Presentation1_1_.pptx_filename_= UTF-8''Presentation1 (1)

8/18/2019 Presentation1_1_.pptx_filename_= UTF-8''Presentation1 (1)

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SpectrumAllocations


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Cellular TelephoneSystems


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bluetooth

RF technology for short range connections between wirelessdevices.

incorporating a radio transceiver

normal range of operation is 10 m (at 1 mW transmit power)

operates in the unregulated 2. !"# fre$uency band% hence it can

be used worldwide

provides 1 data channel at &21 'bps and up to three voicechannels at 'bps for an aggregate bit rate of 1 *bps


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Path loss- power dissipated bytransmitter+ propagation channel

Shadowing- presence of obstacles


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Signal propagation

Undergoes relection, refraction,diraction and scattering.

Path loss models Free space path loss model- A signal

propagating between two points with noattenuation or reection follows the freespace propagation law.

!ay tracing models depend hea"ily onthe geometry and dielectric properties ofthe region through which the signal

propagates.


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#ransmit and !ecei"e Signal $odels-de"elopedmainly for signals in the U%F bands, from .&-&'%( and &-&) '%(, respecti"ely. #his range offre*uencies is *uite fa"orable for wireless system

operation due to its propagation characteristicsand relati"ely small re*uired antenna si(e.

modulators and demodulators are built usingoscillators that generate real sinusoids not

comple eponentials. #hus, the transmittedsignal output from a modulator is a real signal.Similarly, the demodulator only etracts the realpart of the signal at its input.


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model channels as ha"ing a complefre*uency response due to the nature ofthe Fourier transform. As a result, real

modulated and demodulated signals areoften represented as the real part of acomple signal to facilitate analysis.


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Ray tracing

typical urban or indoor environment.

signal transmitted from a fi+ed source will encounter

multiple ob,ects in the environment that produce reflected%diffracted% or scattered copies of the transmitted signal.

-he multipath and transmitted signal are summed together at

the receiver% which often produces distortion in the receivedsignal relative to the transmitted signal.


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n ray tracing we assume a finite number of reflectors with/nown location and dielectric properties.

Ray tracing techni$ues appro+imate the propagation ofelectromagnetic waves by representing the wavefronts assimple particles.

-he error of the ray tracing appro+imation is smallest whenthe receiver is many 0 wavelengths from the nearestscatterer% and all the scatterers are large relative to awavelength and fairly smooth.


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f the transmitter% receiver% and reflectors are all immobilethen the impact of the multiple received signal paths% andtheir delays relative to the 3 path% are fi+ed.

if the source or receiver are moving% then the characteristicsof the multiple paths vary with time.


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Two-Ray Model

a single ground reflection dominates the multipath effect.

-he received signal consists of two components4 the 3component or ray% which is ,ust the transmitted signal

propagating through free space% and a reflected component orray% which is the transmitted signal reflected off the ground.


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5 6 (r7r8 9l):c is the time delay of the ground reflectionrelative to the 3 ray.

;!l 6 ;!a!b is the product of the transmit and receiveantenna field radiation patterns in the 3 direction.

R is the ground reflection coefficient.

;!r 6 ;!c!d is the product of the transmit and receiveantenna field radiation patterns corresponding to the rays oflength r and r8 % respectively


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received signal power isapproximately


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Pr dm / Pt dm + 0) log0)'l + 1)log0)ht hr 2 3) log0)d.


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Dielectric Canyon (Ten-RayModel

-his model assumes rectilinear streets with buildings along both sides of the street and transmitter and receiver antennaheights that are well below the tops of the buildings.

an infinite number of rays can be reflected off the buildingfronts to arrive at the receiver< in addition% rays may also be

bac/=reflected from buildings behind the transmitter orreceiver.

signal paths corresponding to more than three reflections cangenerally be ignored


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-he ten rays incorporate all paths with one% two% orthree reflections4 specifically% there is the 3% thegroundreflected (!R)% the single=wall (3W)

reflected% the double=wall (>W) reflected% the triple=wall (-W) reflected% the wall=ground (W !)reflected and the ground=wall (!W) reflected paths.


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where i denotes the path length of theith reflected ray, 4i / i 2 l5c, and 6'iis the product of the transmit and recei"e

antenna gains corresponding to the ithray


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Shadow fading

ong term shadow fading due to variations in radiosignal power due to encounters with terrainobstructions such as hills or manmade structuressuch as buildings

-he measured signal power differ substantially atdifferent locations even though at the same radialdistance from a transmitter

Represents the medium scale fluctuations of theradio signal strength over distances from tens tohundreds of meters


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Shadow Fading

*any empirical studies demonstrate that the received mean power fluctuates about the average power with a log=normaldistribution

?an be modelled by a gaussian random variable withstandard deviation% @

7e usually model the shadow fading channelgain as log-normal random "ariable then theshadow fading channel gain in d is a 'aussianrandom "ariable


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Shadow !ading

#ypical "alue of 8 range from 9 to 0)d

Shadowing complicates cellular planning #o fully predict shadowing eect, up-to-

date and highly detailed terrain data basesare needed


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"hen the path loss and shadow !adingare combined#the ratio o! received totransmitted power in d$ is given by


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The outage probability is the probability that the received

power at a given distance d the received power is below a

threshold


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Cell coverage

-he cell coverage in cellular system is defined asthe e+pected percentage of locations within a cellwhere the received power is above a given

minimum.


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-he percentage of area within a cell where the received power e+ceeds the minimum re$uired power Pmin isobtained by taking an incremental area dA at radius r fromthe base station (BS) in the cell, as shown in Figure

et Pr(r) be the received power in dA from combined pathloss and shadowing, and let PA = p(Pr(r) Pmin) in dA!"hen the cell co#erage area is gi#en by


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Statistical Multipath Channel Models

f a single pulse is transmitted over a multipath channel the receivedsignal will appear as a pulse train% with each pulse in the traincorresponding to the 3 component or a distinct multipath componentassociated with a distinct scatterer or cluster of scatterers.

:auses delay spread to the recei"ed signal.

delay spread e*uals the time delay between the arri"al ofthe ;rst recei"ed signal component


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f the delay spread is small compared to the inverse of thesignal bandwidth% then there is little time spreading in thereceived signal.

when the delay spread is relatively large% there is significanttime spreading of the received signal which can lead tosubstantial signal distortion


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Another characteristic of the multipath channel is its time=varying nature.

-his time variation arises because either the transmitter orthe receiver is moving% and therefore the location ofreflectors in the transmission path% which give rise tomultipath% will change over time. -hus% if we repeatedlytransmit pulses from a moving transmitter% we will observe

changes in the amplitudes% delays% and the number ofmultipath components corresponding to each pulse


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%arrowband !ading models

where the channel bandwidth is small compared to theinverse delay spread.

narrowband model we will assume a $uasi=staticenvironment with a fi+ed number of multipath componentseach with fi+ed path loss and shadowing. For this $uasi=static environment we then characteri#e the variations overshort distances


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Time-&arying Channel 'mpulseResponse


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two multipath components with delay 1 and 2 are resolvableif their delay difference significantly e+ceeds the inversesignal bandwidth.


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%arrowband adingApproximation


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'n-)hase and *uad SignalComponents


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Signal Envelope and Power Distributions


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$ is the ratio of the power in the %&S component to the power in the other (non'%&S) multipath components.

For $ = we ha#e ayleigh fading, and for $ = * we ha#eno fading, i!e! a channel with no multipath and only a 3component. -he fading parameter

$ is therefore a measure of the severity of the fading4 a small $ implies se#ere fading, a large $ implies more mild fading


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Fade duration


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average time that the signal envelope stays below agiven target level +!

ti denote the duration of the ith fade below le#el +o#er a time interval B0 , ",


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the average fade duration for the signal envelope(amplitude) level with + the target amplitude and-Pr the a#erage en#elope le#el


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%ideband Fading Models

When the signal is not narrowband we get another form ofdistortion due to the multipath delay spread.

n this case a short transmitted pulse of duration " will resultin a received signal that is of duration - 7 -m% where -m isthe multipath delay spread! -hus% the duration of thereceived signal may be significantly increased


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f the multipath delay spread -m CC - then themultipath components are received roughly on topof one another% as shown on the upper right of the

figure. -he resulting constructive and destructiveinterference causes narrowband fading of the pulse% but there is little time=spreading of the pulse andtherefore little interference with a subse$uently

transmitted pulse.


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n the other hand% if the multipath delay spread "m ", then each of the different multipathcomponents can be resolved% as shown in the lower

right of the figure. "owever% these multipathcomponents interfere with subse$uently transmitted pulses. -his effect is called intersymbol interference(3).


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Delay Spread+

signal ta/es different path to reach the destination due tomultiple paths. -his multiple paths cause reflection% refractionand scattering of radio signal. "ence when the signal is

transmitted from one place to the other% multiple copies of thesignal is received with different amplitudes and differentdelays (leads to different time of arrival) at the receiver.

For e+ample% if an impulse is transmitted then it will be nolonger a impulse when it is received at the other end% but itwill become a pulse with spreading effect. -he effect whichma/es this spreading of signal is /nown as >elay spread


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Doppler Spread+

>oppler fre$uency shift is usually different from path to path when signal arrives at the wireless

receiver. "ence transmitted signal fre$uency wille+perience >oppler spreading and is seen as spectralwidening or broadening in received signal powerspectrum. -his width of the spectrum is /nown as

>oppler 3pread or fading bandwidth.

>oppler 3pread is also /nown as fading rate.


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For narrowband signals, the multipathcomponents ha"e a time resolution thatis less than the in"erse of the signal

bandwidth, so the multipath componentscombine at the recei"er to yield theoriginal transmitted signal with amplitudeand phase characteri(ed by random

processes.


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with wideband signals% the received signale+periences distortion due to the delay spread of thedifferent multipath components% so the received

signal can no longer be characteri#ed by ,ust theamplitude and phase random processes. -he effect of multipath on wideband signals must

therefore ta/e into account both the multipath delay

spread and the time=variations associated with thechannel.


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Coherence time

#he time inter"al o"er which the channelcharacteristics will change "ery little.

#ct / 05d Dd is the doppler fre$uency spread


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C!&E'E(CE )*(D%IDT&

*easure of bandwidth over which channelcharacteristics (magnitude and phase ) are highlycorelated.

r in other words all the fre$uency components ofthe signal within the bandwidth will fadesimultaneously.

Dcb 6 1:-m


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Channel spread !actor

#he product #md . C1 channel is underspread. E1 channel is overspread.


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*odule 11

?AA?-G FWRHH33

?"AIIH


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Capacity of a channel

-hese capacity is the ma+imum data rates that can be achieved without any constraints on delay orcomple+ity of the encoder and decoder.


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Fre*uency Selecti"e Fading >igital radio technologies% including ?>*A%

W?>*A% J*D% -H% and Wi*AK% transmit

digital signals in a bandwidth larger than thecoherence bandwidth of the channel. -his meansthat the channel no longer loo/s LflatM across thefre$uency band< rather% the fading is Lfre$uency=

selectiveM with different signal strengths present atdifferent fre$uencies across the band

Capacity of Flat Fading Channels


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Capacity of Flat+Fading Channels

Channel and Syste" Model


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discrete=time channel with stationary and ergodic time=varying gain ;gBiN% AW!I n.i, as shown in Figure!

-he channel gain g.i can change at each time i!

-he system model is also shown in Figure where aninput message w is sent from the transmitter to thereceiver. -he message is encoded into the codeword +%which is transmitted over the time=varying channel as

/.i at time i! "he channel gain gBiN% also called thechannel side information (?3)% changes during thetransmission of the codeword.


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Capacity of flat fading channel

-he capacity of this channel depends on what is /nownabout gBiN at the transmitter and receiver

scenarios.1. Channel Distribution Infor"ation ,CDI-. -he distribution

of g.i is known to the transmitter and receiver.2. 'eceiver CSI. -he value of g.i is known at the recei#er at

time i, and both the transmitter and receiver /now thedistribution of g.i!

. Trans"itter and 'eceiver CSI. -he value of g.i is knownat the transmitter and recei#er at time i, and both thetransmitter and recei#er know the distribution of g.i!

Channel Distribution


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Channel Distribution'n!ormation (CD' ,nown

Channel Side 'n!ormation at


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Channel Side 'n!ormation atReceiver

gBiN is /nown at the receiver at time i. both the transmitter and receiver /now the distribution of

gBiN. there are two channel capacity definitions1. 3hannon capacity% also called ergodic capacity2. ?apacity with utage


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CSI at receiver

3hannon capacity. 3hannon capacity defines the ma+imum data rate that

can be sent over the channel with asymptotically smallerror probability. for 3hannon capacity the ratetransmitted over the channel is constant4 the transmittercannot adapt its transmission strategy relative to the?3.


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Capacity with utage

:apacity with outage applies to slowly-"aryingchannels, where the instantaneous S@! isconstant over a large number of transmissionsa transmission burst and then changes to a

new "alue based on the fading distribution. if the channel has recei"ed S@! during a burst

then data can be sent o"er the channel at ratelog10+ with negligible probability of error.

Since the transmitter does not Bnow the S@!"alue , it must ; a transmission rateindependent of the instantaneous recei"ed S@!.


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the transmitter ;es a minimum recei"edS@! min and encodes for a data rate C =B log2(1 + min !.

Cf the recei"ed S@! is below min then thebits received over that transmission burstcannot be decoded correctly" and therecei"er declares an outage.

Co = (1 # pout !B log2(1 + min ! 0 # pout! is the correctly received

transmissions

Channel Side Infor"ation at the


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Channel Side Infor"ation at the

Trans"itter and 'eceiver


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7hen both the transmitter and recei"erha"e :SC, the transmitter can adapt itstransmission strategy relati"e to this :SC.

the transmitter Bnows the channel andthus will not send bits unless they can bedecoded correctly.


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Shannon Capacity

channel power gain g.i is known to both thetransmitter and receiver at time i!

et s.i be a stationary and ergodic stochastic

process representing the channel state, which takesvalues on a finite set S of discrete memorylesschannels!

et 0s denote the capacity of a particular channel. p(s) denote the probability, or fraction of time, that

the channel is in state s!


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apply this formula to the system model in7e Bnow the capacity of an A7'@channel with a"erage recei"ed S@! is C= B log2(1+!. $et p(! denote the

probability distribution of the recei"edS@!.


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/ ! t C it


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/ero+!utage Capacity

!utage Capacity and Truncated Channel


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!utage Capacity and Truncated Channel

Inversion

y suspending transmission inparticularly bad fading states outagechannel states, we can maintain a higherconstant data rate in the other states andthereby signi;cantly increase capacity.

#he outage capacity is de.ned as themaximum data rate that can be

maintained in all nonoutage channelstates multiplied by probability ofnonoutage.

cutoff fade depth =

The outage capacity associated with a given outage

i i d di ff i i


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probability pout and corresponding cutoff is given

by

>iversity


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y

mitigate the effects of fading.

send the same data over independent fading paths.

-hese independent paths are combined in some waysuch that the fading of the resultant signal isreduced.

independent signal paths have a low probability ofe+periencing deep fades simultaneously.

di ersit


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di"ersity

The chance that two deep fades

occur simultaneously is rare.

0

0

-20

-40

-60

-80

-100

4 8 12 16 d

R e c e i v e d S i n a l ! o w e r

" d # m $

Reali#ation of ndependent Fading aths:


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p gdifferent types of diversities

space diversity

Jse multiple receive antennas% also called anantenna array% where the elements of the array are

separated in distance. 3IR is called array gain. in a uniform scattering environment with isotropic

transmit and receive antennas the minimum antenna

separation re$uired for independent fading on eachantenna is appro+imately one half wavelength ( .OP to be e/act)


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1. polari#ation diversity using either two transmit antennas or two receive

antennas with different polari#ation (e.g. vertically

and hori#ontally polari#ed waves). 3ince the scattering angle relative to each

polari#ation is random% it is highly improbable thatsignals received on the two differently polari#ed

antennas would be simultaneously in deep fades

Contd


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Contd/

>isadvantages of polari#ation diversity

1. can have at most two diversity branches%

corresponding to the two types of polari#ation.

2. polari#ation diversity loses effectively half the power ( dD) since the transmit or receive power isdivided between the two differently polari#edantennas

Angular or directional diversity


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Angular or directional diversity.

>irectional antennas provide angle% or directional%diversity by restricting the receive antenna beamwidth to a given angle.

if the angle is very small then at most one of themultipath rays will fall within the receive beamwidth%so there is no multipath fading from multiple rays.

this diversity techni$ue re$uires either a sufficient

number of directional antennas to span all possibledirections of arrival or a single antenna whosedirectivity can be steered to the angle of arrival ofone of the multipath component


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Fre$uency diversity.

transmitting the same narrowband signal at different

carrier fre$uencies% where the carriers are separated by the coherence bandwidth of the channel.

-his techni$ue re$uires additional transmit power tosend the signal over multiple fre$uency bands


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Receiver diversity


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ysystem model

Receiver diversity


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Receiver diversity

diversity system combines the independent fading paths to obtain a resultant signal that is then passedthrough a standard demodulator.

-he combining can be done in several ways whichvary in comple+ity and overall performance ?o phasing


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Diversity Co"bining Selection


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Diversity Co"bining + Selection

-hreshold ?ombining


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-hreshold ?ombining

avoids the need for a dedicated receiver on each branch by scanning each of the branches inse$uential order and outputting the first signal with3IR above a given threshold Q"!

only one branch output is used at a time.

nce a branch is chosen% as long as the 3IR on that branch remains above the desired threshold% thecombiner outputs that signal

Cntd


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Cntd/

f the 3IR on the selected branch falls below thethreshold% the combiner switches to another branch.

-he simplest criterion is to switch randomly toanother branch

With only two=branch diversity this is e$uivalent toswitching to the other branch when the 3IR on theactive branch falls below " ! "his method is calledswitch and stay combining (SSC).

Maxi"al 'atio Co"bining


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E#ual $ain Co"bining


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E#ual+$ain Co"bining



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Transmitter diversity


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Transmitter diversity

desirable in systems such as cellular systems wheremore space% power% processing capability isavailable on the transmit side versus the receiveside.

>iversity depends upon whether or not the channelgain is /nown to the transmitter.

Without channel /nowledge transmit diversity gainre$uires a combination of space and time diversity.

Channel 1nown at trans"itter


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Channel 1nown at trans"itter

Channel un1nown at trans"itter


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Channel un1nown at trans"itter

Alamouti scheme

Modulation Sche"es


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Modulation Sche"es

Amplitude and phase modulation $PA$ $PSD

$EA$ ierential modulation.

re0uency modulation


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FSD $SD :PFSD

FRHJHI?G SHR3J3 A*-J>H


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*>JA-I


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M)AM

For *A* all of the information is encoded intothe signal amplitude Ai!

-he amplitude of the transmitted signal ta/es on 5different #alues, which implies that each pulsecon#eys log 6 5 = $ bits per symbol time "s

!ray encoding=With this encoding method% if noisecauses the demodulation process to mista/e one

symbol for an ad,acent one (the most li/ely type oferror)% this results in only a single bit error in these$uence of $ bits.


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M)S,


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M)S,

For $PSD all of the information isencoded in the phase of the transmittedsignal.

#ransmitted signal o"er one symbol timeis gi"en by

:ntd..


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:ntd..

C%TD/


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C%TD/


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3' is called 3' and is same as *A* with*6

*uadrature AmplitudeM d l ti (M*AM


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Modulation (M*AM

For *A*% the information bits are encoded in both the amplitude and phase of the transmittedsignal. -hus% whereas both *A* and *3' haveone degree of freedom in which to encode theinformation bits (amplitude or phase)% *A* hastwo degrees of freedom. As a result% *A* is morespectrally efficient than *A* and *3'% in that

it can encode the most number of bits per symbolfor a given average energy


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Di3erential Modulation


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Di3erential Modulation

Cn $PSD and $EA$ signals is carried inthe signal phase.

re*uire coherent demodulation, i.e. thephase of the transmitted signal carrier )must be matched to the phase of therecei"er carrier .

dierential modulation techni*ues do notre*uire a coherent phase reference at therecei"er

Cntd44


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Cntd44

>ifferential modulation is modulation with memory

the symbol transmitted over time Bk"s, (k 4 *)"s)

depends on the bits associated with the currentmessage to be transmitted and the bits transmittedover prior symbol times.

the previous symbol is used as a phase reference forthe current symbol% thus avoiding the need for acoherent phase reference at the receiver

re0uency Modulation


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0 y

encodes information bits into the fre$uency of thetransmitted signal.

at each symbol time " log2

% bits areencoded into the fre&uency of thetransmitted signal s(t!.

transmitted signal si(t! = 'cos(2 fit + i!" where i is the inde of the ith

message and i is the phase associatedwith the ith carrier.

Cntd/


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fre$uency modulation encodes information in thesignal fre$uency% the transmitted signal s(t) has aconstant envelope A!

he modulated signal is less sensitive to amplitudedistortion introduced by the channel or thehardware.

Fre#uency Shift 2eying ,FS2-


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# y y g , -

Cntd44


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F3' modulation assigns different fre$uencies to eachinformation symbol. n the receiving side it is passedthrough a circuit that determines differences in the fre$uencyof the received signal and obtains the original information

signal.

$ oscillators are operating at the dierentfre*uencies fi = fc+i )fc and the modulator

switches between these di*erent oscillatorseach symbol time s

Cntd/


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f the switching timing in the synchroni#ation andmodulating signal of these oscillators is not good%the continuity of the phase between bits cannot bemaintained .

-his discontinuous phase leads to a spectral broadening% which is undesirable

Mini"u" Shift 2eying ,MS2-


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y g , -

$SD is a special case of FSD where0/1

the fre*uency separation is 1Gfc = .,-s.

Since 1Gfc = .,-s is the minimum possible fre&uency separation in FSD, it is

called $SD and therefore it occupies theminimum bandwidth

Continuous+Phase FS2 ,CPFS2-


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, -

A better way to generate F3' that eliminates the phase discontinuity is to fre$uency modulate asingle carrier with a modulating waveform.

?F3' modulation uses a S? (Soltage ?ontrolledscillator). -he S? changes the fre$uency

proportionally to the voltage of the modulating

signal so that the phase between bits is continuous.?F3' (?ontinuous hase F3') is characteri#ed byits low levels of unwanted emissions

Capacity of Fre#uency+Selective Fading

Channels


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Channels

Ti"e+Invariant Channels. ?onsider a time invariant channel with fre$uency

response "(f).

When the channel is time=invariant it is typicallyassumed that "(f) is /nown at both the transmitterand receiver.

Assume a total transmit power .

cntd


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Cntd44


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assume that (f! is bloc/0fading he fre&uency is divided into

subchannels of bandwidth B" where (f!= is constant over each bloc/.

#he fre*uencyselecti"e hfading channelthus consists of a set of A7'@ channelsin parallel with S@!

Cntd/


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The capacity of this parallel set of channels is the

su" of rates associated with each channel with


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power opti"ally allocated over all channels


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Time-&arying Channels -he time=varying fre$uency=selective fading

channel is similar to the time invarient model with 7(f) = 7(f, i), i!e! the channel #aries o#er both fre1uency and time!

difficult to determine the capacity of time=varying

fre$uency=selective fading channels% even when theinstantaneous channel 7(f, i) is /nown perfectly atthe transmitter and receiver

Cntd/


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for channel capacity in time=varying fre$uency=selective fading we ta/e the channel bandwidth B ofinterest and di#ide it up into subchannels the si8e ofthe channel coherence bandwidth Bc!

We then assume that each of the resultingsubchannels is independent% time=varying% and flat=

fading with 7(f, i) = 79 .i on the 9th subchannel .

Cntd/


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Cntd/


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we can obtain the capacity for each of these flat=fading sub channels based on the average power S9that we allocate to each sub channel, sub9ect to atotal power constraint S! Since the channels are

independent% the total channel capacity is ,ust e$ualto the sum of capacities on the individualnarrowband flat=fading channels sub,ect to the totalaverage power constraint% averaged over both timeand fre$uency4


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*odule 1S

*J- ?ARRHR*>JA-I

*J- ?ARRHR *>JA-I


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divides the data stream into multiple substreams

transmitted over different orthogonal subchannelscentered at different subcarrier fre$uencies.

symbol time on each substream much greater thanthe delay spread of the channel

-his insures that the substreams will not e+periencesignificant 3.

P'I(CIPE !F !'T&!$!(*IT3


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orthogonality means two coe+isting signals areindependent of each other in a specified timeinterval and do not interact with each other.

it allows multiple signals to be transmitted perfectlyover a common channel without interference

two signals are orthogonal when the integral of their product over one period is e$ual to #ero or pea/ ofone signal occurs at the null of nearer signal.

Cntd/


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!FDM


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3ingle information stream is split into multiplesymbols and each symbol or group of symbol will

be assigned a separate carrier. All split informationis then transmitted in parallel.

DM TRA%SM'TT5R


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DM R5C5'&5R


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!FDM S3STEM+ DET*IED )!C2

D$M


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D$M

!FDM T'*(SMITTE'


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-he input data stream is modulated by a A*modulator% resulting in a comple+ symbol K( base band modulated signal.

-his symbol stream is passed through a serial to= parallel converter% whose output is a set of I parallel A* symbols corresponding to the

symbols transmitted over each of the subcarriers

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-he I symbols are discrete fre$uency components.

n order to generate s(t)% we therefore need toconvert these fre$uency components into timesamples. We therefore perform an inverse >F- onthese I symbols% which is efficiently implementedusing the FF- algorithm.

output of FF- is called baseband F>*.

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A cyclic prefi+ is inserted in order to eliminate theinter=symbol and inter=bloc/ interference (D). -his cyclic prefi+ of length is a circular e+tension

of the FF-=modulated symbol% obtained by copyingthe last samples of the symbol in front of it.

-he data are bac/=serial converted% forming anF>* symbol that will modulate a high=fre$uency

carrier before its transmission through the channel. d:a conversion is re$uired before rf conversion

stage.

DM R5C5'&5R


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#he data are down-con"erted to thebaseband.


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>ivide the total signalling dimensions into channelsand the assign these channels to different users.

-he most common methods are to divide signalspace along time %fre$uency or code a+is.

->*A F>*A ?>*A 3>*A

TDM*


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C%TD/ TDMA


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n time=division% time is divided into orthogonaltime slots% and each user is assigned a cyclically=repeating timeslot.

only digital data and digital modulation must beused.

cyclically=repeating timeslot implies thattransmission is not continuous for any user

DMA


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C%TD/DMA


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n fre$uency division% the bandwidth is divided intonon overlapping channels. Hach user is thenassigned a different channel for transmission andreception.

>uring the period of the call% no other user can sharethe same fre$uency band.

F>*A is usually implemented in narrowband

systems ?hannel assignment is carried out on a first=come

first= served basis

C%TD/DMA


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?ontinuous transmission 4 the channels% onceassigned% are used on a non=time=sharing basis. -hismeans that both subscriber and D3 can use theircorresponding allotted channels continuously and

simultaneously.

f F>*A channels are not in use% then they sit idle

and cannot be used by other users to increasecapacity.

CDMA


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CDMA


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n code=division% time and bandwidth are usedsimultaneously by different users% modulated byorthogonal or semi=orthogonal codes.

n ?>*A% the narrowband message signal is multiplied

by a very large bandwidth signal called spreading signal(code) before modulation and transmission over the air.-his is called spreading.

Hach user has its own cordword -he receiver correlator distinguishes the senders signal

by e+amining the wideband signal with the same time=synchroni#ed spreading code

SDMA


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spot bea"

antenna

SDMA


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Jse spot bea" antennas The different bea" area can use TDM*5 FDM*5

CDM*

Sectori6ed antenna can be thought of as a

SDM*

*daptive antennas can be used in the future

,si"ultaneously steer energy in the direction of

"any users-

SDMA


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A large number of independently steeredhigh-gain beams can be formed withoutany resulting degradation in S@! ratio.

eams can be assigned to indi"idual

users, thereby assuring that all linBsoperate with maimum gain.

Adapti"e beam forming can be easily

implemented to impro"e the systemcapacity by suppressing co channelinterference

Random access


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A


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n the A"A random access protocol% pac/ets are buffered at each terminal and transmitted over acommon channel to a common hub or base station.

no control is imposed on the channel to synchroni#e

transmission from the various users% and therefore thestart times of pac/ets from different users in thenetwor/ can be modeled as a oisson point process.

they both wait a random amount of time beforeretransmitting. -he goal% of course% is to prevent theusers from colliding once again when they retransmit.

A S8etch o! rame1eneration


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@ote that all pacBets ha"e the same length because thethroughput of A


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begin transmitting at the start of a time slot. -he use of such time slots increases the ma+imum

possible throughput of the channel but alsointroduces the need for synchroni#ation of all nodes

in the networ/

CSMA


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:ollisions can be reduced by :arrierSense $ultiple Access :S$A, whereusers sense the channel and delaytransmission if they detect that another

user is currently transmitting. if the channel is busy,the user wait till the

channel is free.

!#S :#S

Ad-7oc "ireless %etwor8s


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An ad hoc wireless networ/ is a collection ofwireless mobile nodes that self=configure to form anetwor/ without the aid of any establishedinfrastructure.

handle the necessary control and networ/ing tas/s by themselves% generally through the use ofdistributed control algorithms.

-hey can be tailored to specific applications. -hey can be formed from whatever networ/

available.

Cntd/


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#ypes ata networBs %ome networBs

e"ice networBs Sensor networBs istributed control system.

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Data %etwor8s Ad hoc wireless networ/s can support data e+change

between laptops% palmtops% personal digitalassistants (>As)% and other information devices.

AIs with coverage over a relatively small area (aroom% floor% or building.

*AIs with coverage over several s$uare miles (a

metropolitan area or battlefield). energy eIciency is a maJor issue in the

design of wireless data networBs.

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7ome %etwor8s "ome networ/s support communication between ?s%

laptops% >As% cordless phones%smart appliances% securityand monitoring systems% consumer electronics% and

entertainment systemsanywhere in and around the home. different devices accessing a home networ/ have verydifferent power constraints4 some will have a fi+ed powersource and be effectively unconstrained% while others willhave very limited battery power and may not berechargeable. -hus% one of the biggest challenges in homenetwor/ design is to leverage power in unconstraineddevices

Cntd/


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Device (etwor1s >evice networ/s support short=range wireless

connections between devices -hus% the need for cables and the corresponding

connectors between cell phones% modems% headsets%>As% computers% printers%pro,ectors% networ/access points% and other such devices is eliminated.

bluetooth


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M'M


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systems with multiple antennas at the transmitterand receiver are commonly referred to as multipleinput multiple output (**) systems.

ncrease data rates through multiple+ing or to

improve performance through diversity. n ** systems the transmit and receive antennas

can both be used for diversity gain.

*ultiple+ing is obtained by e+ploiting the structureof the channel gain matri+ to obtain independentsignalling paths that can be used to sendindependent data.

(arrowband MIM! Model


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A narrowband point=to=point communication systemof %t transmit and %r receive antennas. -his isrepresented as


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x represents the 5t =dimensional transmitted symbol. n is the 5r =dimensional noise vector. & is the 5r : 5t matri+ of channel gains

hi9representing the gain from transmit antenna 9 toreceive antenna i.

-rasmit power and noise power . Average 3IR T 6:.

Parallel Deco"position of the MIM!

Channel


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Cntd7


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When both the transmitter and receiver have multipleantennas% there is another mechanism for performancegain called "ultiplexing gain0

-he multiple+ing gain of a ** system is achieved

by decomposing into a number R of parallelindependent channels.

R=fold increase in data rate in comparison to a systemwith ,ust one antenna at the transmitter and receiver.

-his increased data rate is called the multiple+ing gain

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a ** channel with 5r : 5t channel gain matri/" /nown to both the transmitter and the receiver. et 7 denote the rank of H. " 6 JUS" where the 5r:5r matri/ and the5t:5t matri/ ;

are unitary matrices. transform the ** channel into 7 parallel

single'input single=output (33) channels withinput V+ and output Vy

** ?hannel ?apacity


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capacity of a ** channel% which e$uals thema+imum data rate that can be transmitted over thechannel with arbitrarily small error probability.

?hannel capacity depends on what is /nown aboutthe channel gain matri+ or its distribution at the

transmitter and:or receiver

Static MIM! Channels


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2 conditions for capacity80 Channel 2nown at Trans"itter

90 Channel 4n1nown at Trans"itter. 4nifor"

Power *llocation

?hannel 'nown at -ransmitter


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the capacity e$uals the sum of capacities on each ofthe independent parallel channels with the transmit power optimally allocated between these channels.

?I->


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R " parallel channels4R " degrees of freedom. X is the noise power. Y average 3IR. At high 3IR channel capacity increases linearly

with number of degrees of freedom in the channel. At low 3IR all power will be alloted to the parallel

channel with largest 3IR

?hannel Jn/nown at -ransmitter4 Jniformower Allocation


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receiver /nows the channel but the transmitter doesnot. transmitter cannot optimi#e its power allocation. best strategy should be to allocate e$ual power to

each transmit antenna. transmitter does not /now at what rate to transmit

such that the data will be received correctly.

capacity definition is capacity with outage

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transmitter fi+es a transmission rate 0, and theoutage probability associated with 0 is the probability that the transmitted data will not bereceived correctly or% e$uivalently% the probability

that the channel " has mutual information less than0!

Cntd/


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% / min%t"%r , this implies that as %grows large, the ** channel capacity in theabsence of ?3- approaches

hence grows linearly in %.

Fading MIM! Channels


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0. :hannel Dnown at #ransmitter.1. :hannel UnBnown at #ransmitter

&. @o :SC at the #ransmitter or !ecei"er

Channel 2nown at Trans"itter


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transmitter optimi#es its transmission strategy foreach fading channel reali#ation as in the case of astatic channel

-he capacity is then ,ust the average of capacities

associated with each channel reali#ation% with poweroptimally allocated. -his average capacity is calledthe ergodic capacity of the channel.

A short-term power constraint A long-term power constraint


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Channel 4n1nown at Trans"itter


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same as capacity with outage for static **channel.

(o CSI at the Trans"itter or 'eceiver


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When there is no ?3 at either the transmitter orreceiver% the linear growth in capacity as a functionof the number of transmit and receive antennasdisappears% and in some cases adding additional

antennas provides negligible capacity gain. channel capacity becomes heavily dependent on the

underlying channel model

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At low 3IR capacity is limited by noise and growslinearly with the channel degree of freedom.

At high 3IR capacity grows only doubly

logarithmically with 3IR.

n other words% there is no multiple+ing gain

associated with multiple antennas when there is notransmitter or receiver ?3

*daptive Modulation


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spectrally=efficient transmission over time=varyingchannel. estimate the channel at the receiver and feed this estimate

bac/ to the transmitter% so that the transmission scheme

can be adapted relative to the channel characteristics. increase average throughput% reduce re$uired transmit power% or reduce average

probability of bit error by ta/ing advantage of favorable

channel conditions to send at higher data rates or lower power% and reducing the data rate or increasing power asthe channel degrades

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practical constraints1. Adaptive modulation re$uires a feedbac/ path

between the transmitter and receiver% which maynot be feasible for some systems.

2. if the channel is changing faster than it can bereliably estimated and fed bac/ to the transmitter%adaptive techni$ues will perform poorly.

. ften only the slow variations can be trac/ed andadapted to

*daptive Trans"ission Syste"


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estimate Z g BiN of the channel power gain g BiN at timei is available to the receiver after an estimation timedelay of ie and that this same estimate is available tothe transmitter after a combined estimation and

feedbac/ path delay of id 6 ie 7 i f . -he availability of this channel information at the

transmitter allows it to adapt its transmissionscheme relative to the channel variation.

treat LgMiN as the true gain


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feedbac/ path does not introduce any errors. -he rate of channel variation will dictate how often

the transmitter must adapt its transmission parameters.

When the channel gain consists of both fast andslow fading components% the adaptive transmissionmay adapt to both if g BiN changes sufficiently slowly%or it may adapt to ,ust the slow fading.


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if g BiN corresponds to shadowing and multipathfading% then at low speeds the shadowing isessentially constant% and the multipath fading issufficiently slow so that it can be estimated and fed

bac/ to the transmitter with an estimation error anddelay that does not significantly degrade performance

MIM! Diversity $ain. )ea"for"ing


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-he multiple antennas at the transmitter and receivercan be used to obtain diversity gain instead ofcapacity gain.

the same symbol% weighted by a comple+ scale

factor% is sent over each transmit antenna% so that theinput covariance matri+ has unit ran/.

-his scheme is also referred to as MIM!bea"for"ing

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-he diversity gain then depends on whether or notthe channel is /nown at the transmitter. When thechannel matri+ 7 is /nown% -he correspondingreceived 3IR can be shown to e$ual / ma 3.

C / B log10 + ma 3% corresponding to thecapacity of a 33 channel with channel power gain

ma . When the channel is not /nown at the transmitter

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