Data Transmission Mobile Telephony

8/14/2019 Data Transmission Mobile Telephony

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COMPUTER AIDED DESIGNCOMPUTER AIDED DESIGN

Data TransmissionData Transmission


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Spectral BandwidthsSpectral Bandwidths


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Cellular Access TechnologiesCellular Access Technologies

Cellular access technologies are theCellular access technologies are the

methods used by mobile phone networksmethods used by mobile phone networksfor transmitting information.for transmitting information.

There are three technologies used by cellThere are three technologies used by cell--phone networks for transmittingphone networks for transmitting

information:information:

FDMAFDMA

TDMATDMA

CDMACDMA


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Frequency Division Multiple AccessFrequency Division Multiple Access

Each signal is modulated in a telephone officeEach signal is modulated in a telephone office

The signals are demodulated in a telephone office in theThe signals are demodulated in a telephone office in the

receiving area.receiving area.

FDMA is not an efficient method for digital transmission.FDMA is not an efficient method for digital transmission.


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Time Division Multiple AccessTime Division Multiple Access

TDM multiplexer takes one byte from each channel by rotation,TDM multiplexer takes one byte from each channel by rotation,

compresses it and transmits as a stream in a precompresses it and transmits as a stream in a pre--assigned sequence.assigned sequence. Data isData is demultiplexeddemultiplexed at the receiving end.at the receiving end.

Voice is compressed so that less transmission space is taken.Voice is compressed so that less transmission space is taken.

TDMA systems operate in 800TDMA systems operate in 800--1900 MHz frequency band.1900 MHz frequency band.


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Code Division Multiple AccessCode Division Multiple Access

Each call is spread out over the entire available bandwidth. MulEach call is spread out over the entire available bandwidth. Multipletiple

calls are overlaid in the channel, with each being assigned a uncalls are overlaid in the channel, with each being assigned a uniqueiquecode and is transmitted in small pieces.code and is transmitted in small pieces.

At the receiving end, the same unique code is used to recover thAt the receiving end, the same unique code is used to recover theesignal.signal.

CDMA operates in 800CDMA operates in 800--1900 MHz frequency band.1900 MHz frequency band.


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GSMGSM

GSM, Global System for MobileGSM, Global System for Mobile

Communication is an international standardCommunication is an international standardwhich uses TDMA technology for access.which uses TDMA technology for access.

GSM is in use in much of Asia, includingGSM is in use in much of Asia, including

India, Europe, Australia and Africa.India, Europe, Australia and Africa.


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Mobile Communication CellsMobile Communication Cells

The geographical area is divided into a grid ofThe geographical area is divided into a grid ofsmall hexagonal cells. Each cell has a base stationsmall hexagonal cells. Each cell has a base stationthat consists of a tower and radio equipment. Athat consists of a tower and radio equipment. Amobile telephone switching office controls all themobile telephone switching office controls all thebase stations in a region.base stations in a region.


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Each cell is a group of 50 to 70 radio channels, so thatEach cell is a group of 50 to 70 radio channels, so that

many simultaneous conversations can be made.many simultaneous conversations can be made.

Adjacent cells must use different groups of channels toAdjacent cells must use different groups of channels to

avoid interference.avoid interference.

In busy areas, cells can be of smaller size.In busy areas, cells can be of smaller size.


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As you move towards the edge of one cell, the cells base statioAs you move towards the edge of one cell, the cells base station notesn notesthe diminishing signal strength, while the base station in the cthe diminishing signal strength, while the base station in the cell youell youare moving towards senses the increasing signal strength.are moving towards senses the increasing signal strength.

The base stations coordinate with each other and the mobile is gThe base stations coordinate with each other and the mobile is given aiven a

signal to shift frequencies.signal to shift frequencies. This switches the phone to the new cell without dropping the calThis switches the phone to the new cell without dropping the call.l.


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Mobile FactsMobile Facts

In a walkieIn a walkie--talkie, same frequency is usedtalkie, same frequency is used

by transceivers at both ends and so, onlyby transceivers at both ends and so, onlyone party can talk at one time.one party can talk at one time.

But when two cellBut when two cell--phones are inphones are in

conversation, they use differenceconversation, they use differencefrequencies.frequencies.

On a Complexity per cubic inch scale, cellOn a Complexity per cubic inch scale, cellphones are some of the most intricatephones are some of the most intricatedevices people play with on a daily basis.devices people play with on a daily basis.


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Data Transmission Mobile Telephony