Chapter 3 Radio Propagation Large Scale Effects

8/10/2019 Chapter 3 Radio Propagation Large Scale Effects

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Mobile Radio Propagation :Large Scale Path Loss

The radiopropagationchannel exhibits

many differentforms of channelimpairments, as aresult of time-

varying signalreflections,blockage andmotion.


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Free Space Propagation Loss

Power levels :


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The free-space path loss:


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Free space propagation loss

Assumes far-field (Fraunhofer region)d >> D and d >> , whereD is the largest linear dimension of antenna

is the carrier wavelengthNo interference, no obstructionsBlack board 4.2Effective isotropic radiated powerEffective radiated powerPath lossFraunhofer region/far fieldIn log scale


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ReflectionPerfect conductors reflect with no attenuation

Like light to the mirrorDielectrics reflect a fraction of incident energy

Grazing angles reflect max*

Steep angles transmit max*Like light to the water

Reflection induces 180 phase shift

Why? See yourself in the mirror q q r q

t


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Reflection from smooth surface

'0 jr


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Reflection


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Reflection


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Reflection


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Reflection coefficients

Equation 4.26, example 4.4, Brewster angle,perfect conductors


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Reflection coefficients

A dielectric material is a substance that is apoor conductor of electricity, but an efficientsupporter of electrostatic fields.

For earth, at frequency 100MHz


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Propagation over smooth plane

The received signal is the phase sum of thedirect wave and the reflected wave from theplane (2-ray model).


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One line of sight and one ground bound


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Method of image


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DiffractionDiffraction occurs when waves hit the edge of an obstacleSecondary waves propagated into the shadowed region Water wave exampleDiffraction is caused by the propagation of secondary wavelets intoa shadowed region.Excess path length results in a phase shift

The field strength of a diffracted wave in the shadowed region is thevector sum of the electric field components of all the secondarywavelets in the space around the obstacle.Huygens principle: all points on a wavefront can be considered aspoint sources for the production of secondary wavelets, and that

these wavelets combine to produce a new wavefront in the directionof propagation.


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Diffraction

Estimating the signal attenuation caused bydiffraction of radio waves over hills andbuildings is essential in predicting the field

strength in a given service area. It ismathematically difficult to make very preciseestimates of the diffraction losses overcomplex and irregular terrian. Some caseshave been derived, such as propagation overa knife-edge object.


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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


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Diffraction


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Fresnel diffraction geometry


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Knife-edge diffraction lossGainExam. 4.7Exam. 4.8


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ScatteringRough surfaces

Lamp posts and trees, scatter all directionsCritical height for bumps is f( ,incident angle), 4.62Smooth if its minimum to maximum protuberance h is lessthan critical height.Scattering loss factor modeled with Gaussian distribution,4.63, 4.64.

Nearby metal objects (street signs, etc.)Usually modeled statistically

Large distant objects Analytical model: Radar Cross Section ( RCS )Bistatic radar equation, 4.66


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It is therefore expected that the receivedsignal is stronger than predicted fromreflection and diffraction models alone.


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Measured results


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Measured results


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Propagation ModelsLarge scale models predict behavior averagedover distances >>

Function of distance & significant environmentalfeatures, roughly frequency independentBreaks down as distance decreasesUseful for modeling the range of a radio system andrough capacity planning,Experimental rather than the theoretical for previousthree modelsPath loss models , Outdoor models, Indoor models


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Small scale (fading) models describe signalvariability on a scale of

Multipath effects (phase cancellation) dominate,path attenuation considered constantFrequency and bandwidth dependentFocus is on modeling Fading: rapid change insignal over a short distance or length of time.


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Log Distance Path Loss Models


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Typical large-scale path loss


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For Example

Sometime different values are used for ndepending on the distance from the transmitter.


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For Examplen does not directly reflect the strength of the received power


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Log-Normal Shadowing ModelShadowing occurs when objects block LOSbetween transmitter and receiver

A simple statistical model can account forunpredictable shadowing

PL(d)(dB)=PL(d)+X0, Add a 0-mean Gaussian RV to Log-Distance PL

Variance is usually from 3 to 12.Reason for Gaussian


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Measured large-scale path lossDetermine n and by mean and varianceEqu. 4.70Equ. 4.72Basic of Gaussian

distribution

A i d l i h


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Area versus Distance coverage model withshadowing model

Percentage forSNR larger thana threshold

Equ. 4.79Exam. 4.9


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Okumura ModelThe major disadvantage with the model is its low response to rapid

changes in terrain, therefore the model is fairly good in urban areas,but not as good in rural areas.Common standard deviations between predicted and measured pathloss values are around 10 to 14 dB.G(hre)

m30m1000200

log20)( tete

te hhhG

m33

log10)(

re

rere h

hhG

m3m103

log20)(

rere

re hh

hG


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Hata ModelEmpirical formulation of the graphical data in the Okamura model.

Valid 150MHz to 1500MHz, Used for cellular systemsThe following classification was used by Hata:

Urban area Suburban area Open area

E d B A LdB logC d B A LdB log

Dd B A LdB logbh f A 82.13log16.2655.69

bh B log55.69.44

94.40log33.18)28/log(78.42

f f D

4.5))28/(log(2 2 f C

MHz f h E m 300cities,largefor97.4))75.11(log(2.32

MHz f h E m 300cities,largefor1.1))54.1(log(29.82

citiessmalltomediumfor)8.0log56.1()7.0log11.1( f h f E m


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PCS Extension of Hata ModelCOST-231 Hata Model, European standardHigher frequencies: up to 2GHzSmaller cell sizesLower antenna heights

G E d B F LdB logbh f F log82.13log9.333.46 f >1500MHz

03G

Metropolitan centersMedium sized city and suburban areas

Chapter 3 Radio Propagation Large Scale Effects

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