Lect 11- MIMO and Space-Time coding.pdf
Transcript of 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
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Space-Time coding
Space-Time coding
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Space-Time coding
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Space-Time coding
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Space-Time coding
Space-Time coding
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Space-Time coding
Space-Time coding