Space time coding is used in MIMO wireless systems to improve communication performance by exploiting spatial diversity. It uses multiple transmit and receive antennas. The Alamouti code is a simple and effective space time block code that achieves full transmit diversity without requiring channel state information at the transmitter. It transmits symbols from two transmit antennas in two time slots so that the receiver can recover the symbols with low complexity decoding. MIMO combined with space time coding can provide high data rates, minimize errors, and increase capacity for wireless applications such as 4G networks.

1. SPACE TIME CODING IN
MIMO
PRESENTED BY:
ILA SHARMA
102001, MTECH ECE

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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5. General MIMO
1 1
h11
h21
MT Transmit MR Receive
antenna h12 h22 antenna
2. 2
.
. hn2 .
. .
. h2m hn1 . .
. h1m
hnm
MT MR
Channel
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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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