1. Department of Electrical Engineering, Universidad Politécnica de Madrid, Madrid, Spain. E-mails: agfaura@die.upm.es, fernando.fernandezm@upm.es, ruben.sansegundo@upm.es
2. Institute of Cognitive Science, Universität Osnabrück, Osnabrück, Germany. E-mail: ahernandez@uos.de
3. Signal Theory and Communications, Universidad Carlos III de Madrid, Madrid, Spain. E-mail: fdiaz@tsc.uc3m.es