A base station is wireless equipment that connects mobile phones and smartphones to a carrier's network via radio waves. Consisting of antennas, transceivers, and control units, base stations are installed on building rooftops, towers, and utility poles. Every mobile phone call or data session must pass through the nearest base station.
The area covered by a single base station is called a "cell." Cell size varies greatly by environment - a few hundred meters in urban areas, several kilometers in suburbs, and potentially tens of kilometers in mountainous regions. Urban cells are small because the high density of users requires distributing the load across more stations. This honeycomb-like cell arrangement is the origin of the "cellular" name, and why mobile phones are called "cellular phones" in English.
As users move, the connected base station switches automatically (handover). This switch typically completes in tens of milliseconds, so calls rarely experience noticeable interruption even during movement.
Japan has over 600,000 base stations installed. With the spread of 5G, deployment of millimeter-wave small cells is advancing in urban areas. Millimeter wave enables high-speed communication but has short range, requiring small base stations every few tens of meters - installation on streetlights and traffic signals is being explored. Securing installation sites and addressing aesthetic concerns are ongoing challenges requiring coordination with local governments. Carriers compete to expand coverage, and the number of base stations is a key indicator of service quality. Each carrier's official website provides coverage maps for checking signal conditions at home and work.