[1] Politecn Turin, Dept Elect, Turin, Italy
[2] Univ So Calif, Dept Elect Engn, Viterbi Sch Engn, Los Angeles, CA 90089 USA
[3] Tech Univ Munich, Inst Commun Engn, D-8000 Munich, Germany
[4] Ecole Polytech Fed Lausanne, Lab Informat Theory, CH-1015 Lausanne, Switzerland