Cell phones

Updated May 29, 2009

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New for June 2009! On this page we will (hopefully) start explaining how cell phones work. If anyone has any suggestions please send them our way!

We are not experts on cell phones, but that never stopped us from blabbing on and on about a microwave topic!

Most cell phones use GaAs HBT power amplifiers and pHEMT transmit/receive switches. Silicon germanium may replace GaAs HBT due to its lower cost some day.

Most cell phones operate around 2.45 GHz.

Because cell phones are made with lead-free processes due to ROHAS, don't expect them to last more than a few years.

This info came from Rick... thanks!

CB radios and walkie-talkies are half-duplex devices. One frequency is used for talking and listening. This means that only one person can talk at a time. Cell phones are full-duplex devices which means that one frequency is used for talking and a separate frequency for listening.

The word "cell" in cell phones comes from the fact that an area is divided into cells each with a tower in the middle. Each cell is typically 10 sq. mi. A cell-phone carrier gets about 832 frequencies to use in a city: 790 for voice and 42 for data. A channel is a pair of frequencies. So there are 395 voice channels and 21 data channels. Each cell uses a seventh of these channels (56 people can talk on their cell phones at once in a cell).

Each cell phone has special codes that identify the service provider (SID--system identification code is a 5-digit number given to each carrier by the FCC), the phone (ESN--an electronic serial number is a 32-bit number programmed into each phone by the manufacturer), and the owner (MIN--mobile identification number is a 10-digit number based on the phone's number).

When you turn on your phone, it listens for its SID on the control channel (a channel used for organizing channel changes during a call and for other data transfers) and transmits a registration request. Your service provider's Mobile Telephone Switching Office (MTSO) keeps track of your phone's location in its database so that it will know which cell to direct an incoming call to.

If your phone number is dialed, the MTSO gets the call and looks in its database to see which cell the phone is in. Then the MTSO communicates with your phone via the control channel to coordinate the frequencies to use. Once those frequencies are used, the call to your phone is placed. Your base station (cell phone tower) will notice the strength of your phone's signal. If you move to the edge of your cell, your base station will coordinate (through the MTSO)with the base station you are moving towards, and at some point, your phone will receive a signal on a control channel telling it to change frequencies. At this point, you have switched cells.

If you move into a cell which is out-of-range of your service provider's towers, your call is handed to a third-party service provider. If the SID on the control channel does not match the SID programmed to your phone, you are roaming. The MTSO of the cell you are roaming contacts your home MTSO to verify that your SID is valid. If confirmed, the roaming MTSO tracks your phone as you move through cells.