1. Department of Mathematics and Statistics and Hubei Key Laboratory of Intelligent Wireless Communications, South-Central University for Nationalities, Wuhan, China
2. Department of Mathematics and Statistics, South-Central University for Nationalities, Wuhan, China
3. Hubei Key Laboratory of Intelligent Wireless Communications, South-Central University for Nationalities, Wuhan, China
4. Department of Informatics, University of Bergen, Bergen, Norway