2. FADINGFADING
““This is about the phenomenon ofThis is about the phenomenon of
loss of signal in telecommunicationsloss of signal in telecommunications.“.“
3. INDEXINDEX
SMALL SCALE FADINGSMALL SCALE FADING
1.1. Small scale multipath propagationSmall scale multipath propagation
2.2. Multipath fading channelMultipath fading channel
FACTORS INFLUENCING SMALL SCALE FADINGFACTORS INFLUENCING SMALL SCALE FADING
DOPPLER SHIFTDOPPLER SHIFT
TYPES OF SMALL SCALE FADINGTYPES OF SMALL SCALE FADING
1.1. Fading effect due to multipath time delay spreadFading effect due to multipath time delay spread
• Flat FadingFlat Fading
• Frequency Selective FadingFrequency Selective Fading
2. Fading effect due to Doppler Spread2. Fading effect due to Doppler Spread
• Fast FadingFast Fading
• Slow FadingSlow Fading
STATISTICAL MODELS FOR MULTIPATH FADINGSTATISTICAL MODELS FOR MULTIPATH FADING CHANNELSCHANNELS
1.1. Clarke’s modelClarke’s model
2.2. Two ray Rayleigh Fading modelTwo ray Rayleigh Fading model
3.3. Saleh and Valenzuele Indoor statistical modelSaleh and Valenzuele Indoor statistical model
4.4. SIRCIM AND SMRCIM Indoor and Outdoor statistical modelSIRCIM AND SMRCIM Indoor and Outdoor statistical model
4. FADINGFADING
Selective fading causes a cloudy pattern to appear onSelective fading causes a cloudy pattern to appear on
anan FFTFFT display.display.
FadingFading (or(or fading channelsfading channels) refers to mathematical) refers to mathematical
models for the distortion that a carrier-modulatedmodels for the distortion that a carrier-modulated
telecommunication signal experiences over certaintelecommunication signal experiences over certain
propagation media. Short-term fading, also known aspropagation media. Short-term fading, also known as
multipathmultipath induced fadinginduced fading, is due to, is due to multipathmultipath
propagationpropagation. Fading results from the. Fading results from the superpositionsuperposition ofof
transmitted signals that have experienced differences intransmitted signals that have experienced differences in
attenuationattenuation,, delaydelay andand phase shiftphase shift while traveling from thewhile traveling from the
source to the receiver. It may also be caused by attenuationsource to the receiver. It may also be caused by attenuation
of a single signal.of a single signal.
The most common types of fading, known as "slowThe most common types of fading, known as "slow
fading" and "fast fading", as they apply to a mobile radiofading" and "fast fading", as they apply to a mobile radio
environment, are explained below.environment, are explained below.
5. FadingFading refers to the time variation of therefers to the time variation of the
received signal power caused by changes in thereceived signal power caused by changes in the
transmission medium or path.transmission medium or path.
Small scale fading or simply fading is used toSmall scale fading or simply fading is used to
describe the rapid fluctuations of the amplitude,describe the rapid fluctuations of the amplitude,
phases or multipath delays of a radio signal overphases or multipath delays of a radio signal over
a short period of time or travel distance, so thata short period of time or travel distance, so that
large scale path loss effects may be ignored.large scale path loss effects may be ignored.
Fading is caused by interference between two orFading is caused by interference between two or
more versions of the transmitted signal whichmore versions of the transmitted signal which
arrives at the receiver at slightly different times.arrives at the receiver at slightly different times.
These waves, called multipath waves, combine atThese waves, called multipath waves, combine at
the receiver antenna to give a resultant signal,the receiver antenna to give a resultant signal,
which can vary widely in amplitude and phase,which can vary widely in amplitude and phase,
depending on the distribution of the intensity anddepending on the distribution of the intensity and
relative propagation time of the waves and therelative propagation time of the waves and the
bandwidth of the transmitted signal.bandwidth of the transmitted signal.
6. EXAMPLEEXAMPLE
For example, consider the common experience ofFor example, consider the common experience of
stopping at traffic lights and hearing a lot of staticstopping at traffic lights and hearing a lot of static
on your FM broadcast radio, which is immediatelyon your FM broadcast radio, which is immediately
corrected if you move less than a meter. Cellularcorrected if you move less than a meter. Cellular
phones also exhibit similar momentary fades. Thephones also exhibit similar momentary fades. The
reason for these losses of signal is the destructivereason for these losses of signal is the destructive
interference that multiple reflected copies of theinterference that multiple reflected copies of the
signal make with itself. To understand how asignal make with itself. To understand how a
signal can destructively interfere with itself,signal can destructively interfere with itself,
consider the sum of twoconsider the sum of two sinusoidalsinusoidal waveformswaveforms
(which are similar to modulated carrier signals)(which are similar to modulated carrier signals)
with different phases.with different phases.
7. FADING IN WIRELESS COMMUNICATIONSFADING IN WIRELESS COMMUNICATIONS
In wireless communications, signal fading isIn wireless communications, signal fading is
caused by multi-path effect. Multi-path effectcaused by multi-path effect. Multi-path effect
means that a signal transmitted from ameans that a signal transmitted from a
transmitter may have multiple copies traversingtransmitter may have multiple copies traversing
different paths to reach a receiver.different paths to reach a receiver.
At the receiver, the received signal should be theAt the receiver, the received signal should be the
sum of all these multi-path signals. Because thesum of all these multi-path signals. Because the
paths traversed by these signals are different;paths traversed by these signals are different;
some are longer and some are shorter.some are longer and some are shorter.
The one at the direction of light of signal (LOS)The one at the direction of light of signal (LOS)
should be the shortest. These signals interactshould be the shortest. These signals interact
with each other. If signals are in phase, theywith each other. If signals are in phase, they
would intensify the resultant signal; otherwise,would intensify the resultant signal; otherwise,
the resultant signal is weakened due to out ofthe resultant signal is weakened due to out of
phase. This phenomenon is called channel fading.phase. This phenomenon is called channel fading.
8. SMALL SCALE MULTIPATH PROPAGATIONSMALL SCALE MULTIPATH PROPAGATION
Multipath in the radio channel creates smallMultipath in the radio channel creates small
scale fading effects. The three mostscale fading effects. The three most
important effects are :important effects are :
Rapid changes in signal strength over aRapid changes in signal strength over a
small travel distance or time interval.small travel distance or time interval.
Random frequency distribution due toRandom frequency distribution due to
varying Doppler shifts on differentvarying Doppler shifts on different
multipath signals.multipath signals.
Time dispersion (Echoes) caused byTime dispersion (Echoes) caused by
multipath propagation delays.multipath propagation delays.
9. FACTORS INFLUENCING SMALL SCALE FADINGFACTORS INFLUENCING SMALL SCALE FADING
Many physical factors in the radio propagation channelMany physical factors in the radio propagation channel
influence small-scale fading. These include the following :influence small-scale fading. These include the following :
(1)(1) Multipath PropagationMultipath Propagation ::
The presence of reflecting objects and scatters in the channelThe presence of reflecting objects and scatters in the channel
creates a constantly changing environment that dissipates the signalcreates a constantly changing environment that dissipates the signal
energy in amplitude, phase and time. These effects results in multipleenergy in amplitude, phase and time. These effects results in multiple
versions of the transmitted signal that arrive at the receivingversions of the transmitted signal that arrive at the receiving
antenna, displaced with respect to one another in time and spatialantenna, displaced with respect to one another in time and spatial
orientation. The random phase and amplitude of the differentorientation. The random phase and amplitude of the different
multipath components caused fluctuations in signal strength, therebymultipath components caused fluctuations in signal strength, thereby
inducing Small scale fading, Signal distortion, or both. Multipathinducing Small scale fading, Signal distortion, or both. Multipath
propagation often lengthens the time required for the base bandpropagation often lengthens the time required for the base band
portion of the signal to reach the receiver which can cause signalportion of the signal to reach the receiver which can cause signal
smearing due to inter symbol interference.smearing due to inter symbol interference.
(2)(2) Speed of the mobileSpeed of the mobile ::
The relative motion between the base station and the mobileThe relative motion between the base station and the mobile
results in random frequency modulation due to different Dopplerresults in random frequency modulation due to different Doppler
Shifts on each of the multipath components. Doppler Shift will beShifts on each of the multipath components. Doppler Shift will be
positive or negative depending on whether the mobile receiver ispositive or negative depending on whether the mobile receiver is
moving towards or away from the base station.moving towards or away from the base station.
10. (3) Speed of surrounding objects :
If objects in the radio channel are in motion, they induced a time
varying Doppler Shift on multipath components. If the surrounding
objects move at a greater rate than the mobile, then this effect
dominates the Small scale Fading. Otherwise, motion of surrounding
objects may be ignored and only the speed of the mobile need be
considered. The coherence time defines the “static ness” of the
channel, and is directly impacted by the Doppler shift.
(4) The transmission bandwidth of the signal :
If the transmitted radio signal bandwidth is greater than the
“bandwidth” of the multipath channel, the received signal will be
distorted, but the received signal strength will not fade much over a
local area (i.e., the small scale signal fading will not be significant). As
will be shown, the bandwidth of the channel can be quantified by the
coherence bandwidth which is related to the specific multipath structure
of the channel. The coherence bandwidth is a measure of the maximum
frequency difference for which signals are still strongly correlated in
amplitude. If the transmitted signal has a narrow bandwidth as
compared to the channel, the amplitude of the signal change rapidly,
but the signal will not be distorted in time.
12. Consider a mobile moving at aConsider a mobile moving at a
constant velocityconstant velocity vv, along a path, along a path
segment having lengthsegment having length dd betweenbetween
points X and Y, while it receivespoints X and Y, while it receives
signals from a remote source assignals from a remote source as
illustrated in Fig (1) in previous slide :illustrated in Fig (1) in previous slide :
The difference in path length traveledThe difference in path length traveled
by the wave from source S to theby the wave from source S to the
mobile at points X and Y ismobile at points X and Y is
∆∆l = dcosθ = v∆tcosθl = dcosθ = v∆tcosθ
13. The phase change in received signalThe phase change in received signal
due to difference in path lengths isdue to difference in path lengths is
therefore,therefore,
∆∆ Φ =Φ = 2 π ∆2 π ∆ ll == 2 π v ∆2 π v ∆ tt cos θcos θ
λ λλ λ
and hence the apparent change inand hence the apparent change in
frequency or Doppler shift, is givenfrequency or Doppler shift, is given
byby fdfd, where, where
fdfd == 11 .. ∆Φ∆Φ == vv . cosθ. cosθ
2π ∆ t λ2π ∆ t λ
14. TYPES OF SMALL SCALE FADINGTYPES OF SMALL SCALE FADING
Fig. (2) : Types of small scale fading
15. FLAT FADINGFLAT FADING
Flat fadingFlat fading, where the bandwidth of, where the bandwidth of
the signal is less than thethe signal is less than the
coherence bandwidthcoherence bandwidth of the channelof the channel
or theor the delay spreaddelay spread is less than theis less than the
symbol periodsymbol period..
17. FREQUENCY SELECTIVE FADINGFREQUENCY SELECTIVE FADING
Frequency selective fadingFrequency selective fading, where, where
the bandwidth of the signal is greaterthe bandwidth of the signal is greater
than the coherence bandwidth of thethan the coherence bandwidth of the
channel or the delay spread ischannel or the delay spread is
greater than the symbol period.greater than the symbol period.
19. FADING EFFECT DUE TO DOPPLER SPREADFADING EFFECT DUE TO DOPPLER SPREAD
Due to Doppler Effect, if a transmitter isDue to Doppler Effect, if a transmitter is
moving away from a receiver, themoving away from a receiver, the
frequency of the received signal is lowerfrequency of the received signal is lower
than the one sent out from the transmitter;than the one sent out from the transmitter;
otherwise, the frequency is increased.otherwise, the frequency is increased.
In wireless communications, there areIn wireless communications, there are
many factors that can cause relativemany factors that can cause relative
movement between a transmitter and amovement between a transmitter and a
receiver.receiver.
It can be the movement of a mobile suchIt can be the movement of a mobile such
as a cell phone.as a cell phone.
It can be the movement of someIt can be the movement of some
background objectives, which causes thebackground objectives, which causes the
change of path length between thechange of path length between the
transmitter and the receiver.transmitter and the receiver.
20. FASTFAST FADINGFADING
Fast FadingFast Fading is a kind of fading occurring with smallis a kind of fading occurring with small
movements of a mobile or obstacle.movements of a mobile or obstacle.
Depending upon how rapidly the transmitted base bandDepending upon how rapidly the transmitted base band
signal changes as compared to the rate of change of thesignal changes as compared to the rate of change of the
channel.channel.
The channel may be classified either as aThe channel may be classified either as a Flat fadingFlat fading oror
Slow fadingSlow fading channel.channel.
In aIn a Fast fadingFast fading channel, the impulse response changeschannel, the impulse response changes
rapidly within the symbol duration. That is, the coherencerapidly within the symbol duration. That is, the coherence
time of the channel is smaller than the symbol period of thetime of the channel is smaller than the symbol period of the
transmitted signal. This causes frequency dispersion (alsotransmitted signal. This causes frequency dispersion (also
called the selective fading) due to Doppler spreading, whichcalled the selective fading) due to Doppler spreading, which
leads to signal distortion.leads to signal distortion.
21. SLOWSLOW FADINGFADING
Slow FadingSlow Fading is a kind of fading caused by largeris a kind of fading caused by larger
movements of a mobile or obstructions within themovements of a mobile or obstructions within the
propagation environment. This is often modeledpropagation environment. This is often modeled
asas log-normal distributionlog-normal distribution with a standardwith a standard
deviation according to thedeviation according to the
Log Distance Path Loss ModelLog Distance Path Loss Model..
In a slow fading channel, the channel impulseIn a slow fading channel, the channel impulse
response changes at a rate much slower than theresponse changes at a rate much slower than the
transmitted base band signal s(t). In this case,transmitted base band signal s(t). In this case,
channel may be assumed to be static over one orchannel may be assumed to be static over one or
several reciprocal bandwidth intervals.several reciprocal bandwidth intervals.
22. Fig. (5) : Type of fading experienced by a signal as a functionFig. (5) : Type of fading experienced by a signal as a function
(a) Symbol period(a) Symbol period (b) Base band signal bandwidth(b) Base band signal bandwidth
23. STATISTICAL MODELS FOR MULTIPATHSTATISTICAL MODELS FOR MULTIPATH
FADING CHANNELSFADING CHANNELS
Several multipath models have been suggested toSeveral multipath models have been suggested to
explain the observed statistical nature of a mobileexplain the observed statistical nature of a mobile
channel.channel.
The first model presented by ossana was based onThe first model presented by ossana was based on
interference of waves incident and reflected from theinterference of waves incident and reflected from the
flat sides of randomly located buildings.flat sides of randomly located buildings.
Ossana’s model is therefore rather inflexible andOssana’s model is therefore rather inflexible and
inappropriate for urban areas where the direct path isinappropriate for urban areas where the direct path is
almost always blocked by buildings or other obstacles.almost always blocked by buildings or other obstacles.
Clarke’s model is based on scattering and is widelyClarke’s model is based on scattering and is widely
used.used.
24. CLARKE’S MODELCLARKE’S MODEL
Clarke developed a model where the statisticalClarke developed a model where the statistical
characteristics of the electromagnetic fields of thecharacteristics of the electromagnetic fields of the
received signals of the mobile are deduced fromreceived signals of the mobile are deduced from
scattering.scattering.
The model assumes a fixed transmitter with aThe model assumes a fixed transmitter with a
vertically polarized antenna. The field incident on thevertically polarized antenna. The field incident on the
mobile antenna is assumed to be comprised of Nmobile antenna is assumed to be comprised of N
azimuthal plane waves with arbitrary carrier phases,azimuthal plane waves with arbitrary carrier phases,
arbitrary azimuthal angles of arrival, and each wavearbitrary azimuthal angles of arrival, and each wave
having equal average amplitude.having equal average amplitude.
It should be noted that the equal average amplitudeIt should be noted that the equal average amplitude
assumption is based on the fact that in the absence ofassumption is based on the fact that in the absence of
a direct line-of-sight path, the scattered arriving at aa direct line-of-sight path, the scattered arriving at a
receiver will experience similar attenuation overreceiver will experience similar attenuation over
small-scale distances.small-scale distances.
25. TWO-RAY RAYLEIGH FADING MODELTWO-RAY RAYLEIGH FADING MODEL
Clarke’s model and the statistics for RayleighClarke’s model and the statistics for Rayleigh
fading are for that fading conditions and do notfading are for that fading conditions and do not
consider multipath time delay.consider multipath time delay.
In modern mobile communication systems withIn modern mobile communication systems with
high data rates, it has become necessary tohigh data rates, it has become necessary to
model the effects of multipath delay spread asmodel the effects of multipath delay spread as
well as fading.well as fading.
A commonly used multipath model is anA commonly used multipath model is an
independent Rayleigh fading two-ray modelindependent Rayleigh fading two-ray model
(which is a specific implementation of the(which is a specific implementation of the
generic fading simulator shown in figuregeneric fading simulator shown in figure
below).below).
26. The figure shows a block diagram of the two-rayThe figure shows a block diagram of the two-ray
independent Rayleigh fading channel model.independent Rayleigh fading channel model.
INPUTINPUT OUTPUTOUTPUT
27. SALEH AND VALENZUELA INDOOR STATISTICAL MODEL
Saleh and Valenzuela reported the results ofSaleh and Valenzuela reported the results of
indoor propagation measurements between twoindoor propagation measurements between two
vertically polarized omni directional antennasvertically polarized omni directional antennas
located on the same floor of a medium sizedlocated on the same floor of a medium sized
office building. Measurements were made usingoffice building. Measurements were made using
10 ns, 1.5 GHz, radar-like pulses.10 ns, 1.5 GHz, radar-like pulses.
The method involved averaging the square lawThe method involved averaging the square law
detected pulse response while sweeping thedetected pulse response while sweeping the
frequency of the transmitted pulse. Using thisfrequency of the transmitted pulse. Using this
method, multipath components within 5 ns weremethod, multipath components within 5 ns were
resolvable.resolvable.
28. SIRCIM AND SMRCIM INDOOR AND OUTDOORSIRCIM AND SMRCIM INDOOR AND OUTDOOR
STATISTICAL MODELSSTATISTICAL MODELS
Rappaport and Seidel reported measurement at 1300 MHz in fiveRappaport and Seidel reported measurement at 1300 MHz in five
factory buildings and carried out subsequent measurements infactory buildings and carried out subsequent measurements in
other types of buildings. The authors developed an elaborate,other types of buildings. The authors developed an elaborate,
empirically derived statistical model to generate measuredempirically derived statistical model to generate measured
channels based on the discrete impulse response channel modelchannels based on the discrete impulse response channel model
and wrote a computer program calledand wrote a computer program called SIRCIM (simulation ofSIRCIM (simulation of
indoor radio channel impulse-response models)indoor radio channel impulse-response models)..
SIRCIM generates realistic samples of small-scale indoor channelSIRCIM generates realistic samples of small-scale indoor channel
impulse response measurements. Subsequent work by Huangimpulse response measurements. Subsequent work by Huang
producedproduced SMRCIM (simulation of mobile radio channelSMRCIM (simulation of mobile radio channel
impulse response models)impulse response models), a similar program that generates, a similar program that generates
small-scale urban cellular and microcellular channel impulsesmall-scale urban cellular and microcellular channel impulse
responses.responses.
These programs are currently in use at over 100 institutionsThese programs are currently in use at over 100 institutions
throughout the world, and have been updated to include angle ofthroughout the world, and have been updated to include angle of
arrival information for micro cell, indoor, and macro cell channels.arrival information for micro cell, indoor, and macro cell channels.
29. REFERENCESREFERENCES
Wireless CommunicationWireless Communication
(T. S. RAPPAPORT, EEE Pub.)(T. S. RAPPAPORT, EEE Pub.)
Wikipedia, the free encyclopediaWikipedia, the free encyclopedia
Google.co.inGoogle.co.in
