Cooperative STBC-OFDM Transmissions with Imperfect Synchronization in Time and Frequency

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Cooperative STBC-OFDM Transmissions with
Imperfect Synchronization in Time and Frequency

• Fan Ng and Xiaohua(Edward) Li
• Department of Electrical and Computer Engineering
• State University of New York at Binghamton
• fngnone1,xli_at_binghamton.edu
• http//ucesp.ws.binghamton.edu/xli

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• Problems
• In cooperative transmissions, synchronization of
  distributed transmitters is difficult
• Imperfect time synchronization
• Channel dispersion, ISI, STBC structure destroyed
• Imperfect frequency synchronization
• Residue carrier, channel time-variation

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• OFDM
• Good candidate for broadband transmissions
• Inherently tolerate time-asynchronism
• Transmission delays resolved as phase of
  subchannels
• Frequency asynchronism is challenging
• Create inter-carrier interference (ICI)
• Our contributions
• Study time/frequency asynchronism among
  cooperative STBC-OFDM transmitters
• Propose a receive scheme to resolve both
  asynchronism

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Cooperative STBC-OFDM transmissions

• Objective redesign STBC-OFDM receiver to
  tolerate asynchronous transmitters
• Reduce synchronization cost by increasing
  computational complexity of receiver

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A special J2 STBC-OFDM transmission scheme
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Conventional OFDM without frequency offset
Transmitted baseband signal
Received baseband signal
Signal after OFDM demodulation
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Cooperative OFDM with delay and frequency offset
Received signal distorted by delay dj and
frequency offset fj
OFDM demodulation. CP removed prior to this point.
where channels are distorted with delay and
carrier offset
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Analysis of inter-carrier interference (ICI)
Desired Signal Part
ICI Part
OFDM symbol illustration
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Joint OFDM detection and STBC decoding
Construct vector channel model
JNxJN channel matrix due to STBC (J) and OFDM
(N) Estimate symbols by solving the
equations
Ci coefficients of desired signal part. Ci
coefficients of ICI part
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Simulations
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?ICI delay not compensated
?Proposed joint detection method
?Conventional STBC-OFDM with perfect synch.
STBC J2. OFDM N32, CP, Delay2fractional Ca
rrier offset 100 Hz, 80 Hz.
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• Summary
• OFDM resolve delay asynchronism for cooperative
  transmission, but suffer from carrier-offset-induc
  ed ICI
• Analyzed ICI structure, proposed a joint
  detection method
• Simulations show the method successfully resolve
  delay carrier asynchronism among transmitters
• Future work is to develop more efficient
  detection algorithm