[1] Univ Porto, Dept Informat Engn, Fac Engn, P-4099002 Porto, Portugal
[2] Inst Syst & Comp Engn Technol & Sci, P-4200465 Porto, Portugal
[3] Univ Penn, Elect & Syst Engn Dept, Philadelphia, PA 19104 USA
[4] Univ Svizzera Italiana, Fac Informat, CH-6900 Lugano, Switzerland