1. Instituto Politecnico Nacional, SEPI-ESIME Zacatenco, U. P. Adolfo Lopez Mateos, 07738 Mexico City, DF, Mexico
2. Universitat Politecnica de Catalunya, Department of Signal Theory & Communications, Remote Sensing Lab, 08034 Barcelona, Spain
3. CINVESTAV-IPN, Telecommunications Section, San Pedro Zacatenco, 07360 Mexico City, DF, Mexico