Cell phone

The Cell Phone

Cell phones, or mobile phones, were originally used in cars, airliners and trains. Basically cell phones are low-powered radio-telephones. Radio transmitters through which the calls go, are located in small geographical units called cells. Each cell's signals are too weak to interfere with other cells working on the same frequency. Narrow band frequency modulation (FM) is usually used for transmission. Every message gets a carrier that is unique to the phone from which it was sent. The cellular phone was first tested in 1978 but has grown on a large scale since then. In Britain alone the subscribers had grown up to 6 million in 1996 and in Japan up to 1 cell phone for every ten people. From 1995 to 1996 cellular telephony grew by 40% and cell phones are standard equipment in many cars.

Every day approximately 30 000 people in the US sign up for and start using a cell phone. A cell phone, a duplex device, uses one frequency for talking and another for listening. Cell phones can communicate on 1 664 channels. Operating in cells gives cell phones an incredible range. They can switch from cell to cell as they move around. You can keep on having a conversation the whole time, while driving across an entire city, for instance.

Before cell phones, radio telephones were used in cars. There was one central antenna per city with only 25 channels available. This meant that you had to have a big transmitter in your car which was able to transmit 40 to 50 miles. There was also far too few channels available for everyone.

The realisation that a city can be divided into cells that can reuse frequencies was a big breakthrough. Reusing of frequencies allows millions of people to use cell phones problem free. Each cell has a tower and some sort of building holding the radio equipment. Because low-power transmitters are used frequencies can be reused. The using of low-power transmitters helps keeping power consumption low which helps a lot, since cell phones are usually battery operated. Low power also means smaller batteries and that's how handheld cell phones was made possible. The transmissions of the base station and phones in its cell does not go very far outside its boundaries. The same frequencies can then be used over a whole city. A lot of base stations are thus needed in a city. A large city can have hundreds of towers, but luckily costs are kept low per user because so many people use cell phones. There are central offices as well, called MTSO (Mobile Telephone Switching Office). All phone connections to the normal land-based phone system in a region are handled by this office, as well as all base stations.

Your cell phone listens for a system ID (SID) on the control channel when switched on. It also transmits a registration request and the network keeps track of your phone through a location database so that the MTSO knows which cell you are in when someone calls you. If you go from one cell to another the phone detects a change in the control channel's strength and has to re-register itself with the new tower when it changes channels. If no control channels can be found, the phone knows it's out of range and displays the no service message.

Digital cell phones are the latest trend and have very quickly become very easy to come by. Although they use the same radio technology, they convert your voice into digital 1's and 0's and then compress it. Now from 3 to 10 calls occupy the same space as a single analogue call. The digital channel has just about three times the capacity of the analogue channel. In order to do this and keep acceptable voice quality, clever modulation and encoding schemes are required. It is for this reason that cell phones have a lot of processing power.

Personal Communication Services (PCS) place the emphasis on personal service. Services like e-mail, paging and caller ID are included. This is a wide service already used by millions of people around the world. Apart from the fact that the cell phone is completely mobile, it is this service that separates the cell phone from a normal telephone.