1. Department of Electrical Engineering and Automation, Aalto University, FI-00076 Espoo, Finland
2. Department of Mathematics and Systems Analysis, Aalto University, FI-00076 Espoo, Finland
3. Unit of Electrical Engineering, Tampere University, FI-33014 Tampere, Finland
4. Institute of Fundamentals and Theory in Electrical Engineering, Graz University of Technology, Graz A-8010, Austria