Cellular communications
Radio transceivers are used as telephones in the specialized communications system called as cellular. Originally, cellular communications network was unreliable, and was used by traveling business people mainly. Nowadays, cellular telephone units are so common that many people regard them as basic necessities.

How cellular systems work

A cellular telephone set, or cell phone, looks and functions like cross between a cordless telephone receiver and a walkie-talkie. The unit contains radio transmitter and receiver combination called as transceiver. Transmission and reception take place on different frequencies, so you can talk and listen at same time. This capability, which allows you to hear the other person interrupt you if he or she chooses, is known as full duplex.

In the ideal cellular network, all transceivers are always within the range of at least one repeater. The repeaters pick up transmissions from portable units and retransmit the signals to telephone network and to other portable units. The region of coverage for any repeater (also called as a base station) is called as cell.

When the cell phone is in motion, say in a car or on a boat, set goes from cell to cell in network. This situation is shown in the figure given below. The curved line is the path of vehicle. Base stations transfer access to cell phone. This is called as handoff. Solid lines show limits of the transmission and reception range for each base station. All the base stations are connected to regional telephone system. This makes it possible for user of portable unit to place calls to, or receive calls from, anyone else in system, whether those other subscribers have cell phones or regular phones.

                        Figure--In a cellular system, a moving vehicle is always within the range of at least one base station.


Older cellular systems occasionally experience from call loss or breakup when signals are handed off from one repeater to the other. This problem has been largely overcome by a technology called as code-division multiple access (CDMA). In the CDMA, repeater coverage zones overlap notably, but signals do not interfere at all with each other as every phone set is assigned a unique signal code. Instead of abruptly switching from 1 base station zone to next, the signal goes through a region in which it is sent actually through more than one base station at a time. This make before break scheme gets rid of one of the most annoying problems inherent in the cellular communications.

To use a cellular network, you should purchase or rent a transceiver and pay a monthly fee. The fees vary, which depends on the location and amount of time per month you use service. While using such a system, it is significant to keep in  mind that your conversations are not private necessarily  .It is simple for unauthorized people to eavesdrop on wireless communications than to intercept wire or cable communications.