Datacarrier AidedFrequency Offset Estimation for OFDM Systems

1
Data-carrier Aided Frequency Offset Estimation
for OFDM Systems
2
Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

3
Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

4
Motivations

• Motivations
• The conventional carrier frequency offset
  estimation methods pilot, cyclic prefix,
  training symbol
• Our proposed schemes adopting the received
  signal on data-carriers
• Providing more accurate frequency
  synchronization, or reducing the pilot numbers to
  raise transmitted data rate.

5
Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

6
Carrier Frequency Offset

• What result in carrier frequency offset (CFO)?
• Mismatch between the oscillators at the TX and RX
• Doppler frequency
• Carrier frequency offset can be divided into
• Integral part
• Fractional part

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OFDM System Model

• The OFDM system model

S ( n )
X ( n )
x ( k )
Adding Cyclic Prefix P/S
S/P
Signal Mapper
Adding Pilots C(n) IFFT
x ( t )
DAC
Channel h ( t)
z (t )
AWGN w ( t)
Remove Cyclic Prefix S/P
r (t )
P/S
FFT
Signal Demapper
ADC

• C is pilot sequence
• h is time domain channel impulse response
• w is additive white Gaussian noise.
• N data information S(n) which have been
  modulated with N modulation values X(n) on
  every sub-carrier

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OFDM System Model

• The k sample of an OFDM block generated by IFFT

N number of subcarriers Ng length of cyclic
prefix
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UWB Channel Model
cluster decay factor path decay
factor cluster arrival rate the
arrival rate of path within each cluster

• Four environments in this UWB channel model
• CM1 model is based on LOS (0-4m) channel
  measurements in 2
• CM2 model is based on NLOS (0-4m) channel
  measurements in 2
• CM3 model is based on NLOS (4-10m) channel
  measurements in 2,
• and NLOS in 3
• CM4 the model generated to fit a 25nsec RMS delay
  spread.

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Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

11
Sensitivity for Carrier Frequency Offset

• The OFDM system model with CFO

S ( n )
X ( n )
x ( k )
Adding Cyclic Prefix P/S
S/P
Signal Mapper
Adding Pilots C(n) IFFT
x ( t )
DAC
Channel h ( t)
z (t )
AWGN w ( t)
Remove Cyclic Prefix S/P
r (t )
P/S
FFT
Signal Demapper
ADC

• is the ratio of the actual
  frquency offset to the sub-carrier spacing

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Sensitivity for Carrier Frequency Offset

• The k-th received sample of the m-th symbol is
  given by

FFT
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Pilot tone - aided CFO Estimation

• PTA CFO estimation

• Let P denote the set of indexes of the Np
  pilot carriers

f
Q
R1
R2
Rm
RmD
Pilot3 (n3)
Pilot2 (n2)
I
Pilot1 (n1)
t
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Pilot tone - aided CFO Estimation

• PTA with weighting (PTAW) CFO estimation

• Let P denote the set of indexes of the Np
  pilot carriers

f
Q
I
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Cyclic Prefix - based CFO Estimation

• CL is the channel length

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Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

17
Modified PTAW
f
R1
R2
Rm
RmD
t
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Modified PTAW
Step2
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Modified PTAW
Step3
Each data-subcarrier d(n) has M candicates
,i1M
Step4
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Modified CPB
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Modified CPB
Step3
Each data-subcarrier d(n) has M candicates
,i1M
Step4
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Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

23
Optimum L for CPB Method
CM1
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Optimum L for CPB Method
CM3
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Discussion of Pilot Numbers
CM1
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Discussion of Pilot Numbers
CM3
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Performance Comparison
CM1 BPSK
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Performance Comparison
CM1 QPSK
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Performance Comparison
CM1 8PSK
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Performance Comparison
CM3 BPSK
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Performance Comparison
CM3 QPSK
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Performance Comparison
CM3 8PSK
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Outline

• Motivations
• Background knowledge
• Conventional CFO estimation strategies
• Modified CFO estimation strategies
• Simulation results
• Conclusions

34
Conclusions

• Advantages
• The key advantages of our proposed algorithms is
  to provide more accurate frequency
  synchronization and reduce pilot numbers to raise
  bandwidth efficiency.
• Comparison with conventional methods
• The MCPB performs better than CPB (lower MSE).
• The MPTAW performs better than two traditional
  pilot tone-aided methods, and we can achieve the
  same performance as PTAW by less pilot numbers.
• The best choices
• If there is acceptable ISI, the MCPB will be the
  most suitable method to estimate CFO because it
  can provide excellent MSE with its superior
  resistance of ICI and constellation size.
• If there is serious ISI, the MPTAW is the best
  choice under this condition since it is robust to
  time domain interference.

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• Thank you

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Reference

• 1 J. R. Foerster, Ed., Channel Modeling
  Sub-committee Report Final, IEEE P802.15 SG3a
  contribution.
• 2 H. Chen and G.J. Pottie, "A Comparison of
  Frequency Offset Tracking Algorithms for OFDM",
  GLOBECOM '03, vol.2, pp. 1069-1073, Dec. 2003.
• 3 K. Shi, E. Serpedin, and P. Ciblat,
  Decision-directed fine synchronization for coded
  OFDM systems, in Proc. IEEE International Conf.
  on Acoustics, Speech, and Signal Processing.
  (ICASSP04), vol. 4, pp. 365-368, 17-21 May 2004.