Lect 11- MIMO and Space-Time coding.pdf

8/11/2019 Lect 11- MIMO and Space-Time coding.pdf

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MULTIPLE INPUT MULTIPLEOUTPUT SYSTEMS (MIMO)

IntroductionMIMO Systems:

use multiple inputs and multiple outputs from asingle channel

are defined by Spatial Diversity and SpatialMultiplexing


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Spatial Diversity and Spatial Multiplexing

Spatial Diversity

Signal copies are transferred from multiple antennasor received at more than one antenna

redundancy is provided by employing an array ofantennas, with a minimum separation of /2 betweenneighbouring antennas

Spatial Multiplexing

the system is able to carry more than one data streamover one frequency, simultaneously

Why MIMO? There is always a need for increase in performance in

wireless systems

Significant increase in spectral efficiency and datarates

High Quality of Service (QoS)

Wide coverage, etc.

Wireless channel that we are using is very unfriendly

Suffers from Cochannel interference and signal level

fading It provides a limited bandwidth

power falls off with distance


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MIMO System solutions

By using Multiple Output Multiple Input (MIMO) systems

Diversity gain mitigates the fading and increasescoverage and improves QoS

Multiplexing gain increases capacity and spectralefficiency with no additional power or bandwidthexpenditure

Array gain results in an increase in average receiveSNR.

Spatial Diversity and Spatial Multiplexing can beconflicting goals

Spatial Multiplexing MIMO channels can be decomposed into a number of R parallel

independent channels Multiplexing Gain

Principle: Transmit independent data signals from different antennas toincrease the throughput, capacity.

Source: An Overview of MIMO Systems in Wireless Communications

www.iet.ntnu.no/projects/beats/Documents/mimo.pdf


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MIMO capacity on fading channels

The capacity increase can be seen by comparing MIMO

systems with SISO, SIMO, and MISO systems

SISO:capacity is given by Shannons classical formula:

Where B is the BW and h is the fading gain

SIMO (with M transmitting antennas), the capacity is given by

MISO (with M transmitting antennas), the capacity is given by

)2

1(2

log hsnrBC +=

)

1

21(

2log

=

+=

m

n nhsnrBC

)

1

21(

2log

=

+=

N

n nh

N

snrBC

MIMO capacity on fading channels

The capacity for MIMO systems can have the following forms(Assuming Tx antennas = Rx antennas = N):

A) If the channel is not known at the transmitter:

Where Es is the total power, 2 is noise level of AWGN

Hence the power is equally shared by each channel

The capacity grows linearly with the number of antennas

B) If the channel is known at the transmitter

)2

21(

2log

nh

N

sE

NC

+=

=

+=

N

n nhn

EC

1

)2

21(

2log


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Spatial Diversity

Improves the signal quality and achieves a higher SNR at the

receiver-side

Principle of diversity relies on the transmission of structuredredundancy

xiyi

MIMO Diversity and Reliability

The performance improvement in SNR and error probability forMIMO can be compared with SISO, SIMO, and MISO

The detailed calculation for SNR and Pe is shown in [1]

SISO:

Receive Diversity (SIMO): Ninxhy iii ,2,1, =+=

21

1

SNRPe

+

nhxy +=

2

2

2

)( hSNRnE

hxEhSNR == and

=N

i

hSNRhSNR 2

)( NSNRPe

+

21

1

and


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MIMO Diversity and Reliability

The values for SNR and Pe for:

Transmit Diversity (MISO):

Transmit/Receive Diversity (MIMO):

The received signal at antenna i will be:

H is the channel fading matrix i

jij

M

j

i nxhy +==1

MSNR

Pe

+

21

1

nxhy jM

j

j +==1

=

=

M

j

jhSNRhSNR1

2)( and

22

)(},min{

HSNRHSNRMN

HSNR

MN

MN

SNRPe

+

},min{21

1

and

Conclusion

The capacity of Receive or Transmit Diversity grows logarithmicallywith the number of antennas

Capacity of MIMO increases linearly with the number of antennas

Using Spatial Diversity:

The SNR increases and Pe decreases when using MIMO

Spatial Multiplexing and Spatial Diversity are conflicting objectives


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding


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Space-Time coding

Space-Time coding

Lect 11- MIMO and Space-Time coding.pdf

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