1. School of Mathematics, Southwest Jiaotong University, Chengdu, China
2. School of Mathematics and the Key Laboratory of Information Coding and Wireless Communications, Southwest Jiaotong University, Chengdu, China
3. School of Computer Science and Electronics Engineering, University of Essex, Colchester, U.K.
4. Key Laboratory of Information Coding and Wireless Communications, Southwest Jiaotong University, Chengdu, China