Overview of Cell Phone Technology

Analog and digital cell phones both use radio technology, but they do it in different ways.

It would be helpful to give an overview of cell phone technology here, since our setup is similar. How does a cell phone work? What's different about it from a normal phone? What do words like "PCS," "GSM," "CDMA," and "TDMA" mean?

First, let's talk about the basics: In essence, a cell phone is a radio. One of the most interesting things about a cell phone is that it is a radio. Alexander Graham Bell came up with the telephone in 1876, and Nikolai Tesla came up with the radio in the 1880s.

Before there were cell phones, people who needed to be able to talk on the go put radio phones in their cars. In the radio-telephone system, each city had one central antenna tower with maybe 25 channels on it. Because of this central antenna, the phone in your car had to have a powerful transmitter that was big enough to send signals 40 or 50 miles (about 70 km).

The smartest thing about the cellular system is how it divides a city into small parts. This lets a city reuse a lot of frequencies, so that millions of people can use cell phones at the same time. In the United States, a typical analogue cell phone system gives the cell phone carrier about 800 frequencies to use throughout the city. The carrier cuts the city up into little pieces. Most of the time, each cell is about 10 square miles (26 square kilometers). Most of the time, we think of cells as hexagons on a big hexagonal grid:

That is, each of the seven cells on a hexagonal grid uses one-seventh of the available channels, so each cell has its own set of frequencies and there are no collisions:

In a city, a cell phone company usually gets 832 radio frequencies to use.
This means that there are usually 395 voice channels per carrier.
So, about 56 voice channels are available on each cell.
In other words, 56 people can talk on their cell phones at the same time in any given cell. With digital methods of transmission, there are more channels to choose from. For example, a TDMA-based digital system can handle three times as many calls as an analogue system, so each cell has about 168 channels (see this page for lots more information on TDMA, CDMA, GSM and other digital cell-phone techniques).

There are low-power transmitters in cell phones. There are two signal strengths for many cell phones: 0.6 watts and 3 watts There are two good things about low-power transmitters:

The signals from a base station and the phones in its cell don't travel very far outside that cell. So, both of the purple cells in the picture above can use the same 56 frequencies.
In any size city, the cellular method needs a large number of base stations But because so many people have cell phones, the cost per user is still low. The Mobile Telephone Switching Office is a central office that each carrier runs in each city (MTSO). This office is in charge of all the landline phone connections and controls all the base stations in the area.
Now, let's look at what happens as you move from cell to cell with your cell phone.

From Cell to Cell
Every cell phone has a unique code that goes with it.

Let's say you have a cell phone and when you turn it on, someone tries to call you. What happens to the call is as follows:

When you turn on the phone for the first time, it checks the control channel for a SID (see sidebar). The control channel is a special frequency that the phone and base station use to talk to each other about things like setting up calls and changing channels. If the phone can't find any control channels to listen to, it knows it's out of range and shows a "no service" message.
When the phone gets the SID, it compares it to the SID that was already in the phone. If the SIDs are the same, the phone knows that the cell it is talking to is in its home system.
Along with the SID, the phone also sends a registration request. The MTSO keeps track of your phone's location in a database, so when it wants to ring your phone, it knows which cell you are in.
When the call comes in, the MTSO tries to find you. It looks up your cell number in its database.
The MTSO chooses the frequency pair that your phone will use to answer the call in that cell.
The MTSO tells your phone which frequencies to use over the control channel. You are talking to a friend on a two-way radio.
As you move toward the edge of your cell, the base station of your cell notices that the strength of your signal is getting weaker. At the same time, the base station in the cell you are moving toward can tell that your phone's signal strength is getting stronger because it is listening to and measuring signal strength on all frequencies, not just its own seventh. The MTSO is how the two base stations talk to each other. At some point, your phone will get a signal on a control channel telling it to change frequencies.

As you move, the signal moves from one cell to the next.
If the SID on the control channel doesn't match the SID you programmed into your phone, the phone knows it is roaming. When you roam, the MTSO of the cell you are in calls the MTSO of your home system. The MTSO of your home system then checks its database to see if the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then keeps track of your phone as you move through its cells. And it's crazy that all of this happens in a matter of seconds!

CBs and cell phones
Compare a cell phone to a CB radio or a walkie-talkie to get an idea of how advanced it is.

Difference between simplex and duplex: Walkie-talkies and CB radios are both simplex devices. That is, when two people talk on a CB radio, they use the same frequency, which means that only one person can talk at a time. A cell phone is a duplex device. That means that you talk on one frequency and listen on a different frequency. Both callers can talk at the same time.
Channels: Most walkie-talkies have one channel, while a CB radio has 40. Most cell phones can talk on more than 1,664 channels.
Range: A 0.25-watt walkie-talkie can send its signal about 1 mile (1.6 km) away. A 5-watt transmitter can send about 5 miles (8 km) with a CB radio because it has much more power. Cell phones work in cells, and as you move around, you can switch from one cell to another. Cells give cell phones an amazing amount of range. Because of the cellular system, someone with a cell phone can drive hundreds of miles and still talk to someone the whole time.

In simplex radio, the frequency used by both transmitters is the same. One person can only talk at a time.

In duplex radio, the two transmitters use different frequencies so that both parties can talk at the same time.
Cell phones are duplex.
In the next part, you'll see what a digital cell phone looks like on the inside.

Inside a Cell Phone
On a scale of "complexity per cubic inch," cell phones are among the most complicated things people use every day. In order to compress and decompress the voice stream, digital cell phones can do millions of calculations per second.

If you take a cell phone apart, you find that it contains just a few individual parts:

A cool circuit board that holds the phone's brains.
An antenna
A liquid crystal display (LCD)
A keyboard (not unlike the one you find in a TV remote control)
A sound recorder
A talker
A battery
The heart of the system is the circuit board. Here's a typical one from a Nokia digital phone:

The front of the circuit board

The back side of the board
. Let's talk about some of the chips and what they do. The analog-to-digital and digital-to-analog conversion chips turn analogue audio signals into digital ones and digital ones back into analogue ones. Work, you can find out more about A-to-D and D-to-A conversion and why it is important for digital audio.

The microprocessor is the brain of the computer.
The microprocessor is in charge of keeping the keyboard and screen running smoothly. It also handles command and control signals with the base station and coordinates the rest of the board's functions. 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 travelling to and from the antenna.

The contacts on the screen and keypad
As the number of features in cell phones has grown, so has the size of their screens. Most new phones have phone books, calculators, and even games built right in. And many of the phones have PDAs or Web browsers built in.

The memory card on the circuit board is a Flash card.

Some phones use internal Flash memory to store information like the SID and MIN codes, while others use external cards that are similar to SmartMedia cards.

The speaker, microphone, and back-up battery of a cell phone
The speakers and microphones in cell phones are so small that it is hard to believe that most of them can play sound so well. The speaker is about the size of a dime, as you can see in the picture above, and the microphone is no bigger than the watch battery next to it. When it comes to the watch battery, this is what the cell phone's clock chip uses.

It's amazing that all of this functionality, which 30 years ago would have filled an entire floor of an office building, now fits in a package that fits comfortably in the palm of your hand.

In 1983, the FCC approved AMPS (Advanced Mobile Phone System), an analogue cell phone standard that was first used in Chicago. Analog cell phones that use AMPS use a range of frequencies between 824 MHz and 894 MHz. The U.S. government made sure that every market had two carriers, called A and B carriers, so that there would be more competition and prices would stay low. The local-exchange carrier (LEC), which is a fancy way of saying the local phone company, was usually one of the carriers.

Each voice channel has a 45 MHz gap between its send and receive frequencies. This keeps them from interfering with each other.

Narrowband Advanced Mobile Phone Service (NAMPS), a version of AMPS that uses digital technology, can handle about three times as many calls as the original system. AMPS and NAMPS only work in the 800-MHz band and don't have many of the features that digital cell phone service has, like e-mail and browsing the Web.

Along Comes Digital
Analog and digital cell phones both use radio technology, but they do it in different ways. This is why many cable companies are switching to digital: it lets them fit more channels into the same amount of bandwidth. It's hard to believe how much better digital systems can be.

Because of this, three to ten digital cell phone calls can fit into the space of one analogue call. FSK sends digital information between the cell tower and the phone by quickly switching between two frequencies, one for 1s and the other for 0s. To turn analogue information into digital, compress it, and turn it back into analogue while keeping a good level of voice quality, you need to use clever modulation and encoding schemes. All of this means that digital cell phones need to be very powerful.

Technologies for Cellular Access
Cell phone networks usually send information using one of three technologies:

Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Even though these technologies sound scary, you can get a good idea of how they work just by looking at their names.
The access method is shown by the first word. The second word, "division," tells you that it splits calls based on the access method.

With FDMA, each call goes to a different frequency.
With TDMA, each call gets a certain amount of time on a certain frequency.
CDMA gives each call its own code and spreads it out over the frequencies that are available.
Each name ends with "multiple access." .Think about radio stations to better understand FDMA: In the available band, each station sends its signal at a different frequency.

Each phone uses a different frequency in FDMA.
Electronics Industry Alliance and Telecommunications Industry Association use TDMA as the access method for Interim Standard 54 (IS-54) and Interim Standard 136. (IS-136). TDMA divides a narrow band with a width of 30 kHz and a length of 6.7 milliseconds into three time slots.

Narrow band is the traditional way of saying "channels." One-third of the time, each conversation can listen to the radio. So, since TDMA uses the same number of channels as an analogue system, it can handle three times as much data. TDMA systems work in either the 800 MHz (IS-54) or 1900 MHz (IS-136) frequency bands.
Global System for Mobile Communications also uses TDMA as its access technology (GSM). But GSM uses TDMA in a way that is different from IS-136 and doesn't work with it. Think of GSM and IS-136 as two different operating systems that work on the same processor,

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