Topic 1-Multipath Fading

7/31/2019 Topic 1-Multipath Fading

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SMALL SCALESMALL SCALE

FADING ANDFADING ANDMULTIPATHMULTIPATH

Department of Electronics

and Communication

Gyan Vihar University


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Wireless Comes of Age

Guglielmo Marconi invented the wirelesstelegraph in 1896 Communication by encoding alphanumeric characters

in analog signal

Sent telegraphic signals across the Atlantic Ocean

Communications satellites launched in 1960s Advances in wireless technology Radio, television, mobile telephone, communicationsatellites

More recently Satellite communications, wireless networking,cellular technology


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Broadband Wireless Technology

Higher data rates obtainable with broadbandwireless technology Graphics, video, audio

Shares same advantages of all wireless services:convenience and reduced cost Service can be deployed faster than fixed service

No cost of cable plant

Service is mobile, deployed almost anywhere


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Limitations and Difficulties of

Wireless Technologies

Wireless is convenient and less expensive Limitations and political and technical difficultiesinhibit wireless technologies Lack of an industry-wide standard Device limitations

E.g., small LCD on a mobile telephone can onlydisplaying a few lines of text

E.g., browsers of most mobile wireless devices use

wireless markup language (WML) instead of HTML


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Cellular Concept


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The Cellular Concept


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19-cell reuse example (N=19)

Figure 3.2 Method of locating co-channel cells in a cellular system. In this example,N= 19 (i.e.,I= 3,

j = 2). (Adapted from [Oet83] IEEE.)


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What is small scale fading?

Small scale fading is used to describe the rapid

Fluctuation Of the amplitude of radio signal over

a short period of time or travel distance.

How is it caused ?

Fading is caused by interference between two or more

Versions of the transmitted signal which will arrive at theReceiver at slightly different times .This waves called

Multipath waves , combine at receiver antenna to give a

Resultant signal which can vary widely in amplitude and

And phase , depending on the distribution of the intensity

And relative propagation times of wave and bandwidth.


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Multipath Propagation

Reflection - occurs when signal encounters asurface that is large relative to the wavelength ofthe signal

Diffraction - occurs at the edge of animpenetrable body that is large compared towavelength of radio wave

Scattering occurs when incoming signal hits anobject whose size in the order of the wavelengthof the signal or less


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Doppler shift:

Due to relative motion between the mobile and base station

each multipath wave experiences an apparent shift infrequency. The shift in received signal frequency due to motio

is called the Dopplers shift .and it is directly proportional

to the velocity and direction of motion of the mobile withrespect to the direction of arrival of the received multipath

wave.


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The Effects of Multipath

Propagation

Multiple copies of a signal may arrive atdifferent phases If phases add destructively, the signal level

relative to noise declines, making detectionmore difficult

Intersymbol interference (ISI)

One or more delayed copies of a pulse mayarrive at the same time will create interferenceto minimize it Separate symbols in time

with guard periods.
http://en.wikipedia.org/wiki/Guard_periodhttp://en.wikipedia.org/wiki/Guard_periodhttp://en.wikipedia.org/wiki/Guard_periodhttp://en.wikipedia.org/wiki/Guard_period


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Factors influencing small

scale fading

Multi path propagation:

The presence of reflecting objects and scatterers in the

channel creates a constantly changing environment thedissipates the energy in amplitude, phase and time.

Speed of the mobile :

The relative motion between the base station and mobileresults in the random frequency modulation due to thedifferent doppler shifts on each multi path components.

Speed of surrounding objects :

If objects in the radio channel are in motion, they inducea time varying doppler shifts on the multi pathcomponents . If the surrounding objects move at a greater

rate than mobile, then this effect dominates the smallscale fading.


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The Transmission Bandwidth of the Signal:

If the transmitted radio signal bandwidth is greater than the

bandwidth of multi path Channel the received signal will be

distorted, but the received signal strength will not Fade muchover local area.

The bandwidth of of a channel will be quantified by coherence

bandwidth which is related to specific multi path structure ofchannel.

The coherence bandwidth is measure of the maximum

frequency difference for which Signals are strongly correlated in

amplitude .

If the transmitted signal has narrow bandwidth as compared to

the channel, the amplitude of the signal will change rapidly, but

they signal will not be distorted in time. Thus , the statistics of small scale signal strength and likelihood

of signal smearing appearing over small scale distance are very

much related to the specific amplitudes and delays of the multi

path channel, as well as the bandwidth of the transmitted Signal.


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Small scale multi path propagation


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Other Multi-path Concerns

Apart from creating frequency selectfading,

Multi-path can also cause inter-symbol

interference.


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Multipath Propagation


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Types of Fading

Fast fading (Changes in amplitude 20 or 30 db) Slow fading (Slow change due to environment Flat fading Selective fading Rayleigh fading Rician fading


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Flat Fading(Non Selective)

Flat Fading is caused by absorbers between the two

antennae and is countered by antenna placement and

transmit power level. Here all frequency component of the

received signal fluctuate in the same proportion

simultaneously.


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Frequency Selective Fading

Frequency selective fading is caused by reflectors between

the transmitter and receiver creating multi-path effects.

Affects unequally the different spectral components of the

radio signal. This fading is usually significant only to thebandwidth of the overall communication channel


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Effects of Frequency Selective Fading

The dips or fades in the response due to reflection cause

cancellation of certain frequencies at the Receiver.

Reflections off near-by objects (e.g. ground, buildings,

trees, etc) can lead to multi-path signals of similar signal

power to the direct signal.

This can result in deep nulls in the received signal power

due to destructive interference Rayleigh fading occurs when there are multiple indirect

paths between transmitter and receiver and no distinct

dominant path such as LOS path.Rician fading a situation where there is a direct LOS path

in addition to a number of indirect multipath.

K=power in the dominant path/power in thescattered paths


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Two independent fading issues


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When the transmitted signal has bandwidth much greater than

Bandwidth of channel, then the multi path structure is

completely resolved by he received signal at anytime andreceived power varies very little since the the individual multi

path amplitudes do not change rapidly over local area.

However if the transmitted signal has a very narrow

bandwidth the the multi path is resolved by received signal

And large signal fluctuations occur at the receiver due to the

phase shift of the many unresolved multi path components

The End


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Topic 1-Multipath Fading

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