1. Electrical Engineering Division, University of Cambridge, 9 J. J. Thomson Avenue, Cambridge CB3 0FA, UK
2. Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
3. Nokia Research Center, Broers Building, 21 J. J. Thomson Avenue, Cambridge CB3 0FA, UK
4. Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan