1. Department of Information Engineering; University of Brescia; via Branze 38 25123 Brescia Italy
2. Department of Electrical Engineering; Eindhoven University of Technology; P.O. Box 513 5600 MB Eindhoven The Netherlands
3. TNO/Holst Centre; High Tech Campus 4 5656 AE Eindhoven The Netherlands
4. Philips Research Laboratories; High Tech Campus 4 5656 AE Eindhoven The Netherlands