Cellular Networks, Cells, and Base Stations

- Cellular Networks, Cells, and Cell Sites

A cellular network or mobile network is a communication network where the last link is wireless. The network is distributed over land areas called cells, each served by at least one fixed-location transceiver (short for transmitter-receiver - a device that both transmits and receives analog or digital signals), but more normally three cell sites or base transceiver stations. These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. 

In radio communications, a transceiver is a two-way radio that combines both a radio transmitter and a receiver that exchanges information in half-duplex mode. A cell site (or cell tower, or cellular base station) is a cellular-enabled mobile device site where antennae and electronic communications equipment are placed - typically on a radio mast, tower, or other raised structure - to create a cell (or adjacent cells) in a cellular network. To build a 5G cell tower, you need a fiber optic Internet connection, some 5G cell equipment, and something tall to stick the equipment to.

The division of a city into small cells allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in nonadjacent cells. 

- Radio Spectrum

A cell is the geographic area that is covered by a single base station in a cellular network. A network for wireless communications is comprised of a large number of base stations to efficiently use radio spectrum to cover the service area. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell. When joined together, these cells provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (e.g., mobile phones, tablets and laptops equipped with mobile broadband modems, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission. 

In a cellular radio system, a land area to be supplied with radio service is divided into cells, in a pattern which depends on terrain and reception characteristics but which can consist of roughly hexagonal, square, circular or some other regular shapes, although hexagonal cells are conventional. Each of these cells is assigned with multiple frequencies (f1 – f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.

- Frequency Reuse and Mobile Switching Center

The mobile switching center (MSC) connects calls by switching the digital voice data packets from one network path to another (also called routing). The MSC also provides the information that is  needed to support mobile service subscribers, such as user registration and authentication information. Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area that are separated by sufficient distance to cause minimal interference with each other.

The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the mobile communication switching system that permits multiple callers in a given area to use the same frequency by switching calls to the nearest available cellular tower having that frequency available. This strategy is viable because a given radio frequency can be reused in a different area for an unrelated transmission. In contrast, a single transmitter can only handle one transmission for a given frequency. Inevitably, there is some level of interference from the signal from the other cells which use the same frequency. Consequently, there must be at least one cell gap between cells which reuse the same frequency in a standard FDMA system.