[1] Univ Ramon Llull, GTM Grp Recerca Tecnol Media, Quatre Camins 2, Barcelona 08022, Spain
[2] Univ Politecn Cataluna, IEEC UPC, Diagonal 647 ETSEIB, E-08028 Barcelona, Spain
[3] Univ Politecn Cataluna, Dept Matemat, Diagonal 647 ETSEIB, E-08028 Barcelona, Spain
[4] Univ Barcelona, Inst Ciencies Cosmos, IEEC, Marti & Franques 1, E-08028 Barcelona, Spain