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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
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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
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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,
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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:
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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
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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
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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
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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
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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 :
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.
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
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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
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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
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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
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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
( )
( )
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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
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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
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.
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.
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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
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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
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, 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 TimeCoherence Time
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 .
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mfCT
1
Coherence time (TC) is defined as:TS
TC
t=t2 - t1t1 t2
f1f2
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Coherence TimeCoherence Time
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=
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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
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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.
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.
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.
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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
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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
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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
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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
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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
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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
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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
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Fl t f diFl t f di F t f diF t f di
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Flat fadingFlat fading Fast fadingFast fading
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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
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Frequency selective fadingFrequency selective fading fast fadingfast fading
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Frequency selective fadingFrequency selective fading fast fadingfast fading
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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.
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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.
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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 FadingRayleigh Fading
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Rayleigh FadingRayleigh Fading
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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.
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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( )=