1. Signals, Systems and Radiocommunications Department, Telecommunications School, Universidad Politécnica de Madrid, Avda. Complutense 30, 28040, Madrid, Spain. E-mails: luca.bergesio@grpss.ssr.upm.es, abernardos@grpss.ssr.upm.es, imarquinez@grpss.ssr.upm.es, besada@grpss.ssr.upm.es, jramon@grpss.ssr.upm.es