1. Graduate School of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japan
2. Graduate School of Information Science and Technology, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan
3. NTT Communication Science Laboratories, Nippon Telegraph and Telephone Corporation, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan