M. Gómez-Parra, C. Sancho, D. Velasco, P. Munőz- Condes, M. Antonia G. San Andrés, L. Calo - Metro de Madrid; Fco. Javier González Fernández – Politechnical University of Madrid; José Carpio Ibáńez – UNED Dep. Electr. and Computer Engineering Madrid - SPAIN