1. Department of Electrical Engineering, ISEC, Polytechnic Institute of Coimbra, Rua Pedro Nunes – Quinta da Nora, 3030-199 Coimbra, Portugal; INESC Coimbra, Rua Antero de Quental, 3000-033 Coimbra, Portugal
2. Department of Electrical Engineering and Computers, University of Coimbra, Polo II, Pinhal de Marrocos, 3030-290 Coimbra, Portugal; INESC Coimbra, Rua Antero de Quental, 3000-033 Coimbra, Portugal