1. a Department of Electrical and Electronic Engineering (C-724), College of Engineering , Yonsei University , 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749 , Korea
2. b Department of Electrical Engineering, College of Electronics & Information , Kwangwoon University , 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701 , Korea