1. Department of Electrical Engineering and Information Systems The 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. Dept. of Electrical and Electronic Engineering Kanazawa Institute of Technology 7‐1 Ohgigaoka, Nonoichi‐city Ishikawa 921‐8501 Japan