1. Department of Radio-Electronics and Telecommunications, Yuri Gagarin State Technical University of Saratov, 77, Politechnicheskaya, Saratov 410054, Russian Federation
2. Faculty of Information Technology, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland
3. Faculty of Mathematics and Mechanics, St. Petersburg State University, 28, Peterhof, Universitetskiy proezd, St. Petersburg 198504, Russian Federation
4. Department of Physics, Technical University of Denmark, 2800 Lyngby, Denmark