[1] NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan
[2] Department of Electrical and Electronic Engineering, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata-gakuen-machi, Akita-shi, Akita 010-8502, Japan
[3] Graduate School of Information Science and Technology, Hokkaido University, Sapporo, Hokkaido 060-0814, Japan