1. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
2. Department of Computer Science and Engineering, University of Aizu, Aizu-Wakamatsu 965-8580, Japan
3. Department of Electrical Engineering, University at Buffalo, SUNY, Buffalo, New York 14260 USA
4. Department of Electrical Computer and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA