1. University of Aizu, School of Computer Science and Engineering, Tsuruga, Ikki-machi, Aizu-Wakamatsu, Fukushima 965-8580, Japan
2. Toshiba Corporation, Semiconductor and Storage Products Company, 2-5-1 Kasama, Sakae-ku, Yokohama 247-8585, Japan
3. Tokyo Institute of Technology, Department of Social Engineering, 2-12-1-W9-98 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
4. Tokyo Institute of Technology, Department of Communications and Computer Engineering, 2-12-1-S3-58 Ookayama, Meguro-ku, Tokyo 152-8550, Japan