1. a Institute of Physics, Polish Academy of Sciences , Al. Lotnikow 32/46, 02-668 , Warsaw , Poland
2. b Wirtschafsts und Infrastrukturplannung , Sylvensteinstrasse 2, 81396, Munich , Germany
3. c France Telecom Research and Development Centre , Grenoble, PB 98, 38243 , Meylan , France
4. d CEA-G/LETI/DTS , 17 rue des Martyrs, 38054 Grenoble cedex 9, France E-mail: