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How A Cell Phone Works

April 20, 2010

How’s a Cell Phone Work?

Almost everyone owns one. But have you ever wondered how your cell phone works? In this essay, we will attempt to simply the answer to that question to the best of our ability.

To do so, we will separate the hardware of the actual cell phone, from the issues revolving around sending data through the airwaves, to the cell towers that make it possible.

The Air Waves

The cell phone “system” divides a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.

A good way to understand the workings of the cell phone is to compare it to a CB radio or a walkie-talkie. Unlike a CB radio or a walkie-talkie, where only one person at a time can speak, the cell phone system is a full duplex system whereby both parties can speak at the same time. This is accomplished by assigning one frequency for speaking and a second frequency for listening.  While a walkie-talkie typically has one channel, and a CB radio has 40, the cell phone can communicate on 1,664 channels! A walkie-talkie can transmit about one mile. A CB radio can transmit about five miles. Cell phones operate within cells, and they can switch cells as they move around. Since cell phones operate within cells and can switch from one cell to another while moving around, their range is limited only by the cell towers in the area…dead zones, so to speak.  Typically, you can drive hundreds of miles while on one cell phone call and not drop the call thanks to the design of our cellular system.

In half-duplex radio, both transmitters use the same frequency. Only one party can talk at a time.
In full-duplex radio, the two transmitters use different frequencies, so both parties can talk at the same time.
Cell phones are full-duplex.

In a typical analog cell-phone system, the cell-phone carrier receives around 800 frerquencies to use within a city. The carrier separates the city into cells that are typically 10 square miles. Cells are like hexagons on a big hexagonal grid. Since your cell phone is never far away from a grid tower, it does not require a high transmission, thus enabling the same frequencies to be used by neighboring cells. Each cell has a base station that consists of a tower and a small building containing the radio equipment. We’ll discuss that later.

In an analog cell, there are 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. But with newer digital technology, (second generation technology and later) each cell can carry three times as many calls or about 168 channels. ­

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. Each carrier in each city also must operate a Mobile Telephone Switching Office (MTSO). Here, all the connections to landlines are made. As you move toward the edge of your cell, your cell’s base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are heading towards realizes your signal strength is growing. The MTSO coordinates a hand-off, of sorts, from the cell you are leaving to the cell you are entering. This is done without your knowledge and is the key reason why you do not drop the call your on. And this process exists among different carriers as well. This is what is referred to as roaming, where you leave your provider’s cell and “roam” into another provider’s cell. It is a pretty amazing technology but not without its problems (enormously high roaming rates in some cases). Digital phones convert your voice into binary data (1s and 0s) and then compress it. This compression allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.

As most all cell phones in the market today are digital phones, we will concentrate on the inner workings of the digital phone only.

The Hardware

As simple as our cell phones are to operate, they’re really quite intricate pieces of technology. Today’s digital cell phones can process millions of calculations per second in to keep up with the necessary compressions and decompressions of the typical voice stream.

The parts of a cell phone

You have probably never taken a cell phone apart, but if you did you would find the following: there is a compact speaker, a microphone, a keyboard, a display screen, and a powerful circuit board with microprocessors that make each phone a miniature computer. When connected to a wireless network, this allows you to make phone calls or exchange data with other phones and computers around the world. And all this is powered by a very lightweight battery that can last for days (Author unknown 6).

The circuit board is the heart of the system. Here is a typical digital phone circuit board from Nokia:
The front

The back


On each circuit board are a number of computer chips. Some of what they do involve the following: The analog to digital and digital to analog conversion chips translate the outgoing audio signal from analog to digital and the incoming signal from digital back to analog. The digital signal processor (DSP) is a processor designed to perform signal-manipulation calculations at high speed. ­

The microprocessor handles all of the busy work including managing command and control signaling with the base station and coordinating the remaining functions on the board.

The microprocessor

The ROM and flash memory chips provide storage for the operating system and customizable features, such as the phone directory. The radio frequency and power section handles power management and recharging. And the RF amplifiers handle signals traveling to and from the antenna.

The display and keypad contacts

The phone’s display has grown in size as the number of features has increased, such as phone directories, calculators and games. And many of the phones incorporate some type of PDA or Web browser.


The Flash memory card on the circuit board
The cell-phone speaker, microphone and battery backup

The technology that allows all this to work in an appliance that fits in the palm of your hand is incredible, considering that the same technology only 30 years ago would have filled an entire floor of an office building!

A word on 3G technology, the standard in mobile communications. 3G stands for “third generation” and is intended to provide true multimedia in today’s cell phone. These are typically named “smart phones” and feature increased bandwidth and transfer rates to accommodate Web-based applications and phone-based audio and video files.  3G networks have potential transfer speeds of up to three Mbps. These speeds allow for downloading data from the Internet and sending and receiving large, multimedia files. 3G makes today’s cell phone working computers that can accommodate video conferencing, receiving streaming video from the Web, sending and receiving faxes and instantly downloading e-mail messages with attachments.

Cell-phone Towers

None of the technology mentioned above would be possible without the many cell phone towers found throughout our cities. Each tower is typically a steel pole or lattice structure that rises hundreds of feet into the air. This cell-phone tower along I-85 near Greenville, S.C., is typical in the United States: This tower is shared by three different cell-phone providers. At the base of the tower you can see that each provider has its own equipment.

The typical equipment that each provider requires at each tower includes radio transmitters and receivers that let the tower communicate with the phones. The radios connect with the antennae on the tower through a set of thick cables. If you ever wondered why you do not find many of these towers as you drive by them, they come in many shapes and sizes and are often fairly well disguised. This one in Morrisville, North Carolina, is designed to look like a tree!

The creation of today’s cell phone system is a marvel of scientific and engineering design. We are not required to understand how it works, but I do hope that this essay allows you to appreciate this wonderful little appliance and all that goes on behind the scenes in order to make your cell phone experience a good one.

Reference

Author unknown 6. How Does A Cell Phone Work? Cell Phone Accessories. 2010.

http://www.cell-phone-accessories.com/how-does-cell-phone-work.html. April 13, 2010.

All material comes from this site unless otherwise noted:

Marshall, Brian, Tyson, Jeff & Layton, Julia. How Cell Phones Work. How Stuff Works,

2010. http://electronics.howstuffworks.com/cell-phone1.htm April 13, 2010

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