1. Department of Electrical and Information Engineering, Polytechnic University of Bari, Via Orabona, 4, 70125 Bari, Italy
2. Department of Engineering, University of Ferrara and CNIT, Via Saragat, 1, 44122 Ferrara, Italy
3. Department of Electrical, Electronic and Information Engineering “G. Marconi”, University of Bologna, Viale del Risorgimento, 2, 40136 Bologna, Italy