Presented By :Presented By : Er. Rajesh Kumar Er. Rajesh Kumar

INDIAINDIA

FADINGFADING

““This is about the phenomenon of This is about the phenomenon of

loss of signal in telecommunicationsloss of signal in telecommunications.“ .“

INDEXINDEXSMALL SCALE FADINGSMALL SCALE FADING

1.1. Small scale multipath propagationSmall scale multipath propagation2.2. 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 FADING1.1. 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 CHANNELSCHANNELS1.1. Clarke’s modelClarke’s model2.2. Two ray Rayleigh Fading modelTwo ray Rayleigh Fading model3.3. Saleh and Valenzuele Indoor statistical modelSaleh and Valenzuele Indoor statistical model4.4. SIRCIM AND SMRCIM Indoor and Outdoor statistical modelSIRCIM AND SMRCIM Indoor and Outdoor statistical model

FADINGFADINGSelective fading causes a cloudy pattern to appear on Selective fading causes a cloudy pattern to appear on

an an FFT display. display.

FadingFading (or (or fading channelsfading channels) refers to mathematical ) refers to mathematical models for the distortion that a carrier-modulated models for the distortion that a carrier-modulated telecommunication signal experiences over certain telecommunication signal experiences over certain propagation media. Short-term fading, also known as propagation media. Short-term fading, also known as multipath induced fading induced fading, is due to , is due to multipath propagation propagation. . Fading results from the Fading results from the superposition of transmitted signals of transmitted signals that have experienced differences in that have experienced differences in attenuation, , delay and and phase shift while traveling from the source to the receiver. It while traveling from the source to the receiver. It may also be caused by attenuation of a single signal.may also be caused by attenuation of a single signal.

The most common types of fading, known as "slow The most common types of fading, known as "slow fading" and "fast fading", as they apply to a mobile radio fading" and "fast fading", as they apply to a mobile radio environment, are explained below.environment, are explained below.

FadingFading refers to the time variation of the refers to the time variation of the received signal power caused by changes in the received signal power caused by changes in the transmission medium or path.transmission medium or path.

Small scale fading or simply fading is used to Small 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 over phases or multipath delays of a radio signal over a short period of time or travel distance, so that a 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 or Fading is caused by interference between two or more versions of the transmitted signal which more 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 at These 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 and depending on the distribution of the intensity and relative propagation time of the waves and the relative propagation time of the waves and the bandwidth of the transmitted signal. bandwidth of the transmitted signal.

EXAMPLEEXAMPLE

For example, consider the common experience of For example, consider the common experience of stopping at traffic lights and hearing a lot of static stopping at traffic lights and hearing a lot of static on your FM broadcast radio, which is immediately on your FM broadcast radio, which is immediately corrected if you move less than a meter. Cellular corrected if you move less than a meter. Cellular phones also exhibit similar momentary fades. The phones also exhibit similar momentary fades. The reason for these losses of signal is the destructive reason for these losses of signal is the destructive interference that multiple reflected copies of the interference that multiple reflected copies of the signal make with itself. To understand how a signal make with itself. To understand how a signal can destructively interfere with itself, signal can destructively interfere with itself, consider the sum of two consider the sum of two sinusoidalsinusoidal waveforms waveforms (which are similar to modulated carrier signals) (which are similar to modulated carrier signals) with different phases. with different phases.

FADING IN WIRELESS FADING IN WIRELESS COMMUNICATIONSCOMMUNICATIONS

In wireless communications, signal fading is In wireless communications, signal fading is caused by multi-path effect. Multi-path effect caused by multi-path effect. Multi-path effect means that a signal transmitted from a means that a signal transmitted from a transmitter may have multiple copies traversing transmitter 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 the At the receiver, the received signal should be the sum of all these multi-path signals. Because the sum 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 interact should be the shortest. These signals interact with each other. If signals are in phase, they with 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 of the resultant signal is weakened due to out of phase. This phenomenon is called channel fading. phase. This phenomenon is called channel fading.

SMALL SCALE MULTIPATH SMALL SCALE MULTIPATH PROPAGATIONPROPAGATION

Multipath in the radio channel creates small Multipath in the radio channel creates small scale fading effects. The three most scale fading effects. The three most important effects are :important effects are :

Rapid changes in signal strength over a Rapid changes in signal strength over a small travel distance or time interval.small travel distance or time interval.

Random frequency distribution due to Random frequency distribution due to varying Doppler shifts on different varying Doppler shifts on different multipath signals.multipath signals.

Time dispersion (Echoes) caused by Time dispersion (Echoes) caused by multipath propagation delays.multipath propagation delays.

FACTORS INFLUENCING SMALL SCALE FACTORS INFLUENCING SMALL SCALE FADINGFADING

Many physical factors in the radio propagation channel Many 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 channel The presence of reflecting objects and scatters in the channel

creates a constantly changing environment that dissipates the signal creates a constantly changing environment that dissipates the signal energy in amplitude, phase and time. These effects results in multiple energy in amplitude, phase and time. These effects results in multiple versions of the transmitted signal that arrive at the receiving antenna, versions of the transmitted signal that arrive at the receiving antenna, displaced with respect to one another in time and spatial orientation. displaced with respect to one another in time and spatial orientation. The random phase and amplitude of the different multipath The random phase and amplitude of the different multipath components caused fluctuations in signal strength, thereby inducing components caused fluctuations in signal strength, thereby inducing Small scale fading, Signal distortion, or both. Multipath propagation Small scale fading, Signal distortion, or both. Multipath propagation often lengthens the time required for the base band portion of the often lengthens the time required for the base band portion of the signal to reach the receiver which can cause signal smearing due to signal to reach the receiver which can cause signal smearing due to inter symbol interference. inter symbol interference.

(2) (2) Speed of the mobileSpeed of the mobile : : The relative motion between the base station and the mobile The relative motion between the base station and the mobile

results in random frequency modulation due to different Doppler results in random frequency modulation due to different Doppler Shifts on each of the multipath components. Doppler Shift will be Shifts on each of the multipath components. Doppler Shift will be positive or negative depending on whether the mobile receiver is positive 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.

(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.

DOPPLER SHIFTDOPPLER SHIFT

Consider a mobile moving at a Consider a mobile moving at a constant velocity constant velocity vv, along a path , along a path segment having length segment having length dd between between points X and Y, while it receives points X and Y, while it receives signals from a remote source as signals from a remote source as illustrated in Fig (1) in previous slide :illustrated in Fig (1) in previous slide :

The difference in path length traveled The difference in path length traveled by the wave from source S to the by the wave from source S to the mobile at points X and Y is mobile at points X and Y is

∆∆l = dcosθ = v∆tcosθ l = dcosθ = v∆tcosθ

The phase change in received signal The phase change in received signal due to difference in path lengths is due to difference in path lengths is therefore,therefore,

∆ ∆ Φ = Φ = 2 π ∆ 2 π ∆ l l = = 2 π v ∆ 2 π v ∆ tt cos θ cos θ λ λ λ λ

and hence the apparent change in and hence the apparent change in frequency or Doppler shift, is given frequency or Doppler shift, is given by by fdfd, where, where

fd fd = = 1 1 . . ∆Φ∆Φ = = v v . cosθ. cosθ 2π ∆ t λ2π ∆ t λ

TYPES OF SMALL SCALE TYPES OF SMALL SCALE FADINGFADING

Fig. (2) : Types of small scale fading

FLAT FADINGFLAT FADING

Flat fadingFlat fading, where the bandwidth of , where the bandwidth of the signal is less than the the signal is less than the coherence bandwidthcoherence bandwidth of the channel of the channel or the or the delay spreaddelay spread is less than the is less than the symbol periodsymbol period. .

Fig. (3): Fig. (3): Flat Fading channel characteristicsFlat Fading channel characteristics

FREQUENCY SELECTIVE FREQUENCY SELECTIVE FADINGFADING

Frequency selective fadingFrequency selective fading, where , where the bandwidth of the signal is greater the bandwidth of the signal is greater than the coherence bandwidth of the than the coherence bandwidth of the channel or the delay spread is channel or the delay spread is greater than the symbol period. greater than the symbol period.

Fig. (4) : Fig. (4) : Frequency selective fading channel characteristicsFrequency selective fading channel characteristics

FADING EFFECT DUE TO DOPPLER FADING EFFECT DUE TO DOPPLER SPREADSPREAD

Due to Doppler Effect, if a transmitter is Due to Doppler Effect, if a transmitter is moving away from a receiver, the moving away from a receiver, the frequency of the received signal is lower frequency 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 are In wireless communications, there are many factors that can cause relative many factors that can cause relative movement between a transmitter and a movement between a transmitter and a receiver. receiver.

It can be the movement of a mobile such as It can be the movement of a mobile such as a cell phone. a cell phone.

It can be the movement of some It can be the movement of some background objectives, which causes the background objectives, which causes the change of path length between the change of path length between the transmitter and the receiver. transmitter and the receiver.

FASTFAST FADINGFADING

Fast FadingFast Fading is a kind of fading occurring with small is 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 band Depending upon how rapidly the transmitted base band signal changes as compared to the rate of change of the signal changes as compared to the rate of change of the channel.channel.

The channel may be classified either as a The channel may be classified either as a Flat fadingFlat fading or or Slow fadingSlow fading channel. channel.

In a In a Fast fadingFast fading channel, the impulse response changes channel, the impulse response changes rapidly within the symbol duration. That is, the coherence rapidly within the symbol duration. That is, the coherence time of the channel is smaller than the symbol period of the time of the channel is smaller than the symbol period of the transmitted signal. This causes frequency dispersion (also transmitted signal. This causes frequency dispersion (also called the selective fading) due to Doppler spreading, which called the selective fading) due to Doppler spreading, which leads to signal distortion. leads to signal distortion.

SLOWSLOW FADINGFADING

Slow Fading Slow Fading is a kind of fading caused by larger is a kind of fading caused by larger movements of a mobile or obstructions within the movements of a mobile or obstructions within the propagation environment. This is often modeled propagation environment. This is often modeled as as log-normal distributionlog-normal distribution with a standard with a standard deviation according to the deviation according to the Log Distance Path Loss ModelLog Distance Path Loss Model..

In a slow fading channel, the channel impulse In a slow fading channel, the channel impulse response changes at a rate much slower than the response 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 or channel may be assumed to be static over one or several reciprocal bandwidth intervals. several reciprocal bandwidth intervals.

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

STATISTICAL MODELS FOR MULTIPATH STATISTICAL MODELS FOR MULTIPATH FADING CHANNELSFADING CHANNELS

Several multipath models have been suggested to Several multipath models have been suggested to explain the observed statistical nature of a mobile explain the observed statistical nature of a mobile channel. channel.

The first model presented by ossana was based on The first model presented by ossana was based on interference of waves incident and reflected from the interference 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 and Ossana’s model is therefore rather inflexible and inappropriate for urban areas where the direct path is inappropriate 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 widely Clarke’s model is based on scattering and is widely used.used.

CLARKE’S MODELCLARKE’S MODEL Clarke developed a model where the statistical Clarke developed a model where the statistical

characteristics of the electromagnetic fields of the characteristics of the electromagnetic fields of the received signals of the mobile are deduced from received signals of the mobile are deduced from scattering. scattering.

The model assumes a fixed transmitter with a The model assumes a fixed transmitter with a vertically polarized antenna. The field incident on the vertically polarized antenna. The field incident on the mobile antenna is assumed to be comprised of N mobile 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 wave arbitrary azimuthal angles of arrival, and each wave having equal average amplitude. having equal average amplitude.

It should be noted that the equal average amplitude It should be noted that the equal average amplitude assumption is based on the fact that in the absence of assumption is based on the fact that in the absence of a direct line-of-sight path, the scattered arriving at a a direct line-of-sight path, the scattered arriving at a receiver will experience similar attenuation over small-receiver will experience similar attenuation over small-scale distances. scale distances.

TWO-RAY RAYLEIGH FADING TWO-RAY RAYLEIGH FADING MODELMODEL

Clarke’s model and the statistics for Rayleigh Clarke’s model and the statistics for Rayleigh fading are for that fading conditions and do not fading are for that fading conditions and do not consider multipath time delay. consider multipath time delay.

In modern mobile communication systems with In modern mobile communication systems with high data rates, it has become necessary to high data rates, it has become necessary to model the effects of multipath delay spread as model the effects of multipath delay spread as well as fading. well as fading.

A commonly used multipath model is an A commonly used multipath model is an independent Rayleigh fading two-ray model independent Rayleigh fading two-ray model (which is a specific implementation of the (which is a specific implementation of the generic fading simulator shown in figure below). generic fading simulator shown in figure below).

The figure shows a block diagram of the two-ray The figure shows a block diagram of the two-ray

independent Rayleigh fading channel model.independent Rayleigh fading channel model.

INPUTINPUT OUTPUTOUTPUT

SALEH AND VALENZUELA INDOOR STATISTICAL MODEL

Saleh and Valenzuela reported the results of Saleh and Valenzuela reported the results of indoor propagation measurements between two indoor propagation measurements between two vertically polarized omni directional antennas vertically polarized omni directional antennas located on the same floor of a medium sized located on the same floor of a medium sized office building. Measurements were made using office 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 law The method involved averaging the square law detected pulse response while sweeping the detected pulse response while sweeping the frequency of the transmitted pulse. Using this frequency of the transmitted pulse. Using this method, multipath components within 5 ns were method, multipath components within 5 ns were resolvable.resolvable.

SIRCIM AND SMRCIM INDOOR AND SIRCIM AND SMRCIM INDOOR AND OUTDOOR STATISTICAL MODELSOUTDOOR STATISTICAL MODELS

Rappaport and Seidel reported measurement at 1300 MHz in five Rappaport and Seidel reported measurement at 1300 MHz in five factory buildings and carried out subsequent measurements in factory 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 measured empirically derived statistical model to generate measured channels based on the discrete impulse response channel model channels based on the discrete impulse response channel model and wrote a computer program called and wrote a computer program called SIRCIM (simulation of SIRCIM (simulation of indoor radio channel impulse-response models)indoor radio channel impulse-response models). .

SIRCIM generates realistic samples of small-scale indoor channel SIRCIM generates realistic samples of small-scale indoor channel impulse response measurements. Subsequent work by Huang impulse response measurements. Subsequent work by Huang produced produced SMRCIM (simulation of mobile radio channel SMRCIM (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 impulse small-scale urban cellular and microcellular channel impulse responses. responses.

These programs are currently in use at over 100 institutions These programs are currently in use at over 100 institutions throughout the world, and have been updated to include angle of throughout 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.

REFERENCESREFERENCES

Wireless Communication Wireless Communication

(T. S. RAPPAPORT, EEE Pub.)(T. S. RAPPAPORT, EEE Pub.)

Wikipedia, the free encyclopediaWikipedia, the free encyclopedia

Google.co.inGoogle.co.in

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