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8/22/2019 Quadrature Amplitutde Modulation
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QUADRATUREAMPLIDUDEMODULATION
Preparedby
Deepa.T,
Asst.Prof.
/TCE
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INTRODUCTIONQAM
Quadrature Amplitude Modulation or QAM is a form ofmodulation which is widely used for modulating data
signals onto a carrier used for radio communications. It iswidely used because it offers advantages over other formsof data modulation such as PSK, although many forms ofdata modulation operate along side each other.
Quadrature Amplitude Modulation, QAM is a signal inwhich two carriers shifted in phase by 90 degrees aremodulated and the resultant output consists of both
amplitude and phase variations. In view of the fact thatboth amplitude and phase variations are present it mayalso be considered as a mixture of amplitude and phasemodulation.
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DefinitionQAM
Formsofphaseshiftkeying Whatisphaseshiftkeying?
Althoughphasemodulationisusedforsomeanalogtransmissions,itisfarmorewidelyusedasadigitalformofmodulationwhereitswitchesbetweendifferentphases.Thisisknownasphaseshiftkeying,PSK,andtherearemanyflavours ofthis.
Quadrature amplitudemodulation(QAM)
Itisevenpossibletocombinephaseshiftkeyingandamplitudekeyinginaformofmodulationknownasquadrature amplitudemodulation,QAM.
Thelistbelowgivessomeofthemorecommonlyusedformsofphaseshiftkeying,PSK,andrelatedformsofmodulationthatareused:
PSKPhaseShiftKeying
BPSK
Binary
Phase
Shift
Keying
QPSKQuadrature PhaseShiftKeying
OQPSKOffsetQuadrature PhaseShiftKeying
8PSK8PointPhaseShiftKeying
16PSK16PointPhaseShiftKeying
QAMQuadrature AmplitudeModulation
16QAM16PointQuadrature AmplitudeModulation
64QAM
64
Point
Quadrature Amplitude
Modulation
MSKMinimumShiftKeying
GMSKGaussianfilteredMinimumShiftKeying
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ANALOG&DIGITALQAM
Quadrature amplitude modulation, QAM may exist in what may betermed either analog digital formats. The analog versions of QAM aretypically used to allow multiple analog signals to be carried on a singlecarrier. For example it is used in PAL and NTSC television systems,where the different channels provided by QAM enable it to carry thecomponents of chroma or colour information. In radio applications asystem known as C-QUAM is used for AM stereo radio. Here the
different channels enable the two channels required for stereo to becarried on the single carrier.
Digital formats of QAM are often referred to as "Quantised QAM" andthey are being increasingly used for data communications often withinradio communications systems. Radio communications systems rangingfrom cellular technology through wireless systems including WiMAX,and Wi-Fi 802.11 use a variety of forms of QAM, and the use of QAMwill only increase within the field of radio communications.
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AdvantagesandDisadvantagesofQAM
Advantages:
QAM appears to increase the efficiency of transmission for radiocommunications systems by utilising both amplitude and phase variations,
Drawbacks
more susceptible to noise because the states are closer together so that a lowerlevel of noise is needed to move the signal to a different decision point.Receivers for use with phase or frequency modulation are both able to use
limiting amplifiers that are able to remove any amplitude noise and therebyimprove the noise reliance. This is not the case with QAM.
The second limitation is also associated with the amplitude component of thesignal. When a phase or frequency modulated signal is amplified in a radiotransmitter, there is no need to use linear amplifiers, whereas when using QAM
that contains an amplitude component, linearity must be maintained.Unfortunately linear amplifiers are less efficient and consume more power, andthis makes them less attractive for mobile applications.
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QAMapplications QAM is in many radio communications and data delivery
applications. However some specific variants of QAM are
used in some specific applications and standards. For domestic broadcast applications for example, 64 QAM and
256 QAM are often used in digital cable television and cablemodem applications. In the UK, 16 QAM and 64 QAM are
currently used for digital terrestrial television using DVB -Digital Video Broadcasting. In the US, 64 QAM and 256QAM are the mandated modulation schemes for digital cableas standardised by the SCTE in the standard ANSI/SCTE 07
2000. In addition to this, variants of QAM are also used for many
wireless and cellular technology applications.
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ConstellationdiagramsforQAM
Quadrature amplitude modulation, QAM, when used for digital transmissionfor radio communications applications is able to carry higher data rates thanordinary amplitude modulated schemes and phase modulated schemes. As with
phase shift keying, etc, the number of points at which the signal can rest, i.e.the number of points on the constellation is indicated in the modulation formatdescription, e.g. 16QAM uses a 16 point constellation.
When using QAM, the constellation points are normally arranged in a squaregrid with equal vertical and horizontal spacing and as a result the most common
forms of QAM use a constellation with the number of points equal to a powerof 2 i.e. 2, 4, 8, 16 . . . .
By using higher order modulation formats, i.e. more points on the constellation,it is possible to transmit more bits per symbol. However the points are closertogether and they are therefore more susceptible to noise and data errors.
To provide an example of how QAM operates, the table below provides the bitsequences, and the associated amplitude and phase states. From this it can beseen that a continuous bit stream may be grouped into threes and represented asa sequence of eight permissible states.
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WhyQAMcalledcombinedASK
AND
PSK
Quadrature Amplitude Modulation uses
the phase and amplitude of the carriersignal to encode data. QAM finds
widespread use in current and emergingwireless standards, including Wi-Fi,Digital Video Broadcast (DVB), WiMAX,IEEE 802.11n, and HSDPA/HSUPA.
The QAM modulation scheme encodesdata by varying both amplitude and phase
of the carrier signal. Thus, it is sometimesviewed as a combination of ASK and PSKmodulation. A more fundamental way ofviewing QAM thought is that it encodesdata by varying the amplitude of twocarrier signals that are in-quadrature(phase difference of 90). Hence the namequadrature-amplitude modulation. Wewill now leverage our understanding of IQdata to understand this idea. As we haveseen, a modulated carrier signal can beexpressed in terms of its IQ componentsas:
whereandaretheamplitudesoftheinphase
andquadraturephasecomponents
respectively.Thus,
we
can
change
the
amplitude()andphase()ofthecarriersignal
byvaryingtheIandQvalues.
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ConstellationdiagramsforQAM
Theconstellationdiagramsshowthedifferent
positions
for
the
states
within
different
forms
ofQAM,quadrature amplitudemodulation.Astheorderofthemodulationincreases,so
doesthe
number
of
points
on
the
QAM
constellationdiagram.
Thediagramsbelowshowconstellationdiagrams
for
avariety
of
formats
of
modulation:
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ConstellationdiagramsforQAM Letslookatthetimedomain
representationofQAMsignals.Taking4QAMasanexample,
supposewe
wish
to
transmit
the
bitstream 100111.Wemaptheseto4QAMsymbolsrepresenting10,01,11.Theresultingtimedomainwaveformforthisbitstream isshowninFigure53.Eachsymbolisrepresented
by
National
Instruments
RF&CommunicationsHandbook,Page6216/08/200719:00:35,Copyright2007NationalInstrumentsCorporation
oneperiod
of
the
sine
wave
and
has
auniquephaseshift.Inthisrespect,4QAMmightbeconsideredaspecialcaseofQAMwheretheamplitudeisthesameforallsymbols.
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ConstellationdiagramsforQAM Theconstellationplotinthis Figure
showsthephaseandamplitudetransitionsofthecarriersignal.The
rawIQ
data
is
represented
by
the
red
trancewiththewhitedotsrepresentingthosesamplesofIQdatathatoccuronsymbolclockperiodsandthataremappedbacktodigitalbitpatternsbasedonthe4QAM
symbol
map.
We
note
that
the
transitionsgothroughtheorigin.Thiscausesabruptamplitudevariationsbetweenconsecutivesymbolsandcausesnoisetobeinjectedinthe
transmittedsymbol
due
to
the
amplifierturningoffandbackonabruptly.ThisproblemcanbefixedbyusingoffsetQAM.RefertotheoffsetPSKmodulationschemediscussedearlierformoredetails.
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Conclusion
Quadrature AmplitudeModulationisan
importantmodulation
scheme
with
many
practicalapplications,includingcurrentand
futurewirelesstechnologies.Someexamples
ofcommunicationsystemsthatuseQAMareWiFi,cablemodems,DigitalVideoBroadcast
(DVB)and
WiMAX.
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