1. a Department of Electronics Engineering, Seokyeong University, Jungneung-Dong 16-1, Sungbuk-Gu, Seoul 136-704, South Korea
2. b Department of Electronics Engineering, Seokyeong University, Jungneung-Dong 16-1, Sungbuk-Gu, Seoul 136-704, South Korea
3. c Department of Electrical & Computer Engineering, Michigan State University, East Lansing, MI 48824, USA