BCWS seminar

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Reducing Required Power Back-Off of Nonlinear
Amplifiers in Serial Modulation using SLM method

• BCWS seminar
• Maryam Sabbaghian
• Carleton University, Ottawa, Canada

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Outline

• OFDM SM.
• Effect of nonlinear amplifiers.
• SLM method for OFDM.
• Modified SLM method for SM.

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• OFDM We transmit a high rate data over parallel
  sub-carriers with a lower rate on each
  sub-carrier.

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• Large PAPR is an important drawback of OFDM
• Large PAPR can generate performance degradation
  unless the power amplifier has large power
  back-off.
• The power amplifier cost increases with its
  back-off value.

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• In SM there is no IFFT at the transmitter.
• In Serial Modulation high peaks are generated
  after filtering and Digital to Analog conversion,
  when there are consecutive large magnitude
  symbols in the data sequence.

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Comparison of amplitude distribution of OFDM and
SM
Pr normalized transmitted envelope gt x
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Nonlinear amplifiers

• Rapp model for power amplifier
• Input Back Off
• PsPower when ViVs, Piinput power

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AM/AM curve of Rapp model (Vs1)
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Effect of Nonlinear Amplifier in out of band
radiation, P02
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Effect of Nonlinear Amplifier in out of band
radiation, P010
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PAPR reduction methods for OFDM

• 1-Clipping and filtering
• To reduce the out of band radiation generated by
  clipping, signal should be filtered.
• Side effects Peak regrowth, in-band distortion,
  performance degradation.

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PAPR reduction methods for OFDM

• 2-methods preventing the occurrence of signals
  with high PAPR
• Coding
• PTS (Partial Transmit Signaling)
• SLM (Selected Mapping)

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SLM method for OFDM

• This method is based on generating M
  statistically independent transformed blocks for
  each data block and transmitting the one with the
  lowest PAPR.
• It requires transmitting some side information
  about the identity of the selected block .

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Block diagram of SLM for OFDM

• Each block is multiplied symbol-by-symbol, before
  the IFFT operation, by one of pseudo-random but
  fixed set of vectors ri whose elements are
  complex numbers with the amplitude equal to one
  and a random phase uniformly distributed between

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Improvement achieved by SLM on OFDM amplitude
distribution
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Modified SLM method

• In OFDM high peaks are generated when signals add
  together constructively during the IFFT
  operation.
• In Serial Modulation high peaks are generated
  after filtering and Digital to Analog conversion,
  when there are consecutive large magnitude
  symbols in the data sequence.

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Modified SLM method

• In the modified SLM method, like OFDM, M
  different blocks of data are generated in the
  transmitter, but each one is a permuted version
  of the original sequence to avoid occurrence of
  consecutive high peaks.
• The transmitter does not need the pseudo random
  sequence.

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Two typical OFDM and SM signals
In SM, the best signal selected for transmission
is not the one with the smallest PAPR.? The
signal is selected based on Least Square Criterion
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The metric of modified SLM method

• The transmitter selects the signal with the
  smallest Sum of Squared Error (SSE) between the
  input signal and the output signal of the
  nonlinear amplifier.

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Effect of SLM on CCDF of amplitude of SM
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Out of band radiation, BO5 dB, P010, N256,
16-QAM
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Out of band radiation, BO7 dB, P010
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Conclusion

• SM has smaller PAPR than OFDM yet the out of band
  radiation of SM is not negligible especially for
  larger constellation sizes.
• The modified SLM algorithm can considerably
  reduce out of band radiation of SM.