1. Railway Technical Research Institute, 2‐8‐38 Hikari‐cho, Kokubunji‐shi Tokyo 185‐8540 Japan
2. Electrical Engineering and Chemistry, Graduate School of Integrative Science and Engineering Tokyo City University, 1‐28‐1 Tamazutsumi, Setagaya‐ku Tokyo 158‐8557 Japan
3. Department of Electrical, Electronics and Communication Engineering, Faculty of Science and Engineering Tokyo City University, 1‐28‐1 Tamazutsumi, Setagaya‐ku Tokyo 158‐8557 Japan