Putting Information on Electromagnetic Waves

How do cell phones work?

Cell phones use the UHF part of the electromagnetic spectrum, 800 to 900 MHz, 1,700 to 1,800 MHz, and 2,100 to 2,200 MHz. The service area for a cell phone provider is divided up into hexagonally shaped cells, each one served by base stations with antenna towers at three corners of the cell. The stations can both receive and transmit information to cell phones. The stations are connected to a network that uses fiber optic cables. When a cell phone is turned on it searches for available services according to a list stored in the phone. It selects the correct frequencies to transmit and receive data, then sends its serial and phone numbers to the system, registering itself in that cell. The network makes sure that the phone number is part of its system and that there is money in the account to pay for a call. After registration is complete and a call is made to that phone, the network can direct the call to the correct cell. The cell phone always searches for the strongest signal from a tower. If the phone moves during the conversation, then the signal strengths will change and the phone will “hand off” the call to a different base station.

Cell phones digitize the voice signals. Special circuits, called digital signal processors, then compress the voice signals and insert codes that can detect errors in transmission. Compression is achieved by sending only the changes in the digital signals, not what stays the same. Cell phone systems also send many different conversations at the same time. One method, called TDMA (time division multiple access), splits up three compressed calls and sends them together. CDMA (code division multiple access) uses TDMA to pack three calls together, and then puts six more calls at two other frequencies. Each of the nine (or more) calls is assigned a unique code so that it can be directed to the correct recipient. Spread CDMA systems use a wide band of frequencies that permit more simultaneous calls.

The technology behind the cell phones we take for granted today is amazing. Transmitting and receiving messages in the electromagnetic spectrum, cell phones automatically select and correct appropriate frequencies, send serial numbers to servers, and register themselves in the cell in which they are located at the time.

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