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EELE 5490, Fall, 2009
Wireless CommunicationsAli S. Afana
Department of Electrical Engineering
Class 5
Dec. 4th, 2009
EE 552/452 Spring 2007
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OutlineReviewFree space propagationReceived power is a function
of transmit power times gains of transmitter and receiver
antennasSignal strength is proportional to distance to the power of
-2Reflection: Cause the signal to decay faster. Depends on the
height of transmitter and receiver
antennasDiffractionScattering
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DiffractionDiffraction occurs when waves hit the edge of an
obstacleSecondary waves propagated into the shadowed regionWater
wave exampleDiffraction is caused by the propagation of secondary
wavelets into a shadowed region. Excess path length results in a
phase shiftThe field strength of a diffracted wave in the shadowed
region is the vector sum of the electric field components of all
the secondary wavelets in the space around the obstacle.Huygens
principle: all points on a wavefront can be considered as point
sources for the production of secondary wavelets, and that these
wavelets combine to produce a new wavefront in the direction of
propagation.
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Diffraction
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Diffraction geometryDerive of equation 4.54-4.57
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Diffraction geometry
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Diffraction geometryFresnel-Kirchoff distraction parameters,
4.56
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Diffraction Loss
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Fresnel ScreensFresnel zones relate phase shifts to the
positions of obstacles
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Fresnel ZonesBounded by elliptical loci of constant
delayAlternate zones differ in phase by 180Line of sight (LOS)
corresponds to 1st zoneIf LOS is partially blocked, 2nd zone can
destructively interfere (diffraction loss)How much power is
propagated this way?1st FZ: 5 to 25 dB below free space prop.
Obstruction of Fresnel Zones 1st 2nd
0-10-20-30-40-50-600o90180odBTip of ShadowObstructionLOS
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Fresnel diffraction geometry
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Knife-edge diffraction
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Knife-edge diffraction lossGain
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Multiple knife-edge diffraction
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ScatteringRough surfacesLamp posts and trees, scatter all
directionsCritical height for bumps is f(,incident angle), Smooth
if its minimum to maximum protuberance h is less than critical
height and rough if greater than that.Scattering loss factor
modeled with Gaussian distributionNearby metal objects (street
signs, etc.)Usually modeled statistically
EE 552/452 Spring 2007
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Propagation ModelsLarge scale models predict behavior averaged
over distances >> Function of distance & significant
environmental features, roughly frequency independentBreaks down as
distance decreasesUseful for modeling the range of a radio system
and rough capacity planning, Experimental rather than the
theoretical for previous three modelsPath loss models, Outdoor
models, Indoor modelsSmall scale (fading) models describe signal
variability on a scale of Multipath effects (phase cancellation)
dominate, path attenuation considered constantFrequency and
bandwidth dependent Focus is on modeling Fading: rapid change in
signal over a short distance or length of time.
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Free Space Path LossPath Loss is a measure of attenuation based
only on the distance to the transmitterFree space model only valid
in far-field; Path loss models typically define a close-in point d0
and reference other points from there:
Log-distance generalizes path loss to account for other
environmental factorsChoose a d0 in the far field.Measure PL(d0) or
calculate Free Space Path Loss.Take measurements and derive
empirically.
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Typical large-scale path loss
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Log-Normal Shadowing ModelShadowing occurs when objects block
LOS between transmitter and receiverA simple statistical model can
account for unpredictable shadowing PL(d)(dB)=PL(d)+X0,Add a 0-mean
Gaussian RV to Log-Distance PLVariance is usually from 3 to 12.
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Questions?
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