1. Department of Information Engineering, Computer Science and Mathematics, University of L’Aquila, 67100 L’Aquila, Italy</br>
2. Research Unit L’Aquila, National Inter-University Consortium for Telecommunications (CNIT), 67100 L’Aquila, Italy</br>
3. Center of Excellence DEWS, University of L’Aquila, 67100 L’Aquila, Italy