1. Graduate School of Information Science and Technology, Hokkaido University, Sapporo 060-0814, Japan
2. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi 243-0198, Japan
3. Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
4. Research Institute of Electronics, Shizuoka University, 3-5-1, Johoku, Hamamatsu 432-8011, Japan