So that you can use your cell phone to make or receive phone calls wherever you are, your phone needs a good connection to a base station in the near vicinity.
Each cell phone and also each base station has a transmitter as well as a receiver. High-frequency electromagnetic fields (EMF) are needed to transmit information between a cell phone and a base station. The information is then usually forwarded between base stations, and between a base station and the fixed network, over copper or fiber optic cables.
To ensure high-quality coverage, mobile communications requires a large number of base stations. Each base station can only handle a certain number of calls and transmit a certain volume of data. Each of them therefore only serves a narrowly defined area (known as the radio cell). An increasing number of users and increasing data rates therefore require a denser mobile communications network, i.e., more base stations.