1. Department of Electrical Engineering and Information SystemsThe University of Tokyo, 7‐3‐1, Hongo, Bunkyo‐ku Tokyo 113‐8656 Japan
2. Mitsubishi Electric Corporation, 8‐1‐1, Tsukaguchi‐Honmachi Amagasaki City Hyogo 661‐8661 Japan
3. Department of Electrical and Electronic EngineeringKanazawa Institute of Technology, 7‐1 Ohgigaoka Nonoichi‐City Ishikawa 921‐8501 Japan