1. Department of Electrical Engineering and Information Science, Ruhr-University Bochum, D-44780 Bochum, Germany
2. School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, USA
3. Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
4. School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA