1. Department of Electronics, Telecommunications and Computer Engineering, ISEL, Lisbon, 1959-007, Portugal.
2. CTS-UNINOVA, 2829-516 Caparica, Portugal.
3. Electrical and Computer Engineering Department, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
4. National Institute for Astrophysics, Optics and Electronics, Puebla 72840, Mexico.
5. TF TE Ioffe R&D Center, St-Petersburg, Russia.