1. Physics Department and CICECO; University of Aveiro; 3810-193 Aveiro Portugal
2. Physics Department, CICECO and Instituto de Telecomunicações; University of Aveiro; 3810-193 Aveiro Portugal
3. VTT Technical Research Centre of Finland; FI90571 Oulu Finland
4. Instituto de Telecomunicações and Department of Electrical and Computer Engineering; Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais; Lisboa 1049-001 Portugal