[1] Politecn Milan, Dept Elect Engn & Informat Sci, I-20133 Milan, Italy
[2] Ist Nazl Fis Nucl, I-20133 Milan, Italy
[3] Alenia Marconi Syst, I-00131 Rome, Italy
[4] Univ Modena, Dept Phys, I-41100 Modena, Italy
[5] Ist Nazl Fis Nucl, I-41100 Modena, Italy