A cellular network is a radio network made up of a number of radio cells (or just cells) each served by a fixed transmitter, known as a cell site or base station. These cells are used to cover different areas in order to provide radio coverage over a wider area than the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main transceivers each serving a cell and a set of distributed (generally, but not always, mobile) transceivers which provide services to the network's users.

Cellular networks offer a number of advantages over alternative solutions:

• increased capacity
• reduced power usage
• better coverage

A good (and simple) example of a cellular system is an old taxi driver's radio system where the taxi company will have several transmitters based around a city each operated by an individual operator.

General characteristics

The primary requirement for a network to succeed as a cellular network is for it to have developed a standardised method for each distributed station to distinguish the signal emanating from its own transmitter from the signals received from other transmitters. Presently, there are two standardised solutions to this issue, frequency division multiple access (FDMA) and code division multiple access (CDMA).

FDMA works by using varying frequencies for each neighbouring cell. By tuning to the frequency of a chosen cell the distributed stations can avoid the signal from other cells. The principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it. Other available methods of multiplexing such as polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separate signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a single cell.

In the case of the aforementioned taxi company, each radio has a knob. The knob acts as a channel selector and allows the radio to tune to different frequencies. As the drivers move around, they change from channel to channel. The drivers know which frequency covers approximately what area. When they don't get a signal from the transmitter, they also try other channels until they find one which works. The taxi drivers only speak one at a time, as invited by the operator (in a sense TDMA).

Broadcast messages and paging

Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles, commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one to one communication between the mobile transreceiver and the base station. This is called paging.