1. Department of Technologies of Computers and Communications, Polytechnic Institute, University of Extremadura, Campus Universitario s/n, 10071 Caceres, Spain
2. Department of Applied Physics, School of Industrial Engineering, University of Extremadura, Avenida de Elvas s/n, 06071 Badajoz, Spain
3. Department of Computer Science, Polytechnic Institute of Leiria, Alto do Vieiro, 2401-951 Leiria, Portugal