1. Department of Electrical and Electronic Engineering, Okayama University, Tsushimanaka 3-1-1, Okayama 700, Japan
2. Hiroshima University, Faculty of Integrated Arts and Sciences, Kagamiyama, Higa-shi, Hiroshima, 739, Japan
3. ATR Optical and Radio Communications Research Laboratories, 2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto-fu 619-02, Japan