1. Graduate School of Science and Engineering, University of the Ryukyus, 1 Senbaru, Okinawa 903-0213, Japan
2. Faculty of Engineering, Information and Systems, University of Tsukuba, 1 Chome-1-1 Tennodai, Ibaraki 305-8577, Japan
3. Hawai’i Natural Energy Institute, University of Hawai’i at Manoa, Honolulu, HI 96822, USA