1. New York University-Polytechnic School of Engineering, 5 Metrotech Center, Brooklyn, New York 11201, USA
2. NorthWest Research Associates, P.O. Box 530, Stockton Springs, Maine 04981, USA
3. Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts 02215, USA
4. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA