1. Department of Solid-State Physics, Technische Universitaet Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
2. Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin, Germany
3. Hochschule fuer Telekommunikation Leipzig (HfTL), Gustav-Freytag-Str. 43-45, 04277 Leipzig, Germany