Coded Modulation for Orthogonal Transmit Diversity

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Coded Modulation for Orthogonal Transmit Diversity
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Motivation

• Wireless Communication Environment
• Noise
• Multipath
• Fading
• MAI
• Demands
• Multimedia applications ? High rate
• Data communication ? Reliability

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Challenges

• Problems
• Low achievable rates if single transmit and
  receive antenna systems are used
• Less reliability due to low SNR and fading
• Some Possible Solutions
• Use more bandwidth (limited resource!)
• Use strong codes (computational complexity!)
• Use multiple antennas (hardware complexity!)

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Multiple-Antenna Systems

• Capacity ? ? min(nT, nR) ? Higher rate
• Potential spatial diversity ? More reliability

I. E. Telatar
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Space-Time Coding
Data

• Slowly fading
• Spatial diversity and coding gain
• Fast fading
• Spatial and temporal diversity, and coding gain

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Space-Time Code Design

• Previous approaches
• Jointly maximizing spatial and temporal
  diversity and coding gain
• No systematic code design method, difficult
• Suggested approach
• Decouples the problem into simpler ones
• Simplifies code design procedure
• Provides systematic code construction method
• Performs better than existing codes

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System Model

• Decouples the problems of maximizing
• Spatial diversity
• Temporal diversity and/or coding gain

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Orthogonal Transmit Diversity
S. Alamouti

• Achieves full diversity (2)
• Provides full rate (R 1)
• No capacity loss
• Simple ML decoder

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Slowly Fading Channels

• Upper bound for pairwise error probability
• No temporal diversity

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Design Criteria

• Maximization of coding gain
• Same as design criterion for single antenna
  systems in AWGN channels
• Codes designed for optimum performance in AWGN
  channels are optimum outer codes

(Standard Euclidean distance)
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Simulation Results (1)
Better performance with same complexity
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Simulation Results (2)
Better performance with same complexity
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Fast Fading Channels

• Upper bound for pairwise error probability

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Design Criteria (1)

• Maximization of
• Hamming distance
• Product distance
• between pairs of consecutive symbols

(c2k-1, c2k) , (e2k-1, e2k)
Design for an Expanded Constellation
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Constellation Expansion (1)

• In dimension

• In size

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Design Criteria (2)

• Design for expanded constellation based on
  maximizing
• Symbol Hamming distance
• Product of squared distances
• Same as design criteria for single antenna
  systems in fast fading channels

D. Divsalar
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Simulation Results (1)
Comparison with ATT smart-greedy code
Better performance with same complexity
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Simulation Results (2)
Comparison of simple OTD with concatenated ST
code (Outer code 4-dimensional MLC)
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Generalized OTD

• OTD systems with nTgt2 and nR?1
• Achieve maximum diversity order (nTnR)
• Not full rate (R lt 1)
• Full rate, full diversity, complex orthogonal
  designs exist only if nT2

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Slowly Fading Channels

• Upper bound for pairwise error probability
• Design criteria
• Maximization of free Euclidean distance

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Fast Fading Channels

• Upper bound for pairwise error probability
• Design criteria
• Maximizing Hamming and product distances in
  expanded constellation

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Simulation Results
Slowly fading channel
Fast fading channel
8-state TCM outer code optimum for AWGN
MTCM outer code
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Summary

• Concatenated orthogonal space-time code
• Decouples the problems of maximizing spatial
  diversity, temporal diversity and/or coding gain
• Simplifies code design procedure and provides a
  systematic method for code construction
• Has better performance compared to existing
  space-time codes

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Contact Information

• mohammad_at_rice.edu
• mahsa_at_rice.edu
• aaz_at_rice.edu
• http//www.ece.rice.edu/mohammad