[1] Univ Sheffield, Dept Elect & Elect Engn, Sheffield S1 3JD, England
[2] Ilmenau Univ Technol, Commun Res Lab, D-98684 Ilmenau, Germany
[3] Univ Pisa, Dept Informat Engn, I-56122 Pisa, Italy
[4] TU Wien, Inst Telecommun, A-1040 Vienna, Austria
[5] Univ Connecticut, Dept Elect & Comp Engn, Storrs, CT 06269 USA