lecture 3_propagation_bis [compatibility mode]

7/27/2019 Lecture 3_Propagation_bis [Compatibility Mode]

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Lecture 3:Mobile Radio Propagation

ng L Khoa

Class 2


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OutlineOutline

Large-Scale Path Loss

Type of waves

Large scale/small scale fading Free space model

Reflection, Diffraction, Scatter

Small-Scale Fa ing an Multipath Stochastic models: Log-distance path loss model and log-normal

shadowing

Outdoor and Indoor propagation models

Parameters of Mobile Multipath Channels

Types of Fading

Rayleigh and Ricean Distributions

Faculty of Electronics & Telecommunications


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Speed, Wavelength, FrequencySpeed, Wavelength, Frequency

Light speed = Wavelength x Frequency

= 3 x 108 m/s = 300,000 km/s

System Frequency Wavelength

AC current 60 Hz 5,000 km


FM radio 100 MHz 3 m

Cellular 800 MHz 37.5 cm

Ka band satellite 20 GHz 15 mm

Ultraviolet light 1015 Hz 10-7 m


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Radio PropagationRadio Propagation



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LargeLarge--scale smallscale small--scale propagationscale propagation



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Propagation ModelsPropagation Models

Large scale models predict behavior averaged over distances >> Function of distance & significant environmental features, roughly

frequency independent

Breaks down as distance decreases

Useful for modeling the range of a radio system and rough capacityplanning,


xper men a ra er an e eore ca

Path loss models, Outdoor models, Indoor models

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

considered constant Frequency and bandwidth dependent

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


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Free Space Path LossFree Space Path Loss

Path Loss is a measure of attenuation based only on the distanceto the transmitter

Free space model only valid in far-field; Path loss models typically define a close-in point d0 andreference other points from there:


Log-distance generalizes path loss to account for otherenvironmental factors

Choose a d0 in the far field.

Measure PL(d0) or calculate Free Space Path Loss.

Take measurements and deriveempirically.

00 )()(

=dddPdP rr

dB

dBrdddPLdPdPL

+==

0

0 2)()]([)(

dBdddPLdPL

+=

0

0 )()(


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Typical largeTypical large--scale path lossscale path loss



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Okumura ModelOkumura 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 30m to 100m



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Okumura ModelOkumura 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 path lossvalues are around 10 to 14 dB.

G(hre) teh


200tete

m33

log10)(

= re

rere h

hhG

m3m103

log20)( >>

= re

rere h

hhG


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Okumura and Hatas modelOkumura and Hatas model

Example 4.10



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Hata ModelHata 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 area

Suburban area

O en area

EdBALdB += logCdBALdB += log

DdBALdB += log


bhfA 82.13log16.2655.69 +=

bhB log55.69.44 =

94.40log33.18)28/log(78.4 2 ++= ffD

4.5))28/(log(2 2 += fC

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

=

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


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PCS Extension of Hata ModelPCS Extension of Hata Model

COST-231 Hata Model, European standard

Higher frequencies: up to 2GHz

Smaller cell sizes

Lower antenna heights


dB

bhfF log82.13log9.333.46 += f >1500MHz

0

3=G

Metropolitan centers

Medium sized city and suburban areas


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Partition losses between floorsPartition losses between floors



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SmallSmall--Scale FadingScale Fading

Rapid fluctuations of radio signal amplitude,phase, or delays

Occurs or short time periodor short travel distance

Large-scale path loss effects can be ignored

Caused by arrival of two or more waves from the source

combining at the receiver


Resultant detected signal varies widely in amplitudes and phase

Bandwidth of transmitted signal is important factor


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Determining the impulse response of a channelDetermining the impulse response of a channel

Transmit a narrowband pulse into the channel

Measure replicas of the pulse that traverse different paths

between transmitter and receiver



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SmallSmall--scale Multipath Propagationscale Multipath Propagation

Fading: The rapid fluctuation of the amplitude of a radio signalover a short period of time or travel distance.

Fading is caused by interference between two or more versions of

the transmitted signal, which arrive at slightly different times. Multipath in the radio channel creates small-scale fading effects.

Phenomenon :


.

or time interval.

2. Random frequency modulation due to varying Doppler shiftson different multipath signals.

3. Time dispersion caused by multipath propagation delays.

If objects in the radio channel are static, and motion is consideredto be only due to that of the mobile, then fading is purely a spatialphenomenon.

Antenna space diversity can prevent deep fading nulls.


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Factors influencing SmallFactors influencing Small--scale fadingscale fading

Multipath propagation: multipath propagation often lengthens thetime required for the baseband portion of the signal to reach thereceiver which can cause signal smearing due to inter-symbolinterference.

Draw a figure to explain ISI

Speed of the mobile: generate random Doppler shifts.

Train passing


Speed of surrounding objects: if the surrounding objects move at agreater rate than the mobile, then this effect dominates the small-scale fading.

The transmission bandwidth of the signal: if signals bandwidth >bandwidth of the multipath channel received signal will bedistorted. The coherent bandwidth is a measure of the maximum

frequency difference for which signals are still stronglycorrelated in amplitude.


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Comparison of the BER for a fadingComparison of the BER for a fadingand nonand non--fading channelfading channel



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Illustration of Doppler effectIllustration of Doppler effect



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Doppler ShiftDoppler Shift

Distance difference

Phase difference

Doppler frequency shift

Frequency shift is positive when mobile moves toward

source


In a multipath environment, frequency shift for each ray maybe different, leading to a spread of received frequencies.

For example, for pure sinusoid, the signal blurred in

frequency. Example 5.1


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Parameters of Mobile Multipath ChannelsParameters of Mobile Multipath Channels

Time Dispersion Parameters

Grossly quantifies the multipath channel

Determined from Power Delay Profile (average over different

time, a function of delay)

Parameters include

Mean Access Dela


RMS Delay Spread Excess Delay Spread (X dB)

Coherence Bandwidth

Doppler Spread and Coherence Time


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Power Delay ProfilesPower Delay Profiles

Power delay profiles are generally represented as plots of relative received power as a function of excess delay withrespect to a fixed time delay reference.

Power delay profiles are found by averaging instantaneouspower delay profile measurements over a local area.

Are measured by channel sounding techniques


Plots of relative received power as a function of excess delay They are found by averaging intantenous power delay

measurements over a local area

Local area: no greater than 6m outdoor

Local area: no greater than 2m indoor Samples taken at /4 meters approximately For 450MHz 6 GHz frequency range.


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Impulse Response Model of a Multipath ChannelImpulse Response Model of a Multipath Channel



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Time Dispersion ParametersTime Dispersion Parameters The mean excess delay, rms delay spread, and excess delay spread (X dB)

are multipath channel parameters that can be determined form a power delay

profile.

The mean excess delay is the first moment of the power delay profile and is

defined as

= =

a

a

P

P

k kk

k

k kk

k

2

2

( )

( )


The rms delay spread is the square root of the second central moment of thepower delay profile, where

Typical values of rms delay spread are on the order of microseconds inoutdoor mobile radio channel and on the order of nanoseconds in indoor

radio channel

Example 5.4

k

2

2 2

2

2

= =

a

a

P

P

k kk

k

k

k kk

k

k

( )

( )

22)( =


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Maximum Excess Delay (X dB)Maximum Excess Delay (X dB)

Maximum Excess Delay (X dB): Defined as the time delay valueafter which the multipath energy falls to X dB below the maximummultipath energy (not necesarily belongingto the first arrivingcomponent). It is also called excess delay spread.

The maximum excess delay is defined as (x - 0), where 0 is the firstarriving signal and x is the maximum delay at which a multipathcomponent is within X dB of the strongest arriving multipath signal.The value ofx is sometimes called the excess delay spread of a


In practice, values depend on the choice of noise threshold used toprocess P(). The noise threshold is used to differentiate betweenmultipath components and thermal noise.

Noise Thresholds

The values of time dispersion parameters also depend on the noisethreshold (the level of power below which the signal is considered asnoise).

If noise threshold is set too low, then the noise will be processed asmultipath and thus causing the parameters to be higher.


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RMS Delay SpreadRMS Delay Spread



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Example (Power delay profile)Example (Power delay profile)

-30 dB

-20 dB

-10 dB

0 dB

Pr()

1.37 s

4.38 s

=

+++

+++= s38.4

]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1(_

0 1 2 5 (s)

=

+++

+++= 2

2222_

2 07.21]11.01.001.0[

)0)(01.0()2)(1.0()1)(1.0()5)(1( s

== s

37.1)38.4(07.21 2


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Effect of delay spreadEffect of delay spread



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Coherent bandwidthCoherent bandwidth

Analogous to the delay spread parameters in the time domain,

coherence bandwidth is used to characterize the channel in the

frequency domain.

Coherence bandwidth is a statistical measure of the range offrequencies over which the channel can be considered flat.

Two sinusoids with frequency separation greater than Bc are affected


.

Receiver

f1

f2

Multipath Channel Frequency Separation: |f1-f2|


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Coherence BandwidthCoherence Bandwidth

Frequency correlation between two sinusoids: 0


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ExampleExample

For a multipath channel, is given as 1.37s.

The 50% coherence bandwidth is given as: 1/5 = 146kHz.

This means that, for a good transmission from a transmitter to areceiver, the range of transmission frequency (channel bandwidth)

should not exceed 146kHz, so that all frequencies in this band

experience the same channel characteristics.


Equalizers are needed in order to use transmission frequencies thatare separated larger than this value.

This coherence bandwidth is enough for an AMPS channel

(30kHz band needed for a channel), but is not enough for a GSM

channel (200kHz needed per channel).


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Coherence TimeCoherence Time

Delay spread and Coherence bandwidth describe the time

dispersive nature of the channel in a local area.

They dont offer information about the time varying nature

of the channel caused by relative motion of transmitter andreceiver.

Doppler Spread and Coherence time are parameters which


escr e t e t me vary ng nature o t e c anne n a sma -sca e

region.


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Doppler SpreadDoppler Spread

Measure of spectral broadening caused by motion, the time rate

of change of the mobile radio channel, and is defined as the

range of frequencies over which the received Doppler spectrum

is essentially non-zero.

We know how to compute Doppler shift: fd


, D,

fm = v/

If the baseband signal bandwidth is much less than BD then

effect of Doppler spread is negligible at the receiver.


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Coherence time is the time duration over which the channel

impulse response is essentially invariant.

If the symbol period of the baseband signal (reciprocal of the

baseband signal bandwidth) is greater the coherence time, than

the signal will distort, since channel will change during the

transmission of the signal .


mfCT

1

Coherence time (TC) is defined as:TS

TC

t=t2 - t1t1 t2

f1f2


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Coherence time is also defined as:

Coherence time definition implies that two signals arriving with

a time separation greater than TC are affected differently by the

channel.

Coherence time Tc is the time domain dual of Doppler spread

mfC f

Tm

423.0216

9=


frequency dispersive-ness of the channel in the time domain.

If the coherence time is defined as the time over which the time

correlation function is above 0.5, then the coherence time is

approximately, where Example 5.6

T fc m

9

16 f

v

m=


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Types of SmallTypes of Small--scale Fadingscale Fading

Small-scale Fading(Based on Multipath Tme Delay Spread)

Flat Fading

1. BW Signal < BW of Channel2. Delay Spread < Symbol Period

Frequency Selective Fading

1. BW Signal > Bw of Channel2. Delay Spread > Symbol Period


Small-scale Fading(Based on Doppler Spread)

Fast Fading

1. High Doppler Spread2. Coherence Time < Symbol Period3. Channel variations faster than baseband

signal variations

Slow Fading

1. Low Doppler Spread2. Coherence Time > Symbol Period3. Channel variations smaller than baseband

signal variations


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Flat FadingFlat Fading

Occurs when symbol period of the transmitted signal is much larger than theDelay Spread of the channel

Bandwidth of the applied signal is narrow.

If Bs > Flat fading

May cause deep fades.

require 20 or 30 dB more power to achieve low BER during times ofdeep fades.


.

The spectral characteristics of the transmitted signals are preserved at thereceiver, however the strength of the received signal changes with time.

Flat fading channels are known as amplitude varying channels or narrow-band channels.

Radio channel has a constant gain and linear phase response over abandwidth which is greater than the bandwidth of the transmitted signal.

It is the most common type of fading described in the technical literature.


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Flat FadingFlat Fading

h(t,)s(t) r(t)

0 TS 0 0 TS+

BC: Coherence bandwidthBS: Signal bandwidthTS: Symbol period: Delay Spread


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Frequency Selective FadingFrequency Selective Fading

Occurs when channel multipath delay spread is greater than the symbolperiod. Symbols face time dispersion

Channel induces Intersymbol Interference (ISI)

Bandwidth of the signal s(t) is wider than the channel impulse response.

Radio channel has a constant gain and linear phase response over abandwidth which is smaller than the bandwidth of the transmitted signal.


symbols within the channel. Thus the channel induces inter-symbol-interference.

Statistical impulse response model and computer generated impulseresponses are used for analyzing frequency selective small-scale fading.

Frequency selective fading channels are known as wideband channels since

the BW of the signal is wider than the BW of the channel impulse response. As time varies, the channel varies in gain and amplitude across the spectrum

of s(t), resulting in time varying distortion in the received signal r(t).

If Bs > Bc , and 0.1Ts < Frequency selective fading


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Frequency Selective FadingFrequency Selective Fading

h(t,)s(t) r(t)

0 TS 0 0 TS+

>> TS

TS


Causes distortion of the received baseband signal

Causes Inter-Symbol Interference (ISI)

Occurs when:

BS > BCand

TS <

As a rule of thumb: TS <


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Fast FadingFast Fading

Due to Doppler Spread

Rate of change of the channel characteristics is larger than theRate of change of the transmitted signal

The channel changes during a symbol period.

The channel changes because of receiver motion. Coherence time of the channel is smaller than

the symbol period of the transmitter signal

It causes fre uenc dis ersion due to Do ler s read and leads to

Occurs when:BS < BD

andTS > TC

BS: Bandwidth ofthe signalBD: DopplerSpread

TS: SymbolPeriodTC: CoherenceBandwidth


distortion.

Note that, when a channel is specified as a fast or slow fading channel, itdoes not specify whether the channel is flat or frequency selective

A flat, fast fading channel the amplitude of the delta functionvaries faster than the rate of change of the transmittedbaseband signal.

A frequency selective, fast fading channel the amplitudes,phases, and time delays of any one of the multipathcomponents varies faster than the rate of change of thetransmitted baseband signal.


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Slow FadingSlow Fading

Due to Doppler Spread

Rate of change of the channel characteristics is much smaller

than the rate of change of the transmitted signal


Occurs when:BS >> BD

andTS


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Different Types of FadingDifferent Types of Fading

With Respect To SYMBOL PERIOD

TS

Flat SlowFading

Flat FastFading


Transmitted Symbol Period

Symbol Period ofTransmitting Signal

TS

TC

Frequency SelectiveSlow Fading

Frequency SelectiveFast Fading


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Different Types of FadingDifferent Types of Fading

With Respect To BASEBAND SIGNAL BANDWIDTH

Frequency SelectiveSlow Fading

Frequency SelectiveFast Fading

BS

Transmitted


Transmitted Baseband Signal Bandwidth

BSBD

Flat FastFading

Signal Bandwidth

Flat SlowFading

C


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Types of SmallTypes of Small--scale Fadingscale Fading

Small-scale Fading(Based on Multipath Tme Delay Spread)

Flat Fading

1. BW Signal < BW of Channel2. Delay Spread < Symbol Period

Frequency Selective Fading

1. BW Signal > Bw of Channel2. Delay Spread > Symbol Period


Small-scale Fading(Based on Doppler Spread)

Fast Fading

1. High Doppler Spread2. Coherence Time < Symbol Period3. Channel variations faster than baseband

signal variations

Slow Fading

1. Low Doppler Spread2. Coherence Time > Symbol Period3. Channel variations smaller than baseband

signal variations

Fl t f diFl t f di Sl f diSl f di


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Flat fadingFlat fading -- Slow fadingSlow fading


Fl t f diFl t f di F t f diF t f di


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Flat fadingFlat fading Fast fadingFast fading


F l ti f diF l ti f di Sl f diSl f di


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Frequency selective fadingFrequency selective fading Slow fadingSlow fading


Frequency selective fadingFrequency selective fading fast fadingfast fading


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Frequency selective fadingFrequency selective fading fast fadingfast fading


Fading DistributionsFading Distributions


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Fading DistributionsFading Distributions

Describes how the received signal amplitude changes with time.

Remember that the received signal is combination of multiple signals

arriving from different directions, phases and amplitudes.

With the received signal we mean the baseband signal, namely theenvelope of the received signal (i.e. r(t)).

It is a statistical characterization of the multipath fading.


Two distributions

Rayleigh Fading

Ricean Fading

Rayleigh DistributionsRayleigh Distributions


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Rayleigh DistributionsRayleigh Distributions

Describes the received signal envelope distribution for channels, where allthe components are non-LOS:

i.e. there is no line-ofsight (LOS) component.


Ricean DistributionsRicean Distributions


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Ricean DistributionsRicean Distributions

Describes the received signal envelope distribution for channels where oneof the multipath components is LOS component.

i.e. there is one LOS component.

Rayleigh FadingRayleigh Fading


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Rayleigh Fading DistributionRayleigh Fading Distribution


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Rayleigh Fading DistributionRayleigh Fading Distribution

The Rayleigh distribution is commonly used to describe the

statistical time varying nature of the received envelope of a flat

fading signal, or the envelope of an individual multipath

component. The envelope of the sum of two quadrature Gaussian noise

signals obeys a Rayleigh distribution.


is the rms value of the received voltage before envelope

detection, and 2

is the time-average power of the receivedsignal before envelope detection.

p r r r r

r

( ) exp( )=

lecture 3_propagation_bis [compatibility mode]

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