2. OUTLINE
• INTRODUCTION
• MIMO
• SPACE TIME CODE
• ALAMOUTI CODE
• APPLICATION OF MIMO
• REFERENCES
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3. INTRODUCTION
• Goal of modern wireless communication
systems:
- High Data Rates for Broadband, Video Conferencing etc.
• Challenges to the goal:
- Power, Bandwidth & Complexity.
• Constraints to the goal:
- To deal with noise
- Multipath propagation
- Interference
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4. MIMO
• MIMO, is the use of multiple antennas at both the
transmitter and receiver to improve
communication performance.
• MIMO exploits spatial diversity by having several
transmit and receive antennas.
• It can greatly increases the capacity of channels
by independently sending streams of data across
multiple antennas
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6. Contd…
h h
11 12
.. h
1m
H h h
21 22
.. h
2m
.. .. .. ..
h h
n1 n2
.. h
nm
The received signal can be given as:
y Hx n
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7. MIMO Benefits
• High data rate
• Achieve high channel capacity
• Minimize probability of error
• Bandwidth efficient.
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8. Introduction to Space-Time Coding
• What is Space-Time Coding?
• Space diversity at antenna
• Time diversity
• Why STC?
• Maximum diversity
• Maximum Coding Gain
• Maximum possible throughput
• Space-Time Block Codes:
• These codes are transmitted using a block structure
which enables simple decoding at the receiver.
.
9. Alamouti Codes
• Simple yet very effective
• Space diversity at transmitter end
At t1 At t2
X1, X2 X1 -X2*
Symbol Transmit Ant1
bits Calculation Antenna X2 X1*
Ant2
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10. Alamouti Code
• Assume two transmit antennas and one receive
antenna.
• The space-time block code transmission matrix
is x x*
1 2
G2 *
x2 x
1
• For each pair of symbols transmit
* *
Antenna #1: x1 x2 Antenna #2: x2 x1
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11. CONTD ……….
• The received signals are y1 h1 x1 h2 x2 1
* *
.
y2 hx
1 2 hx
2 1 2
• Calculate the decision variables as
x1 ' h1* y1 h2 y2*
* *
x2 ' h1 y2 h2 y1
• Unfortunately, the technique is sensitive to
delays.
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12. Summary of Alamouti’s Scheme
• No feedback from receiver to transmitter is
required for CSI to obtain full transmit
diversity.
• No bandwidth expansion (as redundancy is
applied in space across multiple antennas, not
in time or frequency).
• Low complexity decoders.
• No need for complete redesign of existing
systems to incorporate this diversity scheme.
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13. Advantages and Disadvantages
of Block STC
• Advantages:
– Space-time block coding utilizes multiple antennas to
create spatial diversity, this allows a system to have better
performance in a fading environment.
– Good performance with minimal decoding complexity.
– Can achieve maximum diversity gain
– Receivers that use only linear processing.
• Disadvantages:
– Does not have much coding gain .
– Can not always achieve the maximum data rates
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14. APPLICATIONS OF MIMO
• Combined with OFDM, where the problems created
by multi-path channel are handled efficiently.
• MIMO gives reliable communication
• MIMO increases capacity
– MIMO uses independent channel fading due to multipath
propagation to increase capacity.
– No extra expensive bandwidth required
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15. REFERENCES
• Hameed Jafarkhani, “Space-Time Coding-Theory & Practices”, 1st
ed., Cambridge University Press.
• V. Tarokh, H. Jafarkhani and A. R. Calderbank, “Space-time block
codes for orthogonal designs,” IEEE Trans. Inform. Theory, vol. 45,
pp. 1456–1466, July 1999.
• V. Tarokh, N. Seshadri and A. R. Calderbank, “Space-time codes for
high data rate wireless communication: performance criterion and
code construction,” IEEE Trans. Inform. Theory, vol. 44, pp. 744–
765, Mar. 1998.
• Ezio Biglieri, Robert Calderbank, Anthony Constentinicks, Andrea
Goldsmith, Arogyaswami Paubraj, “MIMO Wireless
communication”, 2nd edition, Cambridge University press,2007.
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Array gain : improves resistance to noise, thereby improving the coverage and the range of a wireless networkSpatial diversity gain : mitigates fading and is realized by providing the receiver with multiple copies of the transmitted signal in space, frequency or time.Spatial multiplexing gain : The spatial multiplexing gain increases the capacity of a wireless network.Interference reduction and avoidance : It improve the coverage and range of a wireless network.
Space time coding is a method employed to improve the reliability of data transmission in wireless communication system using multiple transmit antenna.STCs rely on transmitting multiple , copies of a data stream to the receiver in the hope that at least some of them may survive the physical path between transmission and reception in a good enough state to allow reliable decoding.Space time codes(STC) provide transmit diversity for the MIMO.Achieve near optimal diversity gain.STC is concerned withMaximize the transmitted information rate.Minimize the error probability.
