1. Department of Signal Theory and Communications, and Telematics Engineering, University of Valladolid, Valladolid, Spain.
2. Department of Informatics, University of Valladolid, Valladolid, Spain.
3. Department of Statistics, University of Valladolid, Valladolid, Spain.
4. Department of Informatics and Numeric Analysis, University of Cordoba, Cordoba, Spain.