fadingseminar-100713035600-phpapp02

7/22/2019 fadingseminar-100713035600-phpapp02

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Presented By :

Er. Rajesh KumarINDIA


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FADING

This is about the phenomenon ofloss of signal in telecommunications.


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INDEX

SMALL SCALE FADING

1. Small scale multipath propagation2. Multipath fading channel

FACTORS INFLUENCING SMALL SCALE FADING

DOPPLER SHIFT

TYPES OF SMALL SCALE FADING1. Fading effect due to multipath time delay spread Flat Fading Frequency Selective Fading

2. Fading effect due to Doppler Spread Fast Fading Slow Fading

STATISTICAL MODELS FOR MULTIPATH FADINGCHANNELS1. Clarkes model2. Two ray Rayleigh Fading model3. Saleh and Valenzuele Indoor statistical model4. SIRCIM AND SMRCIM Indoor and Outdoor statistical model


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FADING

Selective fading causes a cloudy pattern to appear on an FFTdisplay.

Fading(or fading channels) refers to mathematical modelsfor the distortion that a carrier-modulated telecommunicationsignal experiences overcertain propagation media. Short-termfading, also known as multipath induced fading, is due tomultipath propagation. Fading results from thesuperposition of transmitted signals that have experienceddifferences in attenuation, delay and phase shift whiletraveling from the source to the receiver. It may also becaused by attenuation of a single signal.

The most common types of fading, known as "slow fading"and "fast fading", as they apply to a mobile radioenvironment, are explained below.
http://en.wikipedia.org/wiki/FFThttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Superpositionhttp://en.wikipedia.org/wiki/Superpositionhttp://en.wikipedia.org/wiki/Superpositionhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Delayhttp://en.wikipedia.org/wiki/Phase_shifthttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Delayhttp://en.wikipedia.org/wiki/Phase_shifthttp://en.wikipedia.org/wiki/Phase_shifthttp://en.wikipedia.org/wiki/Phase_shifthttp://en.wikipedia.org/wiki/Phase_shifthttp://en.wikipedia.org/wiki/Delayhttp://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Superpositionhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/FFT


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Fading refers to the time variation of the

received signal power caused by changes in thetransmission medium or path.

Small scale fading or simply fading is used todescribe the rapid fluctuations of the amplitude,phases or multipath delays of a radio signal overa short period of time or travel distance, so thatlarge scale path loss effects may be ignored.Fading is caused by interference between two ormore versions of the transmitted signal whicharrives at the receiver at slightly different times.

These waves, called multipath waves, combine atthe receiver antenna to give a resultant signal,which can vary widely in amplitude and phase,depending on the distribution of the intensity andrelative propagation time of the waves and thebandwidth of the transmitted signal.


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EXAMPLE

For example, consider the common experience ofstopping at traffic lights and hearing a lot ofstatic on your FM broadcast radio, which isimmediately corrected if you move less than a

meter. Cellular phones also exhibit similarmomentary fades. The reason for these losses ofsignal is the destructive interference that multiplereflected copies of the signal make with itself. Tounderstand how a signal can destructively

interfere with itself, consider the sum of twosinusoidal waveforms (which are similar tomodulated carrier signals) with different phases.
http://en.wikipedia.org/wiki/Sinusoidalhttp://en.wikipedia.org/wiki/Sinusoidal


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FADING IN WIRELESS COMMUNICATIONS

In wireless communications, signal fading iscaused by multi-path effect. Multi-path effectmeans that a signal transmitted from atransmitter may have multiple copies traversingdifferent paths to reach a receiver.

At the receiver, the received signal should be thesum of all these multi-path signals. Because thepaths traversed by these signals are different;some are longer and some are shorter.

The one at the direction of light of signal (LOS)should be the shortest. These signals interactwith each other. If signals are in phase, theywould intensify the resultant signal; otherwise,the resultant signal is weakened due to out of

phase. This phenomenon is called channel fading.


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SMALL SCALE MULTIPATH PROPAGATION

Multipath in the radio channel creates smallscale fading effects. The three mostimportant effects are :

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

Random frequency distribution due tovarying Doppler shifts on differentmultipath signals.

Time dispersion (Echoes) caused bymultipath propagation delays.


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FACTORS INFLUENCING SMALL SCALE FADING

Many physical factors in the radio propagation channel influence

small-scale fading. These include the following :

(1) Multipath Propagation :

The presence of reflecting objects and scatters in the channel createsa constantly changing environment that dissipates the signal energyin amplitude, phase and time. These effects results in multipleversions of the transmitted signal that arrive at the receivingantenna, displaced with respect to one another in time and spatialorientation. The random phase and amplitude of the differentmultipath components caused fluctuations in signal strength, therebyinducing Small scale fading, Signal distortion, or both. Multipathpropagation often lengthens the time required for the base bandportion of the signal to reach the receiver which can cause signalsmearing due to inter symbol interference.

(2) Speed of the mobile :

The relative motion between the base station and the mobile resultsin random frequency modulation due to different Doppler Shifts oneach of the multipath components. Doppler Shift will be positive ornegative depending on whether the mobile receiver is moving

towards or away from the base station.


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(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 ignoredand only the speed of the mobile need be considered. The coherence

time defines the staticnessof 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 thecoherence 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.


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DOPPLER SHIFT


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Consider a mobile moving at aconstant velocity v, along a path

segment having length d betweenpoints X and Y, while it receivessignals from a remote source asillustrated in Fig (1) in previous slide :

The difference in path length traveled

by the wave from source S to themobile at points X and Y is

l= dcos= vtcos


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The phase change in received signaldue to difference in path lengths istherefore,

= 2 l = 2 v t cos

and hence the apparent change infrequency or Doppler shift, is given

by fd, wherefd = 1 . = v . cos

2 t


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TYPES OF SMALL SCALE FADING

Fig. (2) : Types of small scale fading


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FLAT FADING

Flat fading, where the bandwidth ofthe signal is less than the coherencebandwidth of the channel or thedelay spreadis less than the symbolperiod.
http://en.wikipedia.org/wiki/Coherence_bandwidthhttp://en.wikipedia.org/wiki/Coherence_bandwidthhttp://en.wikipedia.org/wiki/Delay_spreadhttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Symbol_ratehttp://en.wikipedia.org/wiki/Delay_spreadhttp://en.wikipedia.org/wiki/Delay_spreadhttp://en.wikipedia.org/wiki/Delay_spreadhttp://en.wikipedia.org/wiki/Coherence_bandwidthhttp://en.wikipedia.org/wiki/Coherence_bandwidthhttp://en.wikipedia.org/wiki/Coherence_bandwidth


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Fig. (3): Flat Fading channel characteristics


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FREQUENCY SELECTIVE FADING

Frequency selective fading, wherethe bandwidth of the signal is greaterthan the coherence bandwidth of thechannel or the delay spread isgreater than the symbol period.
http://en.wikipedia.org/wiki/Frequency_selective_fadinghttp://en.wikipedia.org/wiki/Frequency_selective_fadinghttp://en.wikipedia.org/wiki/Frequency_selective_fadinghttp://en.wikipedia.org/wiki/Frequency_selective_fadinghttp://en.wikipedia.org/wiki/Frequency_selective_fadinghttp://en.wikipedia.org/wiki/Frequency_selective_fading


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Fig. (4) : Frequency selective fading channel characteristics


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FADING EFFECT DUE TO DOPPLER SPREAD

Due to Doppler Effect, if a transmitter ismoving away from a receiver, thefrequency of the received signal is lowerthan the one sent out from the transmitter;otherwise, the frequency is increased.

In wireless communications, there aremany factors that can cause relativemovement between a transmitter and areceiver.

It can be the movement of a mobile suchas a cell phone. It can be the movement of some

background objectives, which causes thechange of path length between the

transmitter and the receiver.


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FASTFADING

Fast Fading is a kind of fading occurring with small

movements of a mobile or obstacle.

Depending upon how rapidly the transmitted base band

signal changes as compared to the rate of change of the

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

Slow fadingchannel.

In a Fast fading channel, the impulse response changes

rapidly within the symbol duration. That is, the coherence

time of the channel is smaller than the symbol period of thetransmitted signal. This causes frequency dispersion (also

called the selective fading) due to Doppler spreading, which

leads to signal distortion.


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SLOWFADING

Slow Fading is a kind of fading caused by larger

movements of a mobile or obstructions within the

propagation environment. This is often modeled

as log-normal distribution with a standard

deviation according to the Log Distance Path Loss

Model.

In a slow fading channel, the channel impulse

response changes at a rate much slower than the

transmitted base band signal s(t). In this case,

channel may be assumed to be static over one or

several reciprocal bandwidth intervals.
http://en.wikipedia.org/wiki/Log-normal_distributionhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log_Distance_Path_Loss_Modelhttp://en.wikipedia.org/wiki/Log-normal_distributionhttp://en.wikipedia.org/wiki/Log-normal_distributionhttp://en.wikipedia.org/wiki/Log-normal_distributionhttp://en.wikipedia.org/wiki/Log-normal_distributionhttp://en.wikipedia.org/wiki/Log-normal_distribution


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Fig. (5) : Type of fading experienced by a signal as a function

(a) Symbol period (b) Base band signal bandwidth


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STATISTICAL MODELS FOR MULTIPATH

FADING CHANNELS

Several multipath models have been suggested toexplain the observed statistical nature of a mobilechannel.

The first model presented by ossana was based oninterference of waves incident and reflected from theflat sides of randomly located buildings.

Ossanas model is therefore rather inflexible andinappropriate for urban areas where the direct path isalmost always blocked by buildings or other obstacles.Clarkes model is based on scattering and is widelyused.


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CLARKES MODEL

Clarke developed a model where the statisticalcharacteristics of the electromagnetic fields of thereceived signals of the mobile are deduced fromscattering.

The model assumes a fixed transmitter with a

vertically polarized antenna. The field incident on themobile antenna is assumed to be comprised of Nazimuthal plane waves with arbitrary carrier phases,arbitrary azimuthal angles of arrival, and each wavehaving equal average amplitude.

It should be noted that the equal average amplitudeassumption is based on the fact that in the absence ofa direct line-of-sight path, the scattered arriving at areceiver will experience similar attenuation over

small-scale distances.


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TWO-RAY RAYLEIGH FADING MODEL

Clarkes model and the statistics for Rayleighfading are for that fading conditions and do not

consider multipath time delay.

In modern mobile communication systems with

high data rates, it has become necessary tomodel the effects of multipath delay spread as

well as fading.

A commonly used multipath model is an

independent Rayleigh fading two-ray model(which is a specific implementation of the

generic fading simulator shown in figure

below).


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The figure shows a block diagram of the two-ray

independent Rayleigh fading channel model.

INPUT OUTPUT


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SALEH AND VALENZUELA INDOOR STATISTICAL MODEL

Saleh and Valenzuela reported the results of

indoor propagation measurements between two

vertically polarized omni directional antennas

located on the same floor of a medium sized

office building. Measurements were made using10 ns, 1.5 GHz, radar-like pulses.

The method involved averaging the square law

detected pulse response while sweeping the

frequency of the transmitted pulse. Using thismethod, multipath components within 5 ns were

resolvable.

SIRCIM AND SMRCIM INDOOR AND OUTDOOR


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SIRCIM AND SMRCIM INDOOR AND OUTDOOR

STATISTICAL MODELS

Rappaport and Seidel reported measurement at 1300 MHz in five

factory buildings and carried out subsequent measurements inother types of buildings. The authors developed an elaborate,

empirically derived statistical model to generate measured

channels based on the discrete impulse response channel model

and wrote a computer program called SIRCIM (simulation of

indoor radio channel impulse-response models). SIRCIM generates realistic samples of small-scale indoor channel

impulse response measurements. Subsequent work by Huang

produced SMRCIM (simulation of mobile radio channel

impulse response models), a similar program that generates

small-scale urban cellular and microcellular channel impulseresponses.

These programs are currently in use at over 100 institutions

throughout the world, and have been updated to include angle of

arrival information for micro cell, indoor, and macro cell channels.


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REFERENCES

Wireless Communication

(T. S. RAPPAPORT, EEE Pub.)

Wikipedia, the free encyclopedia

Google.co.in


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ANY QUERIES PLEASE ?


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