8/2/2019 Wc Lecture 1 2011

1/61

Introduction What is communication ?

Significance of guided and unguided

communication Wireless / Mobile Communication

Radio Propagation Effects

8/2/2019 Wc Lecture 1 2011

2/61

Propagation Scenario

8/2/2019 Wc Lecture 1 2011

3/61

Mobile Radio Propagation Large Scale Propagation Effects Distance dependent loss

Reflection

Diffraction Scattering

Useful in estimating radio coverage

Small Scale Propagation Effects

Rapid fluctuations of received signal strength overshort durations or short distances Multipath propagation time dispersive

Mobility frequency dispersive

8/2/2019 Wc Lecture 1 2011

4/61

Large-scale /small-scale

propagation

8/2/2019 Wc Lecture 1 2011

5/61

Propagation phenomena

8/2/2019 Wc Lecture 1 2011

6/61

50-100

1 2 Km

8/2/2019 Wc Lecture 1 2011

7/61

Free space propagation

model - LOS Pr(d) = Pt Gt Gr2/ (4d)2 L Pr(d0)(d0/d)2 Assumes far-field (Fraunhofer region)

Far field distance df>> D and df>> , where D is the largest linear dimension of antenna

is the carrier wavelength df= 2D

2/ do > df Fraunhofer region/far field (1m -1 km)

No interference, no obstructions

Effective isotropic radiated power EIRP = Pt Gt Effective radiated power ERP

Antenna gains dBi, dBd Path loss PL(dB) = 10log (Pt/Pr)

8/2/2019 Wc Lecture 1 2011

8/61

Radio Propagation Mechanisms Reflection

Propagating EM wave impinges on an object which is large ascompared to its wavelength- e.g., the surface of the Earth, buildings, walls, etc.

Conductors & Dielectric materials (refraction) Diffraction

Radio path between transmitter and receiver is obstructed by asurface with sharp irregular edges

Waves bend around the obstacle, even when LOS (line of sight)does not exist

Fresnel zones Scattering

Objects smaller than the wavelength of the propagating wave- e.g. foliage, street signs, lamp posts

Clutter is small relative to wavelength

8/2/2019 Wc Lecture 1 2011

9/61

Reflection Perfect conductors reflect with no attenuation

Light on the mirror

Dielectrics reflect a fraction of incident energy

Grazing angles reflect max* Steep angles transmit max*

Light on the water

Reflection induces 180 phase shift Why? See yourself in the mirror

Reflected field intensity

Fresnel reflection coefficient Brewster angle = 0 ?

q qr

qt

8/2/2019 Wc Lecture 1 2011

10/61

Classical 2-ray Ground reflection

model

8/2/2019 Wc Lecture 1 2011

11/61

2-Ray Model For d > do, the free space propagating field

E(d,t) = (Eodo/d) cos { c(t d/c)} ELOS(d,t) = (Eodo/d) cos { c(td/c)} Eg(d,t) = (Eodo/d) cos { c(td/c)} qi = qo Eg = Ei ; Et = (1+) Ei

8/2/2019 Wc Lecture 1 2011

12/61

2-Ray Model

For small grazing angles, assuming perfecthorizontal E-field polarization and groundreflection , | =-1, Et = 0 |ETOT| = |ELOS + Eg |

ETOT(d,t) = (Eodo/d) cos { c(td/c)}+(-1)(Eodo/d) cos { c(td/c)}

8/2/2019 Wc Lecture 1 2011

13/61

Method of images

8/2/2019 Wc Lecture 1 2011

14/61

2-Ray model Using method of images,

Path difference

= d d = {(ht+hr)2+d2} 1/2{(ht-hr)2+d2} 1/2 If d >> (h

t

+hr

) ; using Taylor series expansion,

= d d 2hthr /d q = 2 / = c / c and d = / c = q/2 fc

8/2/2019 Wc Lecture 1 2011

15/61

2-Ray model At time t = d/c ;

ETOT

(d, t) = (Eo

do

/d) cos { c

((d d)/c)} - (Eo

do

/d) cos0

= (Eodo/d) q - (Eodo/d) (Eodo/d) [ q - 1]

|ETOT(d) |

= { (Eodo/d)2 cos (q - 1)2 + (Eodo/d)2 sin (q )2 }

8/2/2019 Wc Lecture 1 2011

16/61

2-Ray model |ETOT(d) |= { (Eodo/d)

2 cos (q - 1)2 + (Eodo/d)2 sin (q )2 }

|ETOT(d) |= (Eodo/d) (2-2cos q ) = (2Eodo/d) sin( q /2) sin( q /2) ( q /2) < 0.3 rad d > 20 ht hr / ETOT(d) (2Eodo/d) (2 ht hr / d) k / d 2 V/m Pr= Pt Gt Gr (ht hr )

2/ d 4

PL(dB) = 40 log d ( 10logGt + 10logGr + 20loght + 20loghr )

8/2/2019 Wc Lecture 1 2011

17/61

2-Ray model Summary For small T-R separation

ETOT(d,t) = (Eodo/d) cos { c(td/c)}+(-1)(Eodo/d) cos { c(td/c)}

For large T-R separation

ETOT(d) (2Eodo/d) (2 ht hr / d)

8/2/2019 Wc Lecture 1 2011

18/61

2-Ray model Summary For large T-R separation,

ETOT(d) k / d 2 V/m

The free space power is related to square of the electric fieldand hence,

Pr= Pt Gt Gr (ht hr )2/ d 4

Path Loss in dBPL(dB) = 40 log d ( 10logGt + 10logGr + 20loght + 20loghr )

8/2/2019 Wc Lecture 1 2011

19/61

Diffraction Diffraction occurs when waves hit the edge of an obstacle

Secondary waves propagate into the shadowed region

Water wave example

Diffraction is caused by the propagation of secondarywavelets into a shadowed region.

Excess path length results in a phase shift

The field strength of a diffracted wave in the shadowedregion is the vector sum of the electric field components ofall the secondary wavelets in the space around the obstacle.

Huygens principle:all points on a wavefront can beconsidered as point sources for the production of secondarywavelets, and that these wavelets combine to produce anew wavefront in the direction of propagation.

8/2/2019 Wc Lecture 1 2011

20/61

Diffraction geometry

8/2/2019 Wc Lecture 1 2011

21/61

Fresnel Screens

Path difference between successive zones = /2

8/2/2019 Wc Lecture 1 2011

22/61

Fresnel Zones

8/2/2019 Wc Lecture 1 2011

23/61

Fresnel Zone Clearance

8/2/2019 Wc Lecture 1 2011

24/61

Fresnel Zone Clearance

8/2/2019 Wc Lecture 1 2011

25/61

Fresnel Zone Clearance A rule of thumb used for

line-of-sight microwave links55% of the first Fresnel zone

to be cleared.

8/2/2019 Wc Lecture 1 2011

26/61

Fresnel Zone Clearance A rule of thumb used for

line-of-sight microwave links55% of the first Fresnel zone

to be cleared.

8/2/2019 Wc Lecture 1 2011

27/61

Diffraction geometry

8/2/2019 Wc Lecture 1 2011

28/61

Diffraction geometry

8/2/2019 Wc Lecture 1 2011

29/61

Diffraction Parameter Assuming h >,

Excess path length h2 (d1+ d2 )/(2 d1d2)

Phase difference = 2/= 2 / 2 ?

=+ and considering tan ,

h (d1+ d2 )/(d1d2)

Fresnel-Kirchoff diffraction parameter

= h {2(d1+ d2 )/(d1d2)}1/2

= {2 d1d2/((d1+ d2 ))}1/2

8/2/2019 Wc Lecture 1 2011

30/61

Fresnel diffraction geometry

8/2/2019 Wc Lecture 1 2011

31/61

Diffraction Gain Electric Field Strength Ed

Ed = Eo F()

F() complex Fresnel integral evaluatedusing tables / graphs

Diffraction gain Gd(dB) = 20 log |F()|

8/2/2019 Wc Lecture 1 2011

32/61

Approximate solution forGd(dB)

Gd(dB) = 0 -1

Gd(dB) = 20log(0.5-0.62 ) -1 0

Gd(dB) = 20log{0.5exp(-0.95 )} 0 1

Gd(dB) = 20log{0.4-[0.1184-(0.38-0.1 2 )]1/2}1 2.4

Gd(dB) = 20log{0.225/ } > 2.4

8/2/2019 Wc Lecture 1 2011

33/61

Knife-edge diffraction loss

8/2/2019 Wc Lecture 1 2011

34/61

Multiple knife-edge diffraction

8/2/2019 Wc Lecture 1 2011

35/61

Scattering Rough surfaces

Lamp posts and trees, scatter energy in all directions

Critical height for roughness hc = /(8 sinqi) Smooth if its minimum to maximum protuberance

h < hc

For rough surfaces, Scattering loss factor S to bemultiplied with surface reflection coefficient,rough = S ; S is a function of, h, qi

Nearby metal objects (street signs, etc.)

Usually modeled statistically Large distant objects

Analytical model: Radar Cross Section (RCS)

Bistatic radar equation

8/2/2019 Wc Lecture 1 2011

36/61

Measured results

8/2/2019 Wc Lecture 1 2011

37/61

Measured results

8/2/2019 Wc Lecture 1 2011

38/61

Propagation Models Large scale models predict behavior averaged over distances >>

Function of distance & significant environmental features, roughlyfrequency independent

Breaks down as distance decreases

Useful for modeling the range of a radio system and roughcapacity planning,

Path loss models, Outdoor models, Indoor models

Small scale (fading) models describe signal variability on a scale of Multipath effects (phase cancellation) dominate, path attenuationconsidered constant

Frequency and bandwidth dependent

Focus is on modeling Fading: rapid change in signal over a shortdistance or length of time.

8/2/2019 Wc Lecture 1 2011

39/61

Free Space Path Loss Path Loss is a measure of attenuation based only on thedistance to the transmitter

Free space model only valid in far-field;

Path loss models typically define a close-in point d0

and reference other points from there:

2

00 )()(

d

ddPdP rr

dB

dBrd

ddPLdPdPL

0

0 2)()]([)(

8/2/2019 Wc Lecture 1 2011

40/61

Log-distance Path LossLog-distance generalizes path loss to account for other

environmental factors Choose a d0 in the far field. Measure PL(d0) or calculate Free Space Path Loss.

Take measurements and derive nempirically.

dBdddPLdPL

0

0 )()( n

8/2/2019 Wc Lecture 1 2011

41/61

Typical large-scale path loss

8/2/2019 Wc Lecture 1 2011

42/61

Log-Normal Shadowing Model Shadowing occurs when objects block LOS between

transmitter and receiver

A simple statistical model can account for unpredictableshadowing

PL(d)(dB)=PL(d)+X Where X is a zero-mean Gaussian RV (in dB)

(distributed log normally), with standard deviation of

(in dB) is usually from 3 to 12 dB

L l Ri M d l

8/2/2019 Wc Lecture 1 2011

43/61

Longley-Rice Model(ITS Irregular terrain model)

Point-to-point from 40MHz to 100GHz. Predicts median transmission loss, Takes terrain into account, Uses

path geometry, Calculates diffraction losses, forward scatter theory Inputs:

Frequency Path length Polarization and antenna heights Surface refractivity Effective radius of earth Ground conductivity

Ground dielectric constant Climate

Disadvantages Does not take into account details of terrain near the receiver Does not consider Buildings, Foliage, Multipath

Original model modified by Okamura for urban terrain

L l Ri M d l

8/2/2019 Wc Lecture 1 2011

44/61

Longley-Rice ModelOPNET implementation

8/2/2019 Wc Lecture 1 2011

45/61

Durkins Model It is a computer simulator for predicting field

strength contours over irregular terrain.

Line of sight or non-LOS Edge diffractions using Fresnel zone

Disadvantage cannot adequately predict

propagation effects due to foliage, building,and it cannot account for multipathpropagation.

8/2/2019 Wc Lecture 1 2011

46/61

2-D Propagation Raster data

Digital elevation models(DEM) United StatesGeological Survey (USGS)

8/2/2019 Wc Lecture 1 2011

47/61

Algorithm for line of sight

(LOS) Line of sight (LOS) or not

8/2/2019 Wc Lecture 1 2011

48/61

Multiple diffraction

computation

8/2/2019 Wc Lecture 1 2011

49/61

Empirical Models Okumura model

Curves based on extensive measurements(site/freq specific)

Awkward (uses graphs)

Hata model Analytical approximation to Okumura model

Cost 231 Model:

Extends Hata model to higher frequency (2 GHz)

Walfish/Bertoni: Cost 231 extension to include diffraction from rooftops

Commonly used in cellular system simulations

8/2/2019 Wc Lecture 1 2011

50/61

Okumura Model It is one of the most widely used models for signal prediction in urban areas,and it is applicable for frequencies in the range 150 MHz to 1920 MHz

Based totally on measurements (not analytical calculations)

Applicable in the range: 150MHz to ~ 2000MHz, 1km to 100km T-R

separation, Antenna heights of BS upto 200m, MS upto 3m

8/2/2019 Wc Lecture 1 2011

51/61

Okumura Model The 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.

m30m1000200

log20)(

te

tete h

hhG

m33

log10)(

re

rere h

hhG

m3m10

3

log20)(

re

rere h

hhG

8/2/2019 Wc Lecture 1 2011

52/61

OkumuraCorrection factor

GAREA

8/2/2019 Wc Lecture 1 2011

53/61

Hata Model Empirical formulation of the graphical data in the

Okamura model. Valid 150MHz to 1500MHz, Used

for cellular systems

The following classification was used by Hata:

Urban areaSuburban areaOpen area

8/2/2019 Wc Lecture 1 2011

54/61

Hata Model EdBALdB logCdBALdB log

DdBALdB logbhfA 82.13log16.2655.69

bhB log55.69.44

94.40log33.18)28/log(78.42 ffD

4.5))28/(log(22 fC

MHzfhE m 300cities,largefor97.4))75.11(log(2.32

MHzfhE m 300cities,largefor1.1))54.1(log(29.82

citiessmalltomediumfor)8.0log56.1()7.0log11.1(

fhfE m

Urban area

Suburban area

Open area

- E

- E

8/2/2019 Wc Lecture 1 2011

55/61

PCS Extension of Hata Model COST-231 Hata Model, European standard

Higher frequencies: up to 2GHz

Smaller cell sizes

Lower antenna heights

GEdBFLdB log

bhfF log82.13log9.333.46 f >1500MHz

0

3G

Metropolitan centers

Medium sized city and suburban areas

Indoor Propagation

8/2/2019 Wc Lecture 1 2011

56/61

Indoor PropagationModels The distances covered are much smaller

The variability of the environment is much greater

Key variables: layout of the building, construction materials,

building type, where the antenna mounted, etc. In general, indoor channels may be classified either as LOS or

OBS with varying degree of clutter

The losses between floors of a building are determined by theexternal dimensions and materials of the building, as well as the

type of construction used to create the floors and the externalsurroundings.

Floor attenuation factor (FAF)

Signal Penetration into

8/2/2019 Wc Lecture 1 2011

57/61

Signal Penetration intoBuildings RF penetration has been found to be a function of frequency as

well as height within the building. Signal strength receivedinside a building increases with height, and penetration lossdecreases with increasing frequency.

Walkers work shows that building penetration loss decrease at

a rate of 1.9 dB per floor from the ground level up to the 15thfloor and then began increasing above the 15th floor. Theincrease in penetration loss at higher floors was attributed toshadowing effects of adjacent buildings.

Some devices to conduct the signals into the buildings

8/2/2019 Wc Lecture 1 2011

58/61

Repeater Extend coverage range

Directional antenna or distributed antenna systems

R T i d Sit

8/2/2019 Wc Lecture 1 2011

59/61

Ray Tracing and SiteSpecific Modeling Site specific propagation model and graphical information

system. Ray tracing. Deterministic model.

Data base for buildings, trees, etc.

SitePlanner

8/2/2019 Wc Lecture 1 2011

60/61

Simple path loss/shadowing model:

Find Pr:

Find Noise power:

8/2/2019 Wc Lecture 1 2011

61/61

SINR:

Withoutshadowing ( = 0), BPSK works 100%, 16QAM fails all thetime.

Withshadowing (s= 6dB):

BPSK: 16 QAM

75% of users can use BPSK modulation and hence get a PHY data

rate of 10 MHz 1 bit/symbol 1/2 = 5 Mbps Less than 1% of users can reliably use 16QAM (4 bits/symbol) for a

more desirable data rate of 20 Mbps.

Interestingly for BPSK, w/o shadowing, we had 100%; and 16QAM:0%!