Department of Computer Engineering, Bilkent University, Bilkent, 06800 Ankara, Turkey Tel.: +90 312 2903405/ fax: +90 312 2664047;Department of Electrical and Computer Engineering, Mississippi State University, Mississippi State, MS, USA Tel.: +1 662 3253721;Department of Information Engineering and Computer Science, University of Trento, I-38123 Trento, Italy Tel.: +39 0461 882056