How Cell Phones Work

From Tibet to Tanzania to Toronto, no matter where you go you'll see someone talking on his or her cell phone. These days, cell phones provide an incredible array of functions, and new ones are being added at a breakneck pace. Depending on the cell phone model, you can:

• Store contact information
• Make task or to-do lists
• Keep track of appointments and set reminders
• Use the built-in calculator for simple math
• Send or receive e-mail
• Get information (news, entertainment, stock quotes) from the Internet
• Play games
• Watch TV
• Send text messages
• Take photos and videos
• Integrate other devices such as PDAs, MP3 players and GPS receivers

You might hear terms like 4G, LTE, GSM and CDMA thrown around and wonder what they refer to. At its most basic, a cell phone is a radio -- an extremely sophisticated radio, but a radio nonetheless. We'll show you what we mean.

Cell-phone Frequencies

In the dark ages before cell phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit 40 or 50 miles (about 70 kilometers). It also meant that not many people could use radio telephones -- there just were not enough channels.
The genius of the cellular system is the division of 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 sophistication of a cell phone is to compare it to a CB radio or a walkie-talkie.

In half-duplex radio, both transmitters use the same frequency. Only one party can talk at a time.

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• Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once.
• Channels - A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical cell phone can communicate on 1,664 channels or more.
• Range - A walkie-talkie can transmit about 1 mile (1.6 kilometers) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 kilometers) using a 5-watt transmitter. Cell phones operate within cells, and they can switch cells as they move around. Cells give mobile phones incredible range. Someone using a cell phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.

In a typical analog cell phone system in the United States, the cell phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid.

Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in nonadjacent cells. The two purple cells can reuse the same frequencies.
Each cell has a base station that consists of a tower and a small building containing the radio equipment. We'll get into base stations later. First, let's examine the "cells" that make up a cellular system.

Cell-phone Channels

A single cell in an analog mobile phone system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

• A cell phone carrier typically gets 832 radio frequencies to use in a city.
• Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)

Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

• The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure on the previous page, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.
• The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

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. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system and controls all of the base stations in the region.

Cell-phone Codes

All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider.
Let's say you have a cell phone, you turn it on and someone tries to call you. Here's what happens to the call:

• When you first power up the phone, it listens for an SID (see sidebar) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.
• When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.
• Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of the phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.
• The MTSO gets the call, and tries to find you. It looks in its database to see which cell you are in.
• The MTSO picks a frequency pair that your phone will use in that cell to take the call.
• The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. Now, you are talking by two-way radio to a friend.
• 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 moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This handoff switches your phone to the new cell.As you travel, the signal is passed from cell to cell. Let's say you're on the phone and you move from one cell to another -- but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, it'll actually be handed off to the other service provider.If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds.
  The less amazing thing is that you may be charged insane rates for your roaming call. On most phones, the word "roam" will come up on your phone's screen when you leave your provider's coverage area and enter another's. If not, you'd better study your coverage maps carefully -- more than one person has been unpleasantly surprised by the cost of roaming. Check your service contract carefully to find out whether you're paying when you roam. Most of the larger phone companies do not charge for roaming within the U.S., but some of the discount companies do.
  Internationally is another story. The roaming rates can be very high, assuming you have a phone that can work in multiple countries. Different countries use different cellular access technologies. More on those technologies later. First, let's get some background on analog cell phone technology so we can understand how the industry has developed.

 

Analog Cell Phones

In 1983, the analog cell phone standard called AMPS (Advanced Mobile Phone System) was approved by the U.S. Federal Communications Commission (FCC) and first used in Chicago. AMPS uses a range of frequencies between 824 megahertz (MHz) and 894 MHz for analog cell phones. In order to encourage competition and keep prices low, the U. S. government required the presence of two carriers in every market, known as A and B carriers. One of the carriers was normally the local-exchange carrier (LEC), a fancy way of saying the local phone company.
Carriers A and B are each assigned 832 frequencies: 790 for voice and 42 for data. A pair of frequencies (one to transmit and one to receive) is used to create one channel. The frequencies used in analog voice channels are typically 30 kilohertz (kHz) wide -- 30 kHz was chosen as the standard size because it gives you voice quality comparable to a wired telephone.
The transmit and receive frequencies of each voice channel are separated by 45 MHz to keep them from interfering with each other. Each carrier has 395 voice channels, as well as 21 data channels to use for housekeeping activities like registration and paging.
A version of AMPS known as Narrowband Advanced Mobile Phone Service (NAMPS) incorporates some digital technology to allow the system to carry about three times as many calls as the original version. Even though it uses digital technology, it is still considered analog. AMPS and NAMPS only operate in the 800-MHz band and don't offer many of the features common in digital cellular service, such as e-mail and Web browsing.

Along Comes Digital

The first digital cell phones were the second generation (2G) of cellular technology. Digital phones use the same radio technology as analog phones, but they use it differently. Analog systems don't fully use the signal between the phone and the cellular network -- analog signals can't be compressed and manipulated as easily as true digital signals. This is why cable companies switched to digital -- to fit more channels within a given bandwidth.
Digital phones convert your voice into binary information (1s and 0s) and then compress it (see How Analog-Digital Recording Works for details on the conversion process). This compression allows between three and 10 digital cell phone calls to occupy the space of a single analog call.
Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the two to send digital information between the cell tower and the phone. Clever modulation and encoding schemes are required to convert the analog information to digital, compress it and convert it back again while maintaining an acceptable level of voice quality. All of this means that digital cell phones have to contain a lot of processing power.
Let's take a good look inside a digital cell phone.

Inside a Digital Cell Phone

On a "complexity per cubic inch" scale, cell phones are some of the most intricate devices people use on a daily basis. Modern digital cell phones can process millions of calculations per second in order to compress and decompress the voice stream.
If you take a basic digital cell phone apart, you find that it contains just a few individual parts:

• A circuit board containing the brains of the phone
• An antenna
• A liquid crystal display (LCD)
• A keyboard (not unlike the one you find in a TV remote control)
• A microphone
• A speaker
• A battery 
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  Inside a digital cell phone, youll find a circuit board, battery, speaker and more. Look inside a digital cell phone with photos and explanations of each part.

  
  

  The circuit board is the heart of the system. 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. You can learn more about A-to-D and D-to-A conversion and its importance to digital audio in How Compact Discs Work. The digital signal processor (DSP) is a highly customized processor designed to perform signal-manipulation calculations at high speed.
  The microprocessor handles all of the housekeeping chores for the keyboard and display, deals with command and control signaling with the base station and also coordinates the rest of the functions on the board.
  The ROM and flash memory chips provide storage for the phone's operating system and customizable features, such as the phone directory. The radio frequency (RF) and power section handles power management and recharging, and also deals with the hundreds of FM channels. Finally, the RF amplifiers handle signals traveling to and from the antenna.
  
  The display has grown considerably in size as the number of features in cell phones has increased. Most current phones offer built-in phone directories, calculators, games, calendars, notes, Web browsers, and cameras, as well as countless other applications, or apps, to serve practically any need or want.
  
  

   

  The SIM card on the circuit board

  
  

  Some phones store certain information, such as the SID and MIN codes, in internal Flash memory, while others use external cards that are similar to SmartMedia cards.
  Cell phones have such tiny speakers and microphones that it is incredible how well most of them reproduce sound. As you can see in the picture above, the speaker is about the size of a dime and the microphone is no larger than the watch battery beside it. Speaking of the watch battery, this is used by the cell phone's internal clock chip.
  What is amazing is that all of that functionality -- which only 30 years ago would have filled an entire floor of an office building -- now fits into a package that sits comfortably in the palm of your hand

  
  

 

 

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