1. Department of Electrical and Electronic Engineering, University of Cagliari, 09123 Cagliari, Italy
2. Department of Civil, Environmental and Architectural Engineering (DiCAAR), University of Cagliari, 09123 Cagliari, Italy
3. Department of Physics, University of Cagliari, CNR-IOM SLACS and ETSF, Cittadella Universitaria di Monserrato, Strada Prov.le Monserrato-Sestu, km 0.700, Monserrato, 09042 Cagliari, Italy