1. School of Information Science and Electrical Engineering, Shandong Jiaotong University, Jinan, China
2. School of Space Information, Space Engineering University, Beijing, China
3. Shaanxi Key Laboratory of Information Communication Network and Security, Xi’an University of Posts and Telecommunications, Xi’an, China
4. College of Computer Science and Electronic Engineering, Hunan University, Changsha, China
5. Department of Informatics and Telecommunications, University of Thessaly, Lamia, Greece