1. Department of Electronic and Information Engineering, Key Laboratory of Communication and Information Systems, Beijing Municipal Commission of Education; Beijing Jiaotong University; Beijing 100044 China
2. Department of Information Engineering and Computer Science; Feng Chia University; Taichung 40724 Taiwan
3. Department of Computer Science and Information Engineering; Asia University; Taichung 41354 Taiwan
4. Department of Computer Science and Information Engineering; National Chung Cheng University; 160 San-Hsing Ming-Hsiung, Chiayi 621 Taiwan