Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it|Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it|Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it|Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it|Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it|Dept. of Electrical and Electronic Engineering, University of Cagliari Piazza dArmi, I-09123 Cagliari, Italy. armano@diee.unica.it