overview of gsm and cdma

8/3/2019 Overview of GSM and CDMA

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TATA Teleservices Delhi

An Overview on

G.S.M.&

C.D.M.A.


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What is Tele-Communication ?

Tele means DISTANCE and Communication

means INFORMATION TRANSFER So transfer of information between two ormore entities which may be far apart is called

as Tele-Communication


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Types of Communication tech.

Wireline

Wireless


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Communication Path

Wireline Physical layout of wires or optical fiber cables between

entities . A physical contact is essential between the

peer entities for communication Wireless

NO Physical connectivity is required . Communicationis done through radio links ( Electro Magnetic Waves )

and repeaters ( CELL SITES) are placed in betweenpeer entities for efficient communication


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What does one expects

from Wireless ?


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Lower cost

Roaming

Better service and coverage

NO dropped calls Enhanced Privacy

NO Speech Clipping

NO Echo


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Wireless It is type of communication technology

where electromagnetic waves carry thesignal (voice and data) on whole or part

of communication path. Wireless telecommunication involves

converting an audio signal into a

Frequency (RF) signal and broadcastingit using radiating devices calledantennas.


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MSC

ARCHITECTURE


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COMPONENTS OF WIRELESS

Main Switching Center (MSC) Cell Site

Cells

Hand set


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Main Switching Center ( MSC)

The center where the wireless switch issituated is called of main switching

center. All the call are first come to mscthen they are then routed to their

respective handsets through cell sites

locating the shortest path to therespective handset.


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CELL SITE

A Cell site contains a BTS(Base trans-receiver system ) which manages, sends,

and receives traffic from the mobiles in itsgeographical area to a cellular telephoneswitch. It also employs a tower which has

antennas, and provides a microwave linkto the distant cellular switch called MSC

and Handset


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Base Trans-Receiver System(BTS)

The Base Station Subsystem controls the radio

link with the Mobile Station It is physicalsystem which is installed with cell site. It hasReceiver , Transmitter and Power back up. Itreceives signal and add strength to the

signals thus repeating the signal to its originalform and transmitting it again to next entitywhich may be cell site or hand set


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CELL

CELL is the geographical area covered by

a cell site . All the receivers ( handsets)

in this particular area respond to thiscell site i.e. they receives and transmits

signal to a particular cell only


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Techniques Used in Wireless

Global System of Mobile Communication(GSM)

Wireless in Local Loop (WLL)


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G.S.M.GSM is an international standard ensuring that the

same mobile telephony system is used in asmany countries as possible all over the world.

This makes it possible to use the same mobilephone in different locations worldwide.

GSM employs TDMA + FDMA Technique


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FEATURES

Already deployed as a worldwide

standard

National/International roaming

Voice quality comparable to wire line

SIM Facility


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FDMA

In Frequency Division Multiple Access, thefrequency band is divided in slots. Each

user gets one frequency slot assignedthat is used at will.

It could be compared to AM or FM

broadcasting radio where each stationhas a frequency assigned.


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TDMA

In Time Division Multiple Access, the frequencyband is not partitioned but users are allowed to

use it only in predefined intervals of time, oneat a time.

Each caller is assigned a specific time slot fortransmission


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CELL STRUCTURE AND FREQUENCY

ALLOCATION


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A frequency (channel) can be usedagain within an FDMA or TDMA network,

but cells using the same frequencymust be separated by an appropriate

distance. Adjacent cells must beassigned a different set of frequencies.For example, a cell using frequency A

must not be adjacent to another cellusing frequency A.


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How GSM works ?

It works on the principle of

FDMA + TDMA

This means the bandwidth allotted isfirstly divided accordingly cell i.e. eachcell region works on different frequencyallotted to it and each cell frequency is

different from the adjacent cells.

This is FDMA


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So each cell works at a particularfrequency and now this frequency is

divided in time slots ie each user isgiven a specified time in which data of

that respective user is transferred .

This is TDMA


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Wireless in Local Loop

WLL is also a standard evolved andused in USA,It is emerging because

of its advantages over GSM.

WLL uses CDMA as communication

technique


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CDMA

CODE DIVISION MULTIPLE ACCESS

Code Division means the voice analog

signal is converted into itscorresponding digital signal but this

digital signal is accordingly a uniquecode assigned to that user.


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Multiple Accessmeans the same frequencyrange i.e. bandwidth is used by all the users

But there is no interference between the usersbecause each user is is talking to its

respective counterpart in a unique codeassigned to it which is different from the

other users


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CDMA is a ..

modulation and access system that

employs signature codes (rather thantime slots or frequencybands) to

arrange simultaneous and continuousaccess to a network by multiple users.


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SPREAD SPECTRUMTECHNIQUE


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CDMA works on the principle SPREADSPECTRUM Technique, which means

that it spreads the informationcontained in a particular signal of

interest over a much greater bandwidth

than the original signal i.e. large rangeof frequencies are used for informationtransfer


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Due to Spread Spectrum Technique

Capacity increses of 8-10 times that of gsmsystem

Improved call quality,with better and more

consistent sound . Simplified system planning through the use of

same frequency in every sector of cell

Enhanced privacy

Improved coverage


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Cell Structure & Frequency

allocation of CDMA


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Each CELL-SITE in a CDMA networkcan use all available frequencies.

Adjacent Cells can transmit at thesame frequency because users are

separated by Code Channels


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DIFFERENCES

between

GSM & CDMA


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GSM Works on

TDMA +FDMA

Different cell works ondifferent frequencies

Due to change in frequencyfrom one cell to anothercomplex hand off

Due to Complex procedureCall interference is morelikely

WLL Works on CDMA with

Spread SpectrumTechnique

Universal frequency use

Soft Handoff tech leads tolower call drops

Rake receiver technologyleads low call interferenceand .


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Hands-Off

Handoff is the process of

transferring a call from one cell to

another. This is necessary tocontinue the call as the phone

travels.


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Hands-Off in GSM

TDMA and FDMA systems use a hard handoff

when the mobile is moving from one cell siteto another. These technologies do not allow

for any type of make-before-break handoff. Ahard handoff can increase the likelihood of a

dropped call. A hard handoff requires themobile to break the connection with the oldBTS prior to making the connection with the

new one. Hard handoffs are also calledBreak-Before-Make


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Hard Hands-Off

Break before Make


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Soft Hands-OffMake before Break

CDMA


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Call Interference leads to

Disturbance ,Low Speech,Echo

Signals sent over the air can take a direct

path to the receiver, or they can bounce offobjects and then travel to the receiver. Thesedifferent paths, called multi-paths, can resultin the receiver getting several versions of the

same signal but at slightly different times.Multi-paths can cause a loss of signal through

cancellation in other technologies


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GSM

When a signal travels to a handset through

different paths ,in GSM handset ,it will pickthe first signal reaching to it, no matter how

weak that signal is. Thus leading to CallDisturbance, Low Speech and some times

Echo


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Rake Receiver in CDMA

CDMA's rake receiver is multiple receivers inone. The rake receiver identifies the three

strongest multi-path signals and combinesthem to produce one very strong signal. Therake receiver therefore uses multi path to

reduce the power the transmitter must send.

Both the MOBILE and the CELL SITES use rakereceivers


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Power Adjustments

If all mobiles transmitted at the same powerlevel, the base station would receive

unnecessarily strong signals from mobilesnearby and extremely weak signals frommobiles that are far away. This would reduce

the capacity of the system.

This problem is called the NEAR-FAR problem


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Power Adjustment in GSMIf all mobiles transmitted at the same power

level, the base station would receive

unnecessarily strong signals from mobilesnearby and extremely weak signals from

mobiles that are far away. Thiswould reduce

the capacity of the system. This problem iscalled the near-far problem


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Power Adjustment in CDMA

Power control is a CDMA feature that enablesmobiles to adjust the power at which they

transmit. This ensures that the base stationreceives all signals at the appropriate power.The CDMA network independently controlsthe power at which each mobile transmits.

Both forward and reverse links use powercontrol techniques.


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ADVANTAGES

CDMA capacity is ten to twentytimes that of analog systems, and

it's up to four times that of TDMA. CDMA's universal frequency reuse

CDMA users are separated by code


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CDMA's soft handoff leads to low

or no call drops The Rake Receiver leads to better

voice quality

NO INTERFERENCE


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Power control keeps power at anoptimal level.

Power control helps the networkdynamically expand the coveragearea.

CDMA's wide band signal reducesfading.


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CDMA technology has numerous

advantages including:

Coverage

Capacity

Clarity

Cost

Compatibility


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Coding provide the ability to covermore users for the same amount of

available power used in other systems.


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THANK YOU

overview of gsm and cdma

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