overview of gsm and cdma
TRANSCRIPT
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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