A cell phone is a portable telephone which receives or makes calls through a Cell site, or transmitting tower. Radio waves are used to transfer signals to and from the cell phone. Large geographic areas (representing the coverage range of a service provider) are split up into smaller cells to deal with line-of-sight signal loss and the large number of active phones in an area. Each cell site has a range of 3-15 miles and overlaps other cell sites. All of the cell sites are connected to one or more cellular switching exchanges which can detect the strength of the signal received from the telephone.

As the mobile phone user moves or from one cell area to another, the exchange automatically commands the handset and a cell site with a stronger signal (from the handset) to go to a new radio channels. When the latest mobile phone handset responds through the new cell-site, the exchange switches the connection to the new cell-site.

With CDMA technology, the process is slightly different. Multiple CDMA mobile handsets share a specific "channel"; the signals are separated by sending each bit using a pseudo-random code sequence specific to each phone. As the user moves from one cell to another, the handset actually connects to both sites simultaneously. This is known as a "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.

Modern mobile phones use cells because radio frequencies are a limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that a limited number of radio frequencies can be reused by many callers with less interference. CDMA handsets, in particular, must have strict power controls to avoid interference with each other. An incidental benefit is that the batteries in the handsets need less power.

However, almost all mobile phones use cellular technology, including GSM, CDMA and the old analog mobile phone systems. Hence, many people use the term "cell phone" to mean any mobile telephone system. The exception to mobile phones using cellular technology are satellite phones.

Old systems predating the cellular principle may still be in use in places. The most notable real hold-out is that many amateur radio operators maintain phone patches in their clubs' VHF repeaters.

There are a number of different digital cellular technologies; these include: GSM, CDMA (Code Division Multiple Access), DECT, IS-136, and iDEN.

Rrticle originally published on http://en.wikipedia.org