25513780 Multi Path Concept Diversity Concept Diversity Types

8/7/2019 25513780 Multi Path Concept Diversity Concept Diversity Types

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DIVERISTYDIVERISTY


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CONTENTSCONTENTS Multi Path ConceptMulti Path Concept

Diversity ConceptDiversity Concept

Diversity TypesDiversity Types

Space DiversitySpace Diversity

Polarization DiversityPolarization Diversity

Frequency DiversityFrequency Diversity

Time DiversityTime Diversity


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MULTIPATHMULTIPATH In GSM Networks, Radio / Air interface is used for communicationIn GSM Networks, Radio / Air interface is used for communication betweenbetween

operators fixed infrastructure [BTS] and MS.operators fixed infrastructure [BTS] and MS.

In most of wireless systems, there are numerous objects that canIn most of wireless systems, there are numerous objects that can reflect radio signalsreflect radio signals

and scatters throughout the area in a complicated manner.and scatters throughout the area in a complicated manner. At the receiver end, radio signals from the transmitter arrive vAt the receiver end, radio signals from the transmitter arrive via a number of differentia a number of different

paths. The actual signal picked up by the receiver antenna is thpaths. The actual signal picked up by the receiver antenna is the combination of all ofe combination of all of

these different signals.these different signals.

Wireless systems that do not use Diversity Receivers suffer fromWireless systems that do not use Diversity Receivers suffer from multipathmultipathpropagation. Multipath can cause signal dropouts, resulting in bpropagation. Multipath can cause signal dropouts, resulting in brief periods where therief periods where the

audio from the transmitter becomes noisy or is lost completely.audio from the transmitter becomes noisy or is lost completely.

Multipath is usually described as:Multipath is usually described as:

LineLine--OfOf--SightSight (LOS): the direct connection between the Transmitter [TX] and(LOS): the direct connection between the Transmitter [TX] andReceiver [RX].Receiver [RX].

NonNon--LineLine--OfOf--SightSight (N(N--LOS) the path arriving after reflection from reflectorsLOS) the path arriving after reflection from reflectors

If one path is longer than another, the signals from these two pIf one path is longer than another, the signals from these two paths will have differentaths will have different

phases. In addition, when radio signals reflect, their phases rephases. In addition, when radio signals reflect, their phases reverses. At the receiververses. At the receiver

antenna, some of the arriving signals add to the total and someantenna, some of the arriving signals add to the total and some subtract from the total.subtract from the total.


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MULTIPATHMULTIPATH A dropout occurs when the sum of all the arriving signals is lowA dropout occurs when the sum of all the arriving signals is low and the receiver doesand the receiver does

not have a strong enough signal to provide good audio.not have a strong enough signal to provide good audio.

To avoid the effects of Multipath Fading, Diversity receivers arTo avoid the effects of Multipath Fading, Diversity receivers are used as they have twoe used as they have twoantennas and two receiver channels.antennas and two receiver channels.

Special circuits in the receiver select the audio from the antenSpecial circuits in the receiver select the audio from the antenna and receiver channelna and receiver channel

with the better signal.with the better signal.

Diversity receivers provide almost complete immunity to dropoutsDiversity receivers provide almost complete immunity to dropouts due to multipath.due to multipath.


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DIVERSITYDIVERSITY Diversity is a powerful communication receiver technique that prDiversity is a powerful communication receiver technique that provides wireless linkovides wireless link

improvement at relatively low cost.improvement at relatively low cost.

Diversity concept can be explained simply. If one radio path undDiversity concept can be explained simply. If one radio path undergoes deep fade,ergoes deep fade,another independent path may have a strong signal. By having moranother independent path may have a strong signal. By having more than one path toe than one path to

select from, both the instantaneous and average SNRs at the receselect from, both the instantaneous and average SNRs at the receiver may beiver may be

improved, often by as much asimproved, often by as much as 20 dB to 30 dB20 dB to 30 dB..

In order to prevent deep fades from occurring,In order to prevent deep fades from occurring, Microscopic Diversity TechniquesMicroscopic Diversity Techniquescancanexploit the rapidly changing signal. In case of small scale fadiexploit the rapidly changing signal. In case of small scale fading if two antennas areng if two antennas are

separated by a fraction of a meter, one may receive a null whileseparated by a fraction of a meter, one may receive a null while the other receives athe other receives a

strong signal. By selecting the best signal at all times, a recestrong signal. By selecting the best signal at all times, a receiver can mitigate smalliver can mitigate small--scalescale

fading effects, this is calledfading effects, this is called Antenna DiversityAntenna Diversityoror Space Diversity.Space Diversity.


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DIVERSITY TYPESDIVERSITY TYPES Space DiversitySpace Diversity

Polarization DiversityPolarization Diversity

Frequency DiversityFrequency Diversity Time DiversityTime Diversity


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SPACE DIVERSITYSPACE DIVERSITY Space Diversity is also known asSpace Diversity is also known as Antenna Diversity,Antenna Diversity, is one of the most popular formsis one of the most popular forms

of diversity used in wireless systems.of diversity used in wireless systems.

Conventional wireless systems consist of an elevated base statioConventional wireless systems consist of an elevated base station antenna and a mobilen antenna and a mobileantenna close to the ground. The existence of a direct path betwantenna close to the ground. The existence of a direct path between the transmittereen the transmitter

and the receiver is not guaranteed and the possibility of a numband the receiver is not guaranteed and the possibility of a number of scatters in theer of scatters in the

vicinity of the mobile suggests avicinity of the mobile suggests a Rayleigh Fading Signal.Rayleigh Fading Signal.

The concept of antenna space diversity is also used in base statThe concept of antenna space diversity is also used in base station design. At each cellion design. At each cellsite, multiple base station receiving antennas are used to provisite, multiple base station receiving antennas are used to provide diversity reception.de diversity reception.

Since scatters are generally on the ground in the vicinity of thSince scatters are generally on the ground in the vicinity of the mobile, the base statione mobile, the base station

antennas must be spaced considerably far apart to achieve decorrantennas must be spaced considerably far apart to achieve decorrelation.elation.

Space diversity can thus be used at either the mobile or base stSpace diversity can thus be used at either the mobile or base st

ation or both.ation or both.


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FREQUENCY DIVERSITYFREQUENCY DIVERSITY Frequency Diversity is implemented by transmitting information oFrequency Diversity is implemented by transmitting information on more than onen more than one

carrier frequency.carrier frequency.

The idea behind this technique is that frequencies separated byThe idea behind this technique is that frequencies separated bymore than themore than thecoherence bandwidth of the channel will be uncorrelated and willcoherence bandwidth of the channel will be uncorrelated and will thus not experiencethus not experience

the same fade.the same fade.

Frequency Diversity is often employed in microwave lineFrequency Diversity is often employed in microwave line--ofof--sight links which carrysight links which carry

several channels in a Frequency Division Multiplex Mode [FDM].several channels in a Frequency Division Multiplex Mode [FDM]. In this technique, one frequency is IDLE but is available on staIn this technique, one frequency is IDLE but is available on standnd--by basis to provideby basis to provide

frequency diversity. When Diversity is needed the appropriate trfrequency diversity. When Diversity is needed the appropriate traffic is simplyaffic is simply

switched to the backup frequency.switched to the backup frequency.

Disadvantage however is that it not only requires spare bandwidtDisadvantage however is that it not only requires spare bandwidth but also requiresh but also requiresthat there be as many receivers as there are channels used for tthat there be as many receivers as there are channels used for the frequency diversity.he frequency diversity.


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TIME DIVERSITYTIME DIVERSITY Time Diversity repeatedly transmits information at time spacingsTime Diversity repeatedly transmits information at time spacings that exceeds thethat exceeds the

coherence time of the channel, so that multiple repetitions of tcoherence time of the channel, so that multiple repetitions of the signal will behe signal will be

received with independent fading conditions, thereby providing freceived with independent fading conditions, thereby providing for diversity.or diversity.


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25513780 Multi Path Concept Diversity Concept Diversity Types

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