1. China Research Institute of Radiowave Propagation, Qingdao 266107, China
2. Qingdao Institute for Ocean Technology, Tianjin University, Qingdao 266200, China
3. School of Mathematics, Southwest University for Nationalities, Chengdu 600041, China
4. School of Microelectronics, Tianjin University, Tianjin 300072, China