1. 1) Ministry of Internal Affairs and Communication: https://www.soumu.go.jp/johotsusintokei/statistics/data/190531_1.pdf, (2019).
2. 2) Ministry of Internal Affairs and Communication: https://www.tele.soumu.go.jp/j/sys/fees/purpose/kenkyu/, (2019).
3. 3) Hane, Inc. & Beacon Associates, Inc.: http://wavesafe.jp/blog/2015/12/26 (2015).
4. 4) J. Kato: JSPE., 78 (2012) 767-772.
5. 5) S. Sugimoto, S. Kondo, K. Okayama, H. Nakamura, D. Book, T. Kagotani, M. Homma: IEEE Trans. Magn., 35 (1999) 3154-3156.