1. Institute of Engineering, Polytechnic of Porto Dept. of Electrical Engineering Rua Dr. Ant´onio Bernardino de Almeida, 431 4200 – 072 Porto, PORTUGAL
2. Department of Physics University of Bologna, and INFN-Bologna Via Irnerio 46, I-40126 Bologna, ITALY
3. Institute of Mathematics and Informatics Bulgarian Academy of Sciences “Acad. G. Bontchev” Str., Block 8 Sofia – 1113, BULGARIA