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`Overview of Cell Phone Technology
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`The Cell Phone Technology
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`
`
`It would be useful to give an overview of the cell phone technology here as this is quite
`inline with our installation. Let's see how a cell phone works? What makes it different from
`a regular phone? What do all those confusing terms like PCS, GSM, CDMA and TDMA
`mean?
`
`
`Let's start with the basics: In essence, a cell phone is a radio. One of the most interesting
`things about a cell phone is that it is actually a radio an extremely sophisticated radio,
`but a radio nonetheless. The telephone was invented by Alexander Graham Bell in 1876,
`and wireless communication can trace its roots to the invention of the radio by Nikolai
`Tesla in the 1880s (formally presented in 1894 by a young Italian named Guglielmo
`Marconi). It was only natural that these two great technologies would eventually be
`combined!
`
`
`In the dark ages before cell phones, people who really needed mobilecommunications
`ability installed radio telephones in their cars. In the radiotelephone 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 km). It also meant that not many people
`could use radio telephones there just were not enough channels.
`
`
`A digital cell phone from
`Nokia.
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`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. In a typical analog cellphone system in the United
`States, the cellphone 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, like this:
`
`
`Because cell phones and base stations use lowpower transmitters, the same frequencies can be reused in non
`adjacent cells. The two purple cells can reuse the same frequencies.
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`Each cell has a base station that consists of a tower and a small building containing the radio equipment (more on base
`stations later).
`
` A
`
` single cell in an analog system uses oneseventh of the available duplex voice channels. That is, each cell (of the seven
`on a hexagonal grid) is using oneseventh of the available channels so it has a unique set of frequencies and there are no
`collisions:
`
`
`A cellphone 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 on the next page.)
`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. With digital transmission methods, the
`number of available channels increases. For example, a TDMAbased digital system can carry three times as many calls as
`an analog system, so each cell has about 168 channels available (see this page for lots more information on TDMA, CDMA,
`GSM and other digital cellphone techniques).
`
`Cell phones have lowpower 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. Lowpower 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 above, 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 batteryoperated, 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 landbased phone system, and controls all of the base stations in the region.
`
`Now let's analyses what happens we as you (and your cell phone) move from cell to cell.
`
`From Cell to Cell
`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 is 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 setup 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 your 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 it 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. You are talking by twoway radio to a
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`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 oneseventh) 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 hand off switches your phone to the new cell.
`
`As you travel, the signal is passed from cell to cell.
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`Roaming
`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!
`Cell Phones and CBs
`A good way to understand the sophistication of a cell phone is to compare it to a CB radio or a walkietalkie.
`
`Simplex vs. duplex Both walkietalkies and CB radios are simplex 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 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 walkietalkie 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 walkietalkie can transmit about 1 mile (1.6 km) using a 0.25watt transmitter. A CB radio, because it has
`much higher power, can transmit about 5 miles (8 km) using a 5watt transmitter. Cell phones operate within cells,
`and they can switch cells as they move around. Cells give cell 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.
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`In simplex radio, both transmitters use the same frequency. Only one party
`can talk at a time.
`
`In duplex radio, the two transmitters use different frequencies, so both parties
`can talk at the same time.
`Cell phones are duplex.
`
`In the next section, you'll get a good look inside a digital cell phone.
`
`Inside a Cell Phone
`On a "complexity per cubic inch" scale, cell phones are some of the most intricate devices people play with on a daily basis.
`Modern digital cell phones can process millions of calculations per second in order to compress and decompress the
`voice stream.
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`If you take a cell phone apart, you find that it contains just a few individual parts:
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`The parts of a cell phone
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`An amazing 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
`
`The circuit board is the heart of the system. Here is one from a typical Nokia digital phone:
`
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`The front of the circuit board
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`The back of the circuit board
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`In the photos above, you see several computer chips. Let's talk about what some of the individual chips do. The analogto
`digital and digitaltoanalog 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 AtoD and DtoA 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
`signalmanipulation calculations at high speed.
`
`
`The microprocessor
`
`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.
`
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`The display and keypad contacts
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`The display has grown considerably in size as the number of features in cell phones have increased. Most current phones
`offer builtin phone directories, calculators and even games. And many of the phones incorporate some type of PDA or Web
`browser.
`
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`The Flash memory card on the circuit board
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`The Flash memory card removed
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`Some phones store certain information, such as the SID and MIN codes, in internal Flash memory, while others use external
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`cards that are similar to SmartMedia cards.
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`Overview of Cell Phone Technology
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`The cellphone speaker, microphone and battery backup
`
`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!
`
`AMPS
`In 1983, the analog cellphone standard called AMPS (Advanced Mobile Phone System) was approved by the 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 localexchange 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 for transmit
`and one for receive) is used to create one channel. The frequencies used in analog voice channels are typically 30 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 800MHz band and do not offer many of the
`features common in digital cellular service, such as email and Web browsing.
`Along Comes Digital
`Digital cell phones use the same radio technology as analog phones, but they use it in a different way. Analog systems do
`not fully utilize the signal between the phone and the cellular network analog signals cannot be compressed and
`manipulated as easily as a true digital signal. This is the reason why many cable companies are switching to digital so they
`can fit more channels within a given bandwidth. It is amazing how much more efficient digital systems can be.
`
`Digital phones convert your voice into binary information (1s and 0s) and then compress it (see How AnalogDigital
`Recording Works for details on the conversion process). This compression allows between three and 10 digital cellphone
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`calls to occupy the space of a single analog call.
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`Overview of Cell Phone Technology
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`Many digital cellular systems rely on frequencyshift 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!
`
`Cellular Access Technologies
`There are three common technologies used by cellphone networks for transmitting information:
`
`Frequency division multiple access (FDMA)
`Time division multiple access (TDMA)
`Code division multiple access (CDMA)
`
`Although these technologies sound very intimidating, you can get a good sense of how they work just by breaking down the
`title of each one.
`
`The first word tells you what the access method is. The second word, division, lets you know that it splits calls based on
`that access method.
`
`FDMA puts each call on a separate frequency.
`TDMA assigns each call a certain portion of time on a designated frequency.
`CDMA gives a unique code to each call and spreads it over the available frequencies.
`
`The last part of each name is multiple access. This simply means that more than one user can utilize each cell.
`
`FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better
`understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band.
`FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not
`considered to be an efficient method for digital transmission.
`
`In FDMA, each phone uses a different frequency.
`
`TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for
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`Interim Standard 54 (IS54) and Interim Standard 136 (IS136). Using TDMA, a narrow band that is 30 kHz wide and 6.7
`milliseconds long is split timewise into three time slots.
`
`Narrow band means "channels" in the traditional sense. Each conversation gets the radio for onethird of the time. This is
`possible because voice data that has been converted to digital information is compressed so that it takes up significantly less
`transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of
`channels. TDMA systems operate in either the 800MHz (IS54) or 1900MHz (IS136) frequency bands.
`
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`TDMA splits a frequency into time slots.
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`TDMA is also used as the access technology for Global System for Mobile communications (GSM). However, GSM
`implements TDMA in a somewhat different and incompatible way from IS136. Think of GSM and IS136 as two different
`operating systems that work on the same processor, like Windows and Linux both working on an Intel Pentium III. GSM
`systems use encryption to make phone calls more secure. GSM operates in the 900MHz and 1800MHz bands in Europe
`and Asia, and in the 1900MHz (sometimes referred to as 1.9GHz) band in the United States. It is used in digital cellular and
`PCSbased systems. GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular system introduced
`by Motorola and used by Nextel.
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`http://www.mat.ucsb.edu/~g.legrady/academic/courses/03w200a/projects/wireless/cell_technology.htm
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