[1] Department of Information, University of Rome la Sapienza, 00184 Rome, Italy
[2] Bell Labs., Lucent Technologies, 791 Holmdel-Keyport Road, Holmdel, NJ 07733, United States
[3] Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, United States
[4] Agere Systems, Allentown, PA 18109, United States