zhangkainuc@163.com)
5. Department of Information Engineering and Computer Science, University of Trento, 38123 Trento, Italy.