1. Department of Electrical and Electronic Engineering Utsunomiya University 7‐1‐2, Yoto, Utsunomiya Tochigi 321‐8585 Japan
2. Department of Innovative Electrical and Electronic Engineering National Institute of Technology Oyama College, 771, Nakakuki, Oyama Tochigi 323‐0806 Japan
3. Artificial Intelligence Research Center National Institute of Advanced, Industrial Science and Technology 1‐1‐1, Umezono, Tsukuba Ibaraki 305‐8560 Japan
4. Department of Electrical and Electronic Engineering Tokai University 4‐1‐1, Kitakaname, Hiratsuka Kanagawa 259‐1292 Japan