1. Department of Electronic Engineering, Silla University, San 1-1 Gwaebeop-Dong, Sasang-Gu, Busan 617-736, Republic of Korea
2. Department of Information and Communication Engineering, Kyungnam University, 449 Wolyong-Dong, Masan, Kyungnam 631-701, Republic of Korea
3. School of Electrical and Computer Engineering, Georgia Institute of Technology, North Avenue, Atlanta, GA 30332, USA