Under Article 2 of Japan's Radio Act, a radio wave is an electromagnetic wave with a frequency of 3,000,000 MHz or below. That ceiling is 3THz, and every familiar wireless service (mobile phones, Wi-Fi, television, radio) works somewhere inside that range. Mobile networks in Japan mainly use bands between 700MHz and 3.5GHz, while 5G adds the 3.7GHz and 4.5GHz bands as well as the far higher 28GHz millimetre-wave band.
How a wave behaves depends heavily on its frequency. Low frequencies (700-900MHz, known in Japan as the "platinum band") pass through buildings well and cover a wide area, but carry less capacity. High frequencies (3.5GHz to 28GHz) deliver high capacity and speed, yet they are easily blocked by obstacles and travel a shorter distance. Carriers therefore combine several bands instead of relying on one.
Spectrum in Japan is tightly managed under the Radio Act, and the Ministry of Internal Affairs and Communications allocates frequencies. Mobile base stations must meet the technical standards set under that Act, and limits on human exposure are laid down in the Radio Radiation Protection Guidelines, at a level equivalent to the international ICNIRP guidelines.
Typical problems include being out of range, interference from other equipment on the same frequencies, and disruption caused by devices transmitting without a licence, which is an offence under the Radio Act. Airplane mode shuts down all of a handset's transmission and reception at once, and is used during take-off and landing and in areas of medical facilities where radio use is restricted. On most handsets you can switch Wi-Fi or Bluetooth back on individually while airplane mode stays active, which is how in-flight Wi-Fi is normally used.
The signal bars on the screen are easy to misread. They show only how strongly the base station's signal is arriving, not how busy that base station is. The bars can be full while data crawls, simply because a crowd nearby is sharing the same cell. The reverse also holds: one bar is often enough for a voice call, so it is no reason to give up. When the signal is weak indoors, moving to a window or to a higher floor usually helps, because walls and concrete absorb higher frequencies particularly well.