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Mobile Radio Propagation:

Large-Scale Path Loss(S. Rappaport, wireless communications)

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Introduction to Radio Wave Propagation Reflection

– Large buildings, earth surface Diffraction

– Obstacles with dimensions in order of wavelength Scattering

– Foliage, lamp posts, street signs, walking pedestrian, etc.

transmittedsignal

receivedsignal

Ts

max

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Large-scale propagation models large-scale propagation models

characterize signal strength over large T-R separation distances

small-scale or fading models: characterize the rapid fluctuations of the received signal strength over very short travel distances or short time durations

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0 20 40 60 80 100 120-1

0

1

0 20 40 60 80 100 120-1

0

1

0 20 40 60 80 100 120-1

0

1

0 20 40 60 80 100 120-2

0

2

First Path

Echo path(case 1)

Echo path (case 2)

Constructive addition (case 1)

Destructive addition (case 2)

Multipath Fading

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Large-Scale & Small-Scall Fading

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Large-Scale & Small-Scall Fading (Contd.)

The distance between small scale fades is on the order of /2

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Path Loss

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Propagation Models

Free Space Propagation Model - LOS path exists between T-R

May applicable for satellite communication or microwave LOS links

Frii’s free space equation:

- Pt : Transmitted power- Pr : Received power- Gt : Transmitter gain- Gr: Receiver gain- d: Distance of T-R separation- L: System loss factor L1 : Wavelength in meter

Ld

GGPdP rtt

r 22

2

)4()(

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Antenna Gain

Relationship between antenna gain and effective area

• G = antenna gain

• Ae = effective area

• f = carrier frequency

• c = speed of light (3 * 108 m/s) = carrier wavelength

2

2

2

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c

AfAG ee

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Propagation Models (Contd.)

Path Loss – difference (in dB) between the effective transmitted power and the received power, and may or may not include the effect of the antenna gains

Path loss for the free space model when antenna gains included

PL(dB) = 10 log(Pt/Pr) = -10 log(Gt Gr 2 / (4)2 d2 L)

Path loss for the free space model when antenna gains excluded

PL(dB) = 10 log(Pt/Pr) = -10 log(2 / (4)2 d2 L)

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Fraunhofer distance

22Dd f

Where D is the largest physical linear dimension of the antenna. Additionally, to be in the far-field region, d, must satisfy

ff dDd and

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Propagation Models (Contd.)

Modified free space equation

Pr(d) = Pr(d0)(d0/d)2 d d0 df

Modified free space equation in dB form

Pr(d) dBm = 10 log[Pr(d0)/0.001W] + 20 log(d0/d)

where Pr(d0) is in units of watts.

df is Fraunhofer distance which complies:

df =2D2/where D is the largest physical linear dimension of the

antenna

In practice, reference distance is chosen to be 1m (indoor) and 100m or 1km(outdoor) for low-gain antenna system in 1-2 GHz region.

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Example (link budget)

Free Space Loss Path

Frequency 0.9000 GHzERP 50.0000 WattsERP in dBm 46.9897 dBmTransmission Line Loss 0.0000 dBTx Antenna Gain 0.0000 dBiPath Length 0.1500 KmFree Space Path Loss 75.0484 dBRx Antenna Gain 0.0000 dBiRx Transmission Line Loss 0.0000 dBRx Signal Strength -28.0587 dBmRx Threshold (sensitivity) -85.0000 dBmFade Margin 56.9413 dB

RF Link Budget Calculator

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Relating Power to Electric Field

Pd = EIRP / 4d2 = Pt Gt / 4d2

In free space, the power flux density Pd (in W/m2) is given by

Or in another form

Pd = E2 / Rfs = E2 / W/m2

where Rfs is the intrinsic impedance of free space given by =120 = 377 , then

Pd = E2 / 120 W/m2

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Relating Power to Electric Field (Contd.)

Pr(d) = Pd Ae = Ae (E2 / 120 )

At the end of receiving antenna

Or when L=1, which means no hardware losses are taken into consideration

where Ae is the effective aperture of the receiving antenna

Pr(d) = Pt Gt Gr 2 / (4)2 d2

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Large-scale Path Loss (Part 2)The three basic Propagation Mechanisms

Reflection

Diffraction Scattering

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Reflection, Diffraction and Scattering Reflection occurs when a propagating

electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave.

Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges).

Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large.

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Fresnel Reflection Coefficient (Γ) It gives the relationship between the electric field intensity of the reflected and transmitted waves to the incident wave in the medium of origin.

•The Reflection Coefficient is a function of the material properties

• It depends on

Wave Polarization

Angle of Incidence

Frequency of the propagating wave

Reflection

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Reflection from Dielectrics

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• The behavior for arbitrary directions of polarization is illustrated through

the two distinct cases in the figure

Case 1

• The E - field polarization is parallel with the plane of incidence

i.e. the E - field has a vertical polarization, or normal component

with respect to the reflecting surface

Case 2

• The E - field polarization is perpendicular to the plane of incidence

i.e. the E - field is parallel to the reflecting surface ( normal to the

page and pointing out of it towards the reader)

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•The dielectric constant ε of a perfect (lossless) dielectric is given by ε = ε0 εr

where εr is the relative permittivity

and ε0 = 8.85 * 10-12 F/ m

• The dielectric constant ε for a power absorbing, lossy dielectric is ε = ε0 εr - j ε’ where ε’ = σ / 2π f

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•

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• In the case when the first medium is free space and μ1 =

μ2

the Reflection coefficients for the two cases of vertical and horizontal polarization can be simplified to

irir

irir

2

2

||cossin

cossin

iri

iri

2

2

cossin

cossin

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Brewster Angle

It is the angle at which no reflection occurs in the medium of origin

It occurs when the incident angle θB is such that the Reflection Coefficient Γ| | = 0

For the case when the first medium is free space and the second medium has a relative permittivity εr , the above equation can be expressed as

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1)sin(

B

1

1)sin( 2

r

rB

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Ground Reflection (Two- Ray) Model

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Whenever 321 20

3

20 rthhhhd

The received E-field can be approximated

mVd

k

d

hh

d

dEdE rt

TOT /22

)(2

00

The power received at distance d is given by

4

22

d

hhGGPP rt

rttr

For large T- R distances so received power falls off to the 4th power of d, or at 40 db/ decade

rthhd

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•This power loss is much more than that in free space

•At large values of d, the received power and path loss become independent of frequency.

• The path loss for the 2- ray model in db PL (db) = 40 log d – ( 10 log Gt + 10 log Gr + 20 log ht + 20 log hr )

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Diffraction Phenomena: Radio signal can propagate

around the curved surface of the earth, beyond the horizon and behind obstructions.

Huygen’s principle: All points on a wavefront can be considered as point sources for the production of secondary wavelets and these wavelets combine to produce a new wavefront in the direction of propagation.

The 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 obstacles.

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Fresnel Zone Geometry

The wave propagating from the transmitter to the receiver via the top of the screen travels a longer distance than if a direct line-of-sight path exists.

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Fresnel Zone Geometry(Cont’d)

Angle ,

Fresnel-Kirchoff diffraction parameter

Normalizing ,

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Fresnel Zone Geometry(Cont’d)

The concentric circles on the plane are Fresnel Zones.

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The radius of the nth Fresnel zone circle

The excess total path length traversed by a ray passing through each circle is

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Consider a receiver at point R, located in the shadowed region.

The electric field strength Ed,

where E0 is the free space field strength

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Graphical representation of

The diffraction gain:

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Lee’s approximate solution:

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Multiple Knife-edge Diffraction

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Scattering: When does Scattering occur?

When the medium through which the wave travels consists of objects with dimensions that are small compared to wavelength

The number of obstacles per unit volume is large

Large-scale Path Loss (part 4)

How are these waves produced:By rough surfaces, small objects or by other irregularities

in the channel

Normally street signs, lamp posts, trees induce scattering in mobile communication system

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Rayleigh Criterion: Surface roughness is tested using the Rayleigh

criterion,its given by

hc= /8sini

where, i is the angle of incidence

hc is the critical height of surface protuberance

for a given i

The surface is considered smooth if the minimum to maximum protuberance h <= hc and rough if h> hc

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Radar cross section model: The radar cross section (RCS) of a scattering

object is defined as the ratio of the power density of the signal scattered in the direction of the receiver to the power density of the radio wave incident upon the scattering object, and has units of square meters.

RT

2TTR

20logd -20logd - )30log(4-

]RCS[dBm)20log((dBi)G(dBm)P(dBm)P

Bistatic radar equation

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Practical link budget design using path loss models

Log –distance Path Loss Model

00

0

log10)()(

or

)(

d

dndPLdBPL

d

ddPL

n

n is the path loss exponent which indicates the rate at which the path loss increases with distance,

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Log-normal Shadowing:

Xd

dndPLXdPLdBdPL

00 log10)()(])[(

Xσ is the Zero –mean Gaussian distributed random variable with standard deviation σ(also in dB)PL(d) is a random variable with a normal distribution.Define

2

zerf1

2

1dx

2exp

2π

1Q(z)

z

2

x

)(

)(PrdP

QdP rr

The probability that the received signal level will exceed a certain value γ can be calculated from the cumulative density function as

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2

0 022

)(Pr1

)(Pr1

)(R

rr rdrdrPR

dArPR

U

Determination of Percentage of Coverage Area

berf

bU

11

1exp1

2

1)(

2

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