1Department of Computer Science and Engineering, Helsinki University of Technology,P.O. Box 5400, 02015 HUT, Finland.
2IIMAS, Universidad Nacional Autonoma de México, Apdo. Postal 70-221, México, DF, 04510, México.
3Department of Electrical and Communications Engineering, Helsinki University of Technology, P.O. Box 3000, 02015 HUT, Finland.