[1] Univ Essex, Sch Math Stat & Actuarial Sci, Colchester CO4 3SQ, England
[2] Univ Politecn Cataluna, Signal Theory & Commun Dept, Barcelona 08034, Spain
[3] Univ Pompeu Fabra, Dept Informat & Commun Technol, Barcelona 08018, Spain
[4] Univ Cambridge, Dept Engn, Cambridge CB2 1PZ, England