[1] Politecn Milan, Dipartimento Elettr & Informat, I-20133 Milan, Italy
[2] Fraunhofer Inst Telecommun Heinrich Hertz Inst, D-10587 Berlin, Germany
[3] Univ N Carolina, Dept Comp Sci, Chapel Hill, NC 27599 USA
[4] ETH, Inst Computat Sci, CH-8092 Zurich, Switzerland