1. Department of Electrical and Electronic Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan
2. Department of Computer Science and Electronic Engineering, National Institute of Technology, Tokuyama College, Gakuendai, Shunan, Yamaguchi 745-8585, Japan
3. RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan