1. Institute of Theoretical Physics, Warsaw University, Hoz̊a 69, Warsaw, Poland Department of Electrical Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada
2. Department of Electrical Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada
3. Advanced Technology Laboratory, Nortel Technology, P.O. Box 3511, Station C, Ottawa, Ontario K1Y 4H7, Canada