Diversity [Autosaved]Final

7/31/2019 Diversity [Autosaved]Final

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An Overview of Diversity in Wireless Communication

BY

Sani-e-Zehra Rizvi

Std 13748


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WIRELESSCOMMUNICATIONREFERSTOTECHNOLOGYTHATTRANSMITSDATAOVERDISTANCESWITHOUTTHEUSEOF

CONNECTINGWIRES.

DIAGRAMOFA WIRELESS COMMUNICATION SYSTEM


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CHANNEL IMPAIRMENTS

Noise : Thermal noise

Path Loss : The loss in power as the radio signalpropagates

Shadowing : Due to the presence of fixed obstaclesin the radio signal path

Fading : Combines the effect of multiplepropagation paths , rapid movement of mobile unitsand reflectors.


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LOSSESINTHE TRANSMISSION PATH


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FADING

Multiple propagation paths exist between transmitter andreceiver due to scattering from different objects

Copies of the same signal undergo different distortions,attenuation ,delays and phase shifts.

When destructive interference occurs at the receiver thesignal power may be diminished significantly.

A process called fading.

The performance of a system can be severely degraded

by fading. Special Techniques may be employed to improve the

performance.


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FADING


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SYSTEM PERFORMANCE DEGRADATION(EFFECTOFFADING )


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PARAMETERSOFA FADING CHANNEL

Multipath Spread : It tells us the maximum delaybetween paths of significant power in the channel.

Coherence Bandwidth : Gives us how far apart infrequency for signals to undergo different degreesof fading.

Coherence Time : Time duration over which tworeceived signals have a strong potential foramplitude correlation.

Doppler Spread: It gives us the maximum range ofDoppler spread.


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Transmitter dynamic range

For the transmitter to overcome a certain level of fading,it must increase its power by that same level .

It is not practical because of radiation power limitations

and the size and cost of the amplifiers.


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Channel information

The channel information has to be fed back to thetransmitter.

Throughput degradation and added complexity to boththe transmitter and the receiver.


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DIVERSITY

Provides significant link improvement whileincurring little added cost.

Basic Concept: If one path undergoes a deep fade another might

still have a strong signal.

Create multiple independent paths for the signaland combine them in an optimum way.


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DIVERSITY

SNR at the receiver increased by 20-30 dB.

Can be provided across time, frequency and space.

Requirements :

Multiple branchesLow correlation


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SPACE DIVERSITY

Also known as antenna diversity

Most popular form of diversity

Signals received from spatially separated mobile

antennas would have uncorrelated signalsantenna separation of one half wavelength or more


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SPACE DIVERSITY

Also used in base station design.

At each cell site multiple base station receivingantennas are used to provide diversity reception.

Base station antennas must be spaced far apart toachieve decorrelation.

Separation- on the order of several tens ofwavelength.


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SPACE DIVERSITY


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SELECTIONDIVERSITY

Each antennas SNR is monitored.

The receiver with the highest instantaneous SNR(at any given moment of time) is connected to thedemodulator.

The antenna signals are sampled and the mostappropriate one sent to the demodulator.

In practice (S+N)/N could be used. Since it isdifficult to measure SNR alone.

It is simple and cheap.


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SELECTION DIVERSITY


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FEEDBACK DIVERSITY

Scan each antenna until a signal is found that isabove a predetermined threshold

If the signal fall below a threshold : rescanningstarts

Only one receiver is required (one signal isreceived at a time)


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DIAGRAM: FEEDBACK DIVERSITY


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MAXIMAL RATIO COMBINING

All paths co phased and summed with optimalweighting to maximize combiner output SNR.

Signals from all the branches are weightedaccording to their signal voltages to noise powerratios (variable weighting) and then summed up.

Use linear coherent combining of branch signals sothat the output SNR is maximized.


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DIAGRAM : MAXIMAL RATIO COMBINING


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The affect of correlation can be modeled byintroducing equivalent average SNR.

Coherent combining may be difficult to implement.

Performs best of all combiners in terms of SNR. But requires a lot of circuitry e.g. each receiver

needs additional phasing circuit.

Produces an output with acceptable SNR even

when none of the individual signals are themselvesacceptable.


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EQUAL GAIN DIVERSITY

Simplified method of MRC.

Combine multiple signals to one.

In certain cases variable weighting is not

convenient. Branch weights are all set to unity.

The phase is adjusted for each signal so that :

Signal from each branch are co phased

Vectors add in space


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Much better performance than selection diversity.

Allows the receiver to exploit signals that aresimultaneously received on each branch.

The possibility of producing an acceptable signalfrom many unacceptable signal is still retained.

Performance marginally inferior than MRC.


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COMPARISON: MRC VS EGC


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COMPARISON : SC VS MRC


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CONCLUSION

Among different combining techniques MRC hasthe best performance and highest complexity.

SC has the lowest complexity and the simplesttechnique.


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TIMEDIVERSITY

Repeatedly transmit information at time spacing'sthat exceed the coherence time of the channel.

As a result multiple repetitions of the signal will bereceived with independent fading conditions.

Thus providing for diversity.


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TIME DIVERSITY

The separation between the transmit times shouldbe greater than the coherence time, Tc.

As this results in uncorrelated signals.

Uses coding and interleaving to enhanceperformance.

Hence not all bits fall into deep fade.

Consumes extra transmission time.


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TIME DIVERSITY: INTERLEAVING Time diversity can be obtained by interleaving and coding

over symbols across different coherent time periods.


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Word size depends upon:

Type of speech coder

Source coding rate

Maximum tolerable delay Inherent delay associated with interleaver.

The received message cannot be fully decodeduntil all of the bits arrive at the receiver and are

deinterleaved.


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FREQUENCY DIVERSITY

This is often employed in microwave line of sight linkswhich carry several channels in a FDM mode.

Information is transmitted on more than one carrierfrequency.

The reason being that the frequencies separated bymore than the coherence bandwidth of the channel willnot experience the same fading .

At the receiver, the independently faded copies are

optimally combined to give a statistic for decision. The optimal combiner is the maximum ratio combiner.

Frequency diversity consumes extra bandwidth


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FURTHER STUDY

Different types of diversity techniques are beingused in different technologies .For further readingCDMA Rake Receiver can be considered for TimeDiversity, OFDM for Frequency Diversity and smart

antenna for space diversity. Other techniques such as polarization diversity and

error coding algorithms are also available.


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REFERENCES

http://www.oocities.org/hamsadhwani8/smartantennas/diversity.html

http://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdf

http://www.site.uottawa.ca/~sloyka/elg5133/Lec_9_ELG5133.pdf
http://www.oocities.org/hamsadhwani8/smartantennas/diversity.htmlhttp://www.oocities.org/hamsadhwani8/smartantennas/diversity.htmlhttp://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdfhttp://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdfhttp://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdfhttp://www.comlab.hut.fi/opetus/333/2004_2005_slides/Diversity_text.pdfhttp://www.oocities.org/hamsadhwani8/smartantennas/diversity.htmlhttp://www.oocities.org/hamsadhwani8/smartantennas/diversity.html


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