1. Graduate School of Science and Engineering, University of the Ryukyus, Nishihara 903-0213, Japan
2. Faculty of Engineering, Information and Systems, University of Tsukuba, 1 Chome-1-1 Tennodai, Ibaraki 305-8577, Japan
3. National Institute of Technology, Sasebo College, Sasebo 857-1193, Japan
4. Department of Electrical and Computer Engineering, University of Texas at El Paso, El Paso, TX 79968, USA