1. Division of Electrical Engineering and Computer ScienceKanazawa University Kakumamachi, Kanazawa Ishikawa 920‐1192 Japan
2. Division of Engineering Mechanics and Energy, Faculty of Engineering, Information and SystemsUniversity of Tsukuba 1‐1‐1 Tennoudai, Tsukuba 305‐8573 Japan
3. Department of Electrical, Electronics and Communication Engineering, Faculty of EngineeringTokyo City University 1‐28‐1 Tamazutsumi, Setagaya Tokyo 158‐8557 Japan