MOBILE RADIO

PROPAGATION AND FADING:

Part A: Large Scale Fading

References:Rappaport (Chapter 4 and 5)

Bernhard (Chapter 2)

Garg (Chapter 3)

LECTURE 1

INTRODUCTION

Performance of comm sys governed by the channelenvironment

Comms channel is dynamic and unpredictable,analysis often difficult

Unique characteristic in comms channel is aphenomenon called fading variation of signalamplitude over time and frequency

Fading may either be due to multipath propagation,and/or shadow fading

INTRODUCTION

Radio waves extends from a frequency of 30 kHz to 300 GHz

In free space, radio waves propagate in straight line (LOS) and

are reflected off objects. Radio waves on the earth are affected by

the terrain of the ground, the atmosphere and the natural and

artificial objects on the terrain.

There are 3 main propagation means on the earth:

Ground wave

Ionespheric or Sky wave

Trophospheric Wave

RADIO WAVE PROPAGATION

Ground Wave

travels in contact with earths surface reflection, refraction and scattering by objects on the ground transmitter and receiver need NOT see each other affects all frequencies at VHF or higher, provides more reliable propagation means signal dies off rapidly as distance increases

Tropospheric Wave

bending(refraction) of wave in the lower atmosphere VHF communication possible over a long distance bending increases with frequency so higher frequency more chance

of propagation More of an annoyance for VHF or UHF (cellular)

Ionospheric or Sky Wave

Reflected back to earth by ionospheric layer of the earth atmosphere By repeated reflection, communication can be established over

1000s of miles

Mainly at frequencies below 30MHz More effective at times of high sunspot activity

RADIO WAVE PROPAGATION

Range

Transmission range: communication

possible, low error rate

Detection range: detection of the

signal possible, no communication possible

Interference range: signal may

not be detected, signal adds

to the background noise

Region

Near-field (Fresnel)

The close-in region of an antenna wherein the angular field distribution is dependent upon distance from the antenna

Far-field (Fraunhofer)

The region where the angular field distribution is essentially independent of distance from the source.

If the source has a maximum overall dimension D that is large compared to the wavelength, the far-field region is commonly taken to exist at distances

greater than 2D2/ from the source

For a beam focused at infinity, the far-field region is sometimes referred to as the Fraunhofer region

distance

sender

transmission

detection

interference

No effect

EFFECT OF TRANSMISSION

a free line-of-sight IS NOT EQUAL TO a free Fresnel Zone

Refer Example

4.1, Pg 109

Free Space propagation

Refraction

Conductors & Dielectric materials (refraction)

Diffraction

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

Waves bend around the obstacle, even when LOS does not exist

Fresnel zones

Reflection

Propagating wave impinges on an object which is large compared to wavelength

e.g., the surface of the Earth, buildings, walls, etc.

Scattering

Clutter is small relative to wavelength Objects smaller than the wavelength of the propagating wave

e.g., foliage, street signs, lamp posts

RADIO PROPAGATION MECHANISMS

diffractionshadowing

Radio wave

scattering

Radio wave

reflection

Radio wave

Radio Propagation Models and Mechanisms

(outdoor area)

1

2

3

Radio Propagation Models and Mechanisms

(indoor area)

Tx : Transmitter, Rx : Receiver

REAL WORLD

EXAMPLES

INTRODUCTION

Type of imperfections:

Large-scale fading:

Power varies gradually

Over large distance, terrain contours

Determine by path profile and antenna displacement

Small-scale fading:

Small changes of the reflected, diffracted and scattered signals

Resulting in vector summation of destructive/ constructiveinterference at Rx, known as multipath wave

Rapid changes of amplitudes, phase or angle

Also known as Rayleigh fading [1] or frequency selectivity

[1] J.G. Proakis. Digital Communications. Fourth Edition, The McGraw-Hill Companies, 2001

FADING

Rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance (sub-wavelength)

Large scale

mean signal attenuation versus distance

variation about the mean

Small scale

time spreading: flat fading and frequency selective fading

time variance of channel: fast fading and slow fading

Cause by: multipath waves and Doppler shift

Mobile Small Scale and Large Scale Variations

Distance* Courtesy Prof. Rohling Hamburg Harburg University-Germany

FADING Two major components

Long term fading m(t)

Short term fading r(t)

Received signal, s(t)

s(t) = m(t) r(t)

MULTIPATH FADING

PATH LOSS AND FADING

Short term fading

Also known as fast fading caused by local multipath effect by NLOS

Observed over distance = wave length

30mph will experience several fast fades in a sec

Given by Rayleigh Distribution (Rayleigh fading)

The distribution can be formed using the square root of sum of the squareof two Gaussian functions

r = ( Ac2 + As

2)

Ac and As are two amplitude components of the field intensity of thesignal

Long term fading

Long term variation in mean signal level is also known as slow fading

Caused by movement over large distances, shadowing effects and wavediffraction around buildings, hills etc, moving receivers experience slowvariations of the signal level

The probability density function is given by a log-normal distributioni.e.normal distribution on a log scale (log-normal shadowing)

A small deviation of the power level is advantageous for a goodtransmitting quality. Typical values are 3 to 8 dB

FADING

RAYLEIGH and LOG-NORMAL

FADING CHANNEL

CLASSIFICATIONS

flat effectDistortion: amplitude or phase

baseband signal variation

FADING CHANNEL

CLASSIFICATIONS

Ref: B. Sklar. Rayleigh Fading Channels in Mobile Digital Communications Systems. Part I:Characterization, IEEE Communications Magazine, Vol. 35, No. 7, pp. 90-100, July 1997.

PATH LOSS MODEL

Detail path loss model hard to factor in overall system design

Most important characteristic is power falloff with distanceRadio propagation models

- Analytical models mathematical - Empirical models observation/experimentation- Composite (Semi-empirical)

Applications:- Predict large scale coverage for mobile systems

- Estimate and predict SNR

WHAT IS A PATH LOSS?

R = Pt + Gtot L

L = Pt + Gtot R

Example: for Pt = 39 dBm, Gtot = 7.5 dB, R = -95 dBm, path

loss, L, cant exceed 141.5 dB without violating the R (Rx

sensitivity)

FRIIS TRANSMISSION EQUATIONPower density at any distance, R, in the far field is the total power transmitted divided by the area of the sphere of radius R

** Page 109, Example 4.2

Assumes far-field (Fraunhofer region) d >> D and d >> , where

D is the largest linear dimension of antenna

is the carrier wavelength

Suppose we have unobstructed line-of-sight (LOS), the Free

Space Propagation Loss (FSPL) is denoted by:

FSPL

distance

frequency

)(log20log2044.32

)(4

log20

kmMHz

d

f

dBdf

dBd

FSPL

Try: http://www.qsl.net/pa2ohh/jsffield.htm

ACTIVITY 1

1. The communication system, with total path loss

of 142 dB is operated under free space

propagation conditions at 900 Mhz. Determine

its maximum range

2. Calculate the maximum distance that can be

achieved, Given:

Total Path Loss (PL) = 142 dB

fMHz = 2350 MHz

distance

frequency

)(log20log2044.32 kmMHz

d

f

dBdfFSPL

ACTIVITY 1

1. The communication system, with total path loss

of 148.3 dB is operated under free space

propagation conditions at 900 Mhz. Determine

its maximum range

2. Calculate the maximum distance that can be

achieved, Given:

Total Path Loss (PL) = 142 dB

fMHz = 2350 MHz

distance

frequency

)(log20log2044.32 kmMHz

d

f

dBdfFSPL

d = 689 km

d = 127 km

PEPL Plane Earth Propagation Loss

Path loss for flat reflecting surface One LOS path and one ground (or reflected) bounce

Ground bounce approximately cancels LOS path above critical distance

PEPL is given by (- for loss)

PEPL

(m)receiver andansmiter between tr distance

(m)height (MS)receiver

(m)height (BS)r transmitte

))(log(20)log(20)log(40

log202

d

h

h

dBhhd

d

hhPEPL

r

t

rt

rt

Pg. 125, Eq. 4.53 accurate PEPL equation, look at Example 4.6

ACTIVITY 2

Calculate the maximum range of the communication system

in activity #1 earlier, assuming hr = 1.5m, ht = 8m, f = 2350

MHz and that propagation takes place over a plane earth.

How does this range change if the base station antenna

height is doubled?

(m)receiver andansmiter between tr distance

(m)height (MS)receiver

(m)height (BS)r transmitte

))(log(20)log(20)log(40

log202

d

h

h

dBhhd

d

hhPEPL

r

t

rt

rt

ACTIVITY 2

Calculate the maximum range of the communication system

in activity #1 earlier, assuming hr = 1.5m, hm = 8m, f = 2350

MHz and that propagation takes place over a plane earth.

How does this range change if the base station antenna

height is doubled?

(m)receiver andansmiter between tr distance

(m)height (MS)receiver

(m)height (BS)r transmitte

))(log(20)log(20)log(40

log202

d

h

h

dBhhd

d

hhPEPL

r

t

rt

rt

r = ?? km, when antenna height doubled, range increase by factor of sqrt(2)

for same propagation loss, hence r = ??

)log(10)(][

][][][;)()(

exponent losspath

and distanceally with logrithmic

decreasespower signal received average

model losspath distance-Log

00

0

d

ddPdBP

dBmPdBmPdBPd

ddP

d

LL

rtLL

LOG-DISTANCE MODEL

)log(10][ddBm])[(

[dB])([dBm]dBm])[(

)(]dB)[(

0

0d

dPdP

dPPdP

XdPdP

rr

Ltr

LL

With fading (log-normal) variablerandomdist Gaussian mean zeroX

Refer graph in Pg 141

PATH LOSS EXPONENT

Path loss is a function of- T-R distance (d)- Path loss exponent (n)- Standard deviation (s)

Estimation path model parameters from measured data by linear regressionThe estimation error probability is also available Use path loss models for link budget design Estimate the percentage of coverage area for a signal:

])(y[Probabilit bPr

Diffraction occurs when waves hit the edge of an obstacle- Secondary waves propagated into the shadowed region- Excess path length results in a phase shift

- Fresnel zones relate phase shifts to the positions of obstacles

Model obstructions like hills, building use knife edge diffraction model

Fresnel-Kirchoff diffraction parameter

Single and multiple (Bullington, Millington, Deygout) knife-edge

Diffraction gain (loss) depends on v

DIFFRACTION MODEL

TR

1st Fresnel zone

Obstruction

21

21

21

21

21

21

)(

2)(2

dd

ddh

where

dd

dd

dd

ddhv

RT d1 d2

h

DIFFRACTION MODEL

4.2225.0

log20)(

4.21

)1.038.0(1184.04.0log20)(

10))95.0exp(5.0log(20)(

01)6.05.0log(20)(

10)(

2

vv

dBG

v

vdBG

vvdBG

vvdBG

vdBG

d

d

d

d

d

Refer Pg 132-133, Example 4.7

PERCENTAGE AREA COVERAGE

)(1)(

21

2

1

2exp

2

1)(

)()(

)()(

2

zQzQ

zerfdx

xzQ

dPQdPP

dPQdPP

z

rr

rr

Pg. 143

Predict the signal strength at some point or local area

Consider also the terrain profile, e.g., mountains, trees, buildings,

obstacles.

Obtain models from systematic interpretation of measurement data

Classifications:

Computer based models:

Longley-Rice model

Durkins model

Measurement model

Okumura model

Empirical model

Hata model

PCS extension and wideband PCS microcell models

Walfish and Bertoni Model

OUTDOOR PROPAGATION MODELS

Computer-based models

Longley-Rice model Model point-to-point propagation

Frequency band 40MHz-100GHz

Use Geometric optic techniques:

Two-ray ground reflection, knife edge refraction,scattering

Can use the terrain path profile if available

Can not add environment corrections, no multipath considerations

Case study on Longley-Rice: Durkins Model (Pg 146)

Measurement model

Okumura model

Most widely used model in urban areas

Obtained by extensive measurements

Represented by charts (curves) giving median attenuation relative to free space attenuation

Valid under:

Frequency band: 150-1920 MHz

T-R distance: 1-10 km,

BS antenna height: 30-1000 m

Quasi-smooth terrain (urban & suburban areas)

OUTDOOR PROPAGATION MODELS

Okumura model properties

Based completely on measurement, no analytical explanation and in graphical form

based on extensive measurement in the Tokyo area at frequencies from 150-1920 MHz. Valid for those frequencies and distance from 1

to 100 km

Model is valid for an urban environment over quasi-smooth terrain

Simple, but accurate for predicting path loss of cellular & land mobiles. Practical standard for system planning

Okumuras model is very accurate in cluttered environments, but responds slowly to rapid changes in terrain (as often seen in rural

areas)

Calculate path loss:

Determine free space loss

Look up table for median attenuation A

Add correction factors due to antennas and environments

OKUMURA MODEL

tenvironmen toduefactor gain :

m 310),3/log(20

m 3),3/log(10)(

m 301000 :/200)log(20)(

distance and frequency

n with attenuatiomedian :),(

losspath space free :

losspath median :

)()(),(dB][

:Model LossPath Okumura

3

50

50

A

r

rr

r

ttt

ma

F

ArtmaF

G

hh

hhhG

hhhG

df

dfA

L

L

GhGhGdfALL

OKUMURA MODEL

OKUMURA MODEL

m3m103

log20)(

m33

log10)(

m10m1000200

log20)(

re

re

re

rere

re

tete

te

hh

hG

hh

hG

hh

hG

Empirical formulation to match Okumura model

Validity: fc = 150-1500MHz, ht = 30-200m, hr = 1-10m

Suitable for large cell, not for PCS microcells (

Hata Model - PCS Extension

Setup by EURO-COST: COST-231 committee

Valid for

1.5-2 GHz PCS systems

base station height, ht = 30 - 200 m

Mobile height, hr = 1 - 10 m

Distance, d = 1 - 20 km

Environment: Urban areas

centresan metropolitfor dB 3

density treemoderate with centres

suburban andcity sized mediumfor dB 0

8.0)log(56.17.0)log(1.1)a(

where

log)log55.69.44(

)(log82.13log9.333.46

r

M

r

Mt

rtcp

C

fhfh

Cdh

hahfL

COST231 - HATA MODEL

COST: Cooperative for Sci and Tech

CCIR

An empirical formula for the combined effects of free-space

path loss and terrain induced path loss was published by

the CCIR (Comite' Consultatif International des Radio-

Communication, now ITU-R) and is given by

)buildingsby covered area of (%log2530

8.0)(log56.1]7.0)(log1.1[)(

where

)(10log)](10log55.69.44[

)()(10log82.13)(log16.2655.69

10

1010

10

B

fhfha

Bdh

hahfMHzL

MHzmMHzm

kmt

rtCCIR

Activity 4: for ht = 8 m, fMHz = 2350, hr = 1 m and 25% area covered by

buildings, calculate the max. distance for path loss model based on CCIR

OTHER MODELS

Walfisch-Ikegami Model

Valid between 800 and 2,000 MHz and over distances of 20 m to 5 km

Useful for dense urban canyon-style environments where antenna height is lower than the average building height Signals are guided along the street, like an urban canyon

The Walfisch-Ikegami Model includes a diffraction constant and the street width

Walfish & Bertoni model:

Consider the impact of rooftops and building height

Considered in IMT-2000 evaluation

WALFISCH-IKEGAMI MODEL

Applicable to large, small and micro-cells where antennas are mounted below roof tops,

Assumes radio path is obstructed by buildings,

Considers generalized diffraction.

b

For NLOS path situations, the WIM gives the path loss using the following parameters:

= base antenna height over street level, in meters (4 to 50m)

= mobile station antenna height in meters (1 to 3m)

= nominal height of building roofs in meters

= height of base antenna above rooftops in meters

= height of mobile antenna below rooftops in meters

= building separation in meters (20 to 50m recommended if no data)

= width of street (b/2 recommended if no data)

= angle of incident wave with respect to street (use 90 if no data)

hbhmhBhb = hb-hBhm = nB-hmb

w

hb w

dBase antenna

hB

Buildings

hm

Street level Mobile antenna

WALFISCH-IKEGAMI LOS MODEL STREET CANYON

Walfisch-Ikegami Street Canyon Model is defined when line-of-sight

exists between the mobile and the Base Station.

LLOS = 42.64 + 26log(d) + 20log(f), for d > 20 m

where:

LLOS = path loss (dB)d = distance (Km)f = frequency (MHz)

Activity 5: calculate the max. distance given LLOS for WIM-

LoS is 142 dB and fMHz = 2350

WALFISCH-IKEGAMI MODEL NLOS

The model is the most complex but it has the ability to represent more environments.

In the absence of data, building height in meters may be estimated by three times the number of floors, plus 3m if the

roof is pitched instead of flat.

The model works best for base antennas well above roof height.

The NLOS path loss equation is best presented in sections due to its complexity

where:

LNLOS = path Loss (dB)

Lfs = free space loss = 32.45 + 20.log(d) + 20.log(f)

d = distance from site (Km)

f = frequency (MHz)

Lrts = roof-top-street diffraction and scatter loss

Lmds = multi-screen diffraction loss

0LL ,L

0LL ,LLLL

mdsrtsfs

mdsrtsmdsrtsfs

NLOS

Lrts = -16.9 10.log(w) + 10.log(f) + 20.log(mobile) + Lstreet, for mobile > 0

Lrts = 0, for mobile 0

where:

Lstreet = -10 + 0.354 for 0 < 35

= 2.5 + 0.075(-35) for 35 < 55

= 4.0 0.114(-55) for 55 90

Walfisch-Ikegami Model NLOS

height of mobile antenna

below rooftops in meters

angle of incident wave

with respect to street (use

90if no data)

WALFISCH-IKEGAMI MODEL

MULTI-SCREEN DIFFRACTION LOSS

Lmds = Lmed + ka + kd.log(d) + kf.log(f) 9.log(b)

where :

Lmed = -18 log(1 + base) for base > 0= 0 for base 0

ka = 54 for base > 0= 54 0.8 base for d 0.5 and base 0= 54 1.6 based for d < 0.5 and base 0

kd = 18 for base > 0= 18 15 base/hroof for base 0

kf = -4 + 0.7 [(f/925)-1] for urban and suburban

= -4 + 1.5 [(f/925)-1] for dense urban

b = building separation in meters(20 to 50m recommended if nodata)

base = height of base antenna above rooftops in meters

Activity 6: calculate d given LNLOS based on WIM-

NLoS =142 dB, fMHz = 2350, ht = 8 m, hr = 1 m

PROPAGATION MODEL COMPARISON

Okumura-Hata Walfisch-Ikegami

Frequency Range 150 MHz to 1 GHz

1.5 to 2 GHz

800 MHz to 2 GHz

BTS Antenna Height 30 to 200 meters

above roof-top

4 to 50 meters

above roof-top

UE Antenna Height 1 to 10 meters 1 to 3 meters

Range 1 to 20 kilometers 30 meters to 6 kilometers

OTHER MODELS

Wideband PCS microcell model

Measurement in the microcells

Results:

Two-ray ground reflection model is good for LOS microcells

Log-distance path loss model is good for OBS (obstructed) microcells

Ibrahim and Parsons model - equations developed to best fit data observed at London. (freq. 168-900 MHz)

Lees model - Use at 900MHZ with 3 parameters (median transmission loss, slope of the path loss curve and adjustment

factor)

Summary

From Activity 1 to 6, observe the difference between the calculated distance based on different propagation models

Identify which PL models over-estimate and under-estimate the calculated distance

Is there any PL model which gives a realistic representation of the considered scenario (given ht = 8 m, hr = 1 m, Pt = 39 dBm, fMHz = 2350, Gtot = 7.5 dBm)

Calculated distance values for common example given ht = 8 m, hr = 1 m, Pt = 39 dBm, fMHz = 2350, Gtot = 7.5

dBm

Path Loss Model Calculated distance

Free space 127,000

WIM LoS 16,200

Hata open 5,300

Hata suburban 1,600

WIM NLoS 820

Hata Small/Large City 740

CCIR 550