Cell Phone Technology.docx

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The Cell Phone Technology

It would be useful to give an overview of the cell phone technologyhere as this is quite inline with our installation. Let's see how a cellphone works? What makes it different from a regular phone? What doall those confusing terms like P!" #!$"%$&and %$&mean?

Let's start with the basics( In essence" a cell phone is a radio. )ne ofthe most interesting things about a cell phone is that it is actually aradio ** an e+tremely sophisticated radio" but a radio nonetheless. hetelephone was invented by&le+ander #raham,ell in -/0" andwireless communication can trace its roots to the invention of the radioby 1ikolai esla in the -2s 3formally presented in -45 by a youngItalian named #uglielmo $arconi6. It was only natural that these twogreat technologies would eventually be combined7

In the dark ages before cell phones" people who really needed mobile*communications ability installed radio telephonesin their cars. In theradio*telephone system" there was one central antenna tower per city" and perhaps 25 channelsavailable on that tower. his central antennameant that the phone in your car needed a powerfultransmitter ** big enough to transmit 52 or 82 miles 3about /2 km6. It also meant that not many peoplecould use radio telephones ** there 9ust were not enough channels.
http://www.gsmworld.com/http://www.cdmaonline.com/http://www.cdmaonline.com/http://www.iec.org/online/tutorials/tdma/index.htmlhttp://memory.loc.gov/ammem/bellhtml/bellhome.htmlhttp://www.etedeschi.ndirect.co.uk/marconi/http://www.cdmaonline.com/http://www.iec.org/online/tutorials/tdma/index.htmlhttp://memory.loc.gov/ammem/bellhtml/bellhome.htmlhttp://www.etedeschi.ndirect.co.uk/marconi/http://www.gsmworld.com/


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he genius of the cellular system is the division of a city into small cells. his allows e+tensivefrequency reuseacross a city" so that millions of people can use cell phones simultaneously. In a typicalanalog cell*phone system in the :nited !tates" the cell*phone carrier receives about 800 frequenciestouse across the city. he carrier chops up the city into cells. ;ach cell is typically si


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miles3=0 square kilometers6. ells are normally thought of as he+agons on a big hexagonal grid" likethis(

Because cell phones and base stations use low-power transmitters, the same frequencies can bereused in non-adacent cells! "he two purple cells can reuse the same frequencies!

;ach cell has a base stationthat consists of a tower and a small building containing the radio equipment3more on base stations later6.

& single cell in an analog system uses one*seventh of the available duple+ voice channels. hat is" eachcell 3of the seven on a he+agonal grid6 is using one*seventh of the available channels so it has a uniqueset of frequencies and there are no collisions(

& cell*phone carrier typically gets 8#2 radio frequenciesto use in a city.

;ach cell phone uses two frequencies per call ** a duple+ channel ** so there are typically #$5

%oice channelsper carrier. 3he other 5= frequencies are used for control channels** more onthis on the ne+t page.6

herefore" each cell has about 5& %oice channelsavailable.

In other words" in any cell" 80 people can be talking on their cell phone at one time. With digitaltransmission methods" the number of available channels increases. >or e+ample" a "'()-baseddigitalsystem can carry three times as many calls as an analog system" so each cell has about -0 channelsavailable 3seethis pagefor lots more information on %$&" %$&" #!$ and other digital cell*phonetechniques6.
http://electronics.howstuffworks.com/cell-phone6.htmhttp://electronics.howstuffworks.com/cell-phone6.htmhttp://electronics.howstuffworks.com/cell-phone6.htm


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ell phones have low-power transmittersin them. $any cell phones have two signal strengths( 2.0watts and watts 3for comparison" most , radios transmit at 5 watts6. he base station is alsotransmitting at low power. Low*power transmitters have two advantages(

he transmissionsof a base station and the phones within its cell do not make it very far

outside that cell. herefore" in the figure above" both of the purple cells can reuse the same 5&frequencies. he same frequencies can be reused e+tensively across the city.

he power consumptionof 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.

he cellular approach requires a large number of base stations in a city of any si


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Bou are talking by two*way radio to a friend7

&s you move toward the edge of your cell" your cell's base stationnotes that your signal

strengthis diminishing. $eanwhile" the base station in the cell you are moving toward 3which islistening and measuring signal strength on all frequencies" not 9ust its own one*seventh6 seesyour phone's signal strength increasing. he two base stations coordinate with each otherthrough the $!)" and at some point" your phone gets a signal on a control channel telling it tochange frequencies. his hand offswitches your phone to the new cell.

oamingIf the !I% on the control channel does not match the !I% programmed into your phone" then the phoneknows it is roaming. he $!) of the cell that you are roaming in contacts the $!) of your homesystem" which then checks its database to confirmthat the !I% of the phone you are using is valid. Bourhome system %erifiesyour phone to the local $!)" which then tracks your phone as you move throughits cells. &nd the ama


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.n simplex radio, both transmitters use the same frequency!+nly one party can tal/ at a time!

.n duplex radio, the two transmitters use differentfrequencies, so both parties can tal/ at the same time!

ell phones are duplex!

In the ne+t section" you'll get a good look inside a digital cell phone.


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