1. Department of Electronic and Electrical Engineering, University College, Belfield, Donnybrook, Dublin 4, Ireland
2. Katholieke Universiteit Leuven, Department of Electronic and Engineering, ESAT-SISTA, Kardinaal Mercierlaan 94, B-3001 Leuven (Heverlee), Belgium
3. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, USA