1. Fujitsu Laboratories Ltd., 10‐1 Morinosato‐Wakamiya, Atsugi, Kanagawa 243‐0197, Japan
2. Communications Research Laboratory, 4‐2‐1 Nukui‐kitamachi, Koganei, Tokyo 184‐8795, Japan
3. Osaka University, 1‐3 Machikaneyama, Toyonaka, Osaka 560‐8531, Japan