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 Cell Phone Basic Theory And Principles

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PostSubject: Cell Phone Basic Theory And Principles   Wed Oct 24, 2007 12:16 am

One of the most interesting about cell phones is that it is actually a radio---an extremely sophisticated radio, but a radio never the less. The telephone was invented by Alexander Graham Belle in 1876, and wireless communication can trace its roots to the invention of the radio by Nikolai Tesla in the1880’s formally presented by 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 need mobile communication ability installed radio telephones in there 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 I 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 radiotelephones, there just not enough channels. The genius of cellular system is the division of city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.

The sophistication of cell phone compare to a CB radio or a walkie-talkie.
• Full duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. 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 means that one frequency for talking and for listening.
• 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 using a 0.25-watt transmitter. Cell phones operate within cells, and they can switch cells as they move around.
Cell Engineering
Cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent 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.
A single cell in an analog system uses one-seventh of the available duplex voice channels. Each cell 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 so there are typically 395 voice channels per carrier.
• Each cell has about 56 voice channels available.

Transmission

Cell phones have low-power transmitters. Many cell phones have two signal strengths: 0.6 watts and 3 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 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 battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

Cell Phone Codes
• When you first power up the phone, it listens for an SID 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 SID’s matches, 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. Now, you are talking by two-way radio to a friend.

Inside a 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.


Circuit board contains
• An antenna
• Buzzer
• A Vibrator
• A Liquid Crystal Display (LCD)
• A keyboard
• A microphone
• A speaker
• A battery
• 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. 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.

Digital Technology

Digital cell phones are the second generation (2G) of cellular technology. They 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.
Digital phones convert voice into binary information (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.
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.

2G Technologies

Three common technologies used by 2G cell-phone networks for transmitting information
• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)
The first word tells what the access method is. The second word, division, means 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.
FDMA
Separate the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. FDMA is not considered to be an efficient method for digital transmission.
TDMA

Is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136) Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long are split time-wise into three time slots.
Narrow band means "channels" in the traditional sense. Each conversation gets the radio for one-third 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 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.
GSM
TDMA is also used as the access technology for Global System for Mobile Communication (GSM). However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands. The 850-MHz and 1900-MHz (sometimes referred to as 1.9-GHz) band. It is used in digital cellular and PCS-based systems. GSM is also the basis for Digital Enhanced Network.
To connect to the specific service providers in these different countries, GSM users simply switch subscriber identification module (SIM) cards. SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider.


CDMA
Takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.
3G Technology
3G technologies is the latest in mobile communications. 3G stands for "third generation" -- this makes analog cellular technology generation one and digital/PCS generation two. 3G technologies are intended for the true multimedia cell phone -- typically called smart phones and features increased bandwidth and transfer rates to accommodate Web-based applications and phone-based audio and video files.
3G comprise several cellular access technologies. The three most common ones as of 2005 are:
• CDMA 2000- based on 2G Code Division Multiple Access
• WCDMA- Wideband Code Division Multiple Access
• Time-division Synchronous Code-division Multiple Access TD-SC
3G networks have potential transfer speeds of up to 3 Mbps. 3G's high data rates are ideal for downloading information from the Internet and sending and receiving large, multimedia files. 3G phones are like mini-laptops and can accommodate broadband applications like video conferencing, receiving streaming video from the Web, sending and receiving faxes and instantly downloading e-mail messages with attachments.

Dual Band vs. Dual Mode

• Multiple bands - A phone that has multiple-band capability can switch frequencies. A quad-band GSM phone could use GSM service in the 850-MHz, 900-MHz, 1800-MHz or 1900-MHz band.
• Multiple modes - In cell phones, "mode" refers to the type of transmission technology used. A phone that supported AMPS and TDMA could switch back and forth as needed.
• Multiple band/multiple modes - Allows you to switch between frequency bands and transmission modes as needed.

Phone have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the digital mode(s) first, and then switch to analog.
You can find both dual-mode and tri-mode phones. The term "tri-mode" can be deceptive. It may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog. In that case, it is a true tri-mode phone. But it can also mean that it supports one digital technology in two bands and also offers analog support. A popular version of the tri-mode type of phone for people who do a lot of international traveling has GSM service in the 900-MHz band for Europe and Asia and the 1900-MHz band for the United States, in addition to the analog service.

Problems with Cell Phones

• Generally, non-repairable internal corrosion of parts results if you get the phone wet or uses wet hands to push the buttons. Consider a protective case. If the phone does get wet, be sure it is totally dry before you switch it on so you can try to avoid damaging internal parts.
• Extreme heat in a car can damage the battery or the cell-phone electronics. Extreme cold may cause a momentary loss of the screen display.
• Analog cell phones suffer from a problem known as "cloning." A phone is "cloned" when someone steals its ID numbers and is able to make fraudulent calls on the owner's account.
Cell-phone Towers
A cell-phone tower is typically a steel pole or lattice structure that rises hundreds of feet into the air

The box houses the 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


Tower and all of the cables and equipment at the base of the tower are heavily grounded.
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