Chapter 4 Mobile Radio Propagations

8/9/2019 Chapter 4 Mobile Radio Propagations

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Chapter FourMobile Radio Propagations

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Objectives of the Chapter

In cellular system, calls are occasionally disconnected

Possible cause: Rapid fluctuation of radio signal amplitudeover short time period or travel distance

Reasons for wireless channels to become selective and dispersive both

in frequency and timeSources of signal fluctuation: multipath propagation and mobility

To understand physical parameters such as carrier frequency,

mobile speed, bandwidth, delay spread impact how a wireless channel

behaves from the communication system point of view.

Techni ues to minimize or modify propagation loss.

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!ecture Outline

Introduction

Parameters of Mobile Radio hannel

lassification of !ading hannel

Impulse Response Model of a "ireless

hannel

!ading hannel #istribution Model

$ummary

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"ntroduction to #ireless Channels

%lectromagnetic waves can transmit through&

' guided medium or

'n unguided medium.

(uided mediums such as coa$ial and fiber optic cables are

far less hostile for the information carrying %M" than the

wireless or unguided medium.

It presents limited challenges and problems which are uniquefor this )ind of transmission.

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"hen %M" travels through the wireless channel, it undergoes

many )inds of propagation effects such as reflection% diffraction

and scattering due to the presence of buildings, mountains and otherobstructions.

Reflection: *ccurs when the %M" impinge on ob+ects which has very

large dimension as compared to the wavelength of the wave.

&iffraction: *ccurs when the wave interacts with a surface having

sharp irregularities.

Scattering: *ccurs when the %M" passes through ob+ects whichhas

very small dimension as compared to the wavelength of the wave


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!arge Scale Path !oss Models: predicts the mean

signal strength for arbitrary transmitter(receiver

distances.

It predicts the average signal strength for large T$(R$

separation, typically in hundreds of )m.

Time constants associated with variations are very long.

seful in estimating the coverage area of antenna

More important for cell site planning

"ncludes !ree space path loss, the two/ray model, ata

model, *)umura model, 0

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The power received by a receiver antenna at a distance d isgiven by the !riis free space equation&

where• Pt& transmitted power • Pr& received power • (t, (r& antenna gain• 1& the system loss factor not related to propagation.

2miscellaneous loss, and 1 34• & wavelength in meters

9

λ

≥


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The gain of an antenna

where 'e& the effective aperture related to the physical sizeof antenna.

The wavelength is related to the carrier frequency

where • f& the carrier frequency in ertz• & the carrier frequency in radians per second.• c& the speed of light in meters5sec

cω


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Small Scale Fading Models & describes the signal strength

variation in close spatial pro$imity to a particular

location

Characteri)e the rapid fluctuation of received signalstrength&

Over very short travel distances *a fe+ +avelengths,or

Over very short time durations *in order of seconds,

The received power may vary by -.(/. d0 when the receiver

is moved by fraction of a wavelength.

This is because the received signal is the sum of

many contributions coming from different directions


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1$ample: $mall scale and large scale fading

$ignal variation in an indoor

radio communication system.

$ignal fades rapidly as the receiver

moves by more than 67 d8m

owever, the average signal decays

much more slowly decays much

more slowly with distance 2smoothed

line4

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Representation of multipath wireless propagation

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Major 1ffects of Fading :

1nvelope Fading: Rapid change in signal strength over

small travel distances or short time periods.

Time &ispersion: Multiple signals arriving a different times

when added together at the antenna, signals are spread out in

time. This can cause a smearing of the signal and

interference between bits that are received.

Fre uency &ispersion : hanges in the frequency of signals.

Random frequency modulation due to varying &oppler shifts

on different multipath signals• This #oppler shift is caused by the mobility of mobile

which cause an apparent shift in frequency

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Physical Factors that influence fading :Multipath Propagation: due to the presence of reflecting ob+ectsand clutters

Multiple version of the signal arrives at the receiver

with different amplitude and time delays.

2rban areas with many buildings distributed over large spatialscale

• 1arge number of strong multipath signals some having

large time delayRural areas few multipath signals 21*$ 9 ground reflection4

Relevant terms: #elay spread and coherence bandwidth


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3' Speed of Mobile & due to the relative motion of the base station,

mobile station, and surrounding environment.

Causes &oppler shift 2:9; or :/;4 at each multipathcomponent

Results in random frequency modulation or apparent shift

in frequency

' receiver moving at high speed can pass through several

fades in small period of time

auses time/varying #oppler shift on the multipath

components

The term coherence time determines how :static; the channel

is and depends on the #oppler shift,


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-' The band+idth of the signal: causes frequency selectivity.

The channel bandwidth can be quantified by the term

coherence band+idth*0 c,

< 8 c measures the ma-imum frequency difference in which

signals are strongly correlated in amplitude

If 8" of the signal is greater than 8 c, the received signal will be

distorted *filtered, in frequency

If the transmitted signal has a narro+ band+idth as comparedto the channel, signal will not be distorted in frequency


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Parameters of Mobile Radio Channel

"ireless propagation are mostly governed by a number of

unpredictable factors .• $o, it is preferred to characterize the wireless channel from

statistical point of vie+ using some fundamental parameters.

ere, we will see these fundamental parameters and their

impacts on wireless communication

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4' &oppler Shift: is the change in fre uency of a wave for

an observer moving relative to the source of the wave.aused by movement of T-, R-, and surrounding

environment

Results multiplicative in time rendering the channel

impulse response linear time variant *!T5,'

!or the mobile shown in figure, phase change in the

received signal due to path difference is

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The apparent change in frequency

This is &oppler spreading , which

increase or decrease the signalfrequency at R-

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ence, when transmitter and5or receiver is moving, the

frequency of the received signal will not be the same as that of

the transmitted signal

"hen they are moving towards each other, the frequency of

the received signal is higher than the source.

"hen they are moving opposite to each other, the received

fre uency decreases .

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3' The Coherence Time* Tc,: #elay spread and coherence bandwidth

are parameters which describe the time dispersive nature of the

wireless channel.8ut, they do not give information about the time varying nature of

the channel caused by relative motion between the mobile and base

station.

#oppler spread and coherence time are parameters which describe the

time varying nature of the channel.

&oppler spread f & is a measure of the spectral e$pansion caused by

the time rate of change of the mobile radio channel.

The range of frequencies over which the received &oppler spectrum

is essentially nonzero.


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Coherence time*T c, is the time domain dual of #opplerspread

It is used to characterize the time varying nature of thefre uency dispersiveness of the channel in time domain.

oherence time is the time duration over which two

received signals have strong amplitude correlation.

The coherence time is related with the #oppler spread by

If symbol period *T s, of the baseband signal is greater than

the coherence time of the channel, then the channel will

change during transmission, thus causing distortion at the24


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-' Time &ispersive ParametersThe wireless channel is fully described by its impulse responsemodel as

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-'4 Po+er &elay Profile*P&P,: is a statistical parameter indicating

how the po+er of a &irac delta function is dispersed in time

domain as a result of multipath propagation.

It is usually given in table where the average power

associated with each multipath component is provided along

with the corresponding delay.

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In particular the average power of the = th path is given by

$umming all quantitie power P R In practice the P#P isnormalized so that thesum o

s provides the total average received

f is unityas

8ased on , we define multipath channel parameters thatare

used to characterize time dispersive channel such as& mean

e$cess delay% RMS delay spread% ma$imum e$cess delay and

coherence 0#'

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-'3' The Mean 1$cess &elay*6 ,: is the first moment of the

power delay profile and is defined as

"here is the average power of the delay profiles inlinear

power units and is the relative delay in seconds.

-'- The RMS &elay Spread* Ϭ 6,: is the square root of the

second central moment of the power delay profile and is given by

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These delays are measured relative to the first detectablesignal

arriving at the receiver at > 7 ?7Typical values of RM$ delay spread are on the order of

microseconds in outdoor mobile radio channels and on

the order of nanoseconds in indoor mobile radio channels'

/' The Mean 1$cess &elay *$ d0, : the time delay in which

Multipath energy falls to @d8 below the ma-imum

6 $(6 . where > 7 is the first arrival signal and > - is the ma-imum

signal point at which the Multipath component is 7d0 of

the strongest arrival signal.


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8' Coherence 0and+idth*0 c,: is a statistical measure of the range

of frequencies over which the channel can be considered 9flat .

!lat channel is a channel which passes all spectral

components with appro-imately e ual gain and linear

phase .

The range of frequencies over which two frequency

components have a strong amplitude correlation.

The coherence bandwidth is defined based on therelation derived from the RMS delay spread .

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If the coherence bandwidth is defined as the bandwidth

over which the frequency correlation function is 7.A

If the coherence bandwidth is defined as the bandwidth

over which the frequency correlation function is 7.B

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4' Fading due to Multipath &elay;' Flat Fading: B s CC Bc or

T s DD(enerally$ignal fits easily within the bandwidth of thechannel

hannel 8" DD signal 8"Most commonly occurring type of fading

T s ≥ 37σ τ

σ τ

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!lat fading is generally considered desirable :

%ven though fading in amplitude occurs, the signal is not distorted.

For+ard lin=: can increase mobile receiver gain 2automatic gain

control4

Reverse lin= & can increase mobile transmitter power 2powercontrol4

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0' Fre uency Selective* Time &ispersive ,Fading:

B s D Bc or T s C σ τ

(enerally T s

8 sD8 c, certain frequency components of the signal are

attenuated much more than others.

≤ 37σ τ

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T s > ? 6 : delayed version of transmitted signal arrive during

different symbol periods

1$ample: receiving a 1*$ :3; E multipath :7; 2from

prior symbol4

This results in "S" that is undesirable

It is very difficult to predict mobile reciever performance

with frequency selective channels

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3' Fading due to &opplerSpreadaused by motion of T- and R- and reflecting sources.

;' Fast Fading:0 s > f & or T s < T c

0 s > f & oppler shifts significantly alter spectral 8"

of transmit signal.

• $ignal spreading

Ts < Tc: wireless channel changes within onesymbol period

• Rapid amplitude fluctuations41


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0' Slo+ Fading: T s >> T c or 0 s


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"mpulse Response Model of #ireless Channel

$mall scale variations of a signal is related to the

impulse response of the mobile radio channel

The impulse response is&

' wideband channel characterization that contains

all information necessary to simulate or analy)e any

type of channel

' wireless channel can be modelled as a linear timevarying

21TJ4 filter

The time variation is due to the motion mobile

"e use discrete/time impulse response model 43


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In Multipath channel, the received signal is the sum of

!ine of sight path component and

;ll resolvable Multipath components

Filtering is caused by the summation of amplitudes

and delays of Multipath signals at any instant of time.

ence the received low pass signal can be described by

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Thus the low pass equivalent impulse response of the

wireless channel is given by the !T5 e uation

In this 1TJ model h2>, t4&

t represents the time variations due to motion6 represents the channel Multipath delay for a fi-ed value oft

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Characteristics of MultipathComponent

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Fre uency Selective Fading : alters the signal waveform

and therefore the detection performance.

Channel e uali)ation is utilized to compensate the effect.

Multi(carrier Systems & transferring a broadband signal

into parallel narrowband streams

Time Selective Fading: smears the signal spectrum and

introduces variation too fast for power control.

Coding% time interleaving and diversity techniques are mosteffective means of coping with time selective fading.

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Chapter 4 Mobile Radio Propagations

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