1. a EE Dept. , Sogang Univ. , Korea
2. b RFID R&D, Hynix Semiconductor , Ichon, 467-701, Korea
3. c Department of Electrical Engineering , Korea Univ. , Seoul, 136-701, Korea
4. d Department of Electrical & Electronic Engineering , Kongju National Univ. , Cheonan, 330-717, Korea