1 Laboratory of Electronics and Signal Processing, Department of Physics, Faculty of Sciences, University of Dschang, P. O. Box 067, Dschang, Cameroon.
2 Instituto de Física Teórica, Universidade Estadual Paulista, UNESP, Rua Dr. Bento Teobaldo Ferraz 271, Bloco II, Barra Funda, São Paulo, 01140-070 Brazil.