Incremental Pilots in OFDM

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Incremental Pilots in OFDM

• Parichaya Shrivastava

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What will we discuss?

• Brief overview of OFDM
• Why use OFDM?
• How does it work?
• Pilots
• Why do we need them?
• Incremental Pilots
• Concept
• What am I trying to do?

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Why use OFDM?

• Higher spectral efficiency
• Lower multi-path distortion

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• Higher spectral efficiency

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• Example of an OFDM modem

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• Multipath problem

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• A Multipath solution

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• The Multipath solution

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• Example of an OFDM modem

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OFDM challanges

• Synchronization requirements are much more strict
  
• Requires more complex algorithms for time /
  frequency synch
• Peak-to-average power ratio
• Large signal peaks require higher power
  amplifiers
• Amplifier cost grows nonlinearly with required
  power

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• Pilot based OFDM system model

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• Main equation

Channel estimation equation
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• Frequency domain channel estimation

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• Pilot allocation 1

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• Pilot allocation 2

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Why Incremental Pilots?

• Some channels (e.g. where there is more frequency
  selective fading), increased number of pilots
  will help.
• More pilots
• Interpolation error small -gt Better channel
  estimation -gt Better BER

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• Transmitter with channel encoder

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Incremental pilots trade-off
Loss in redundancy acceptable
Better CE Better BER
Less redundancy Worse BER
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Loss in redundancy. Crude figure
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Work to do

• See the performance under different channel
  conditions
• Different coding schemes (Convolutional, Turbo,
  LDPC)
• Complexity of implementing higher coding rates
  for different coding schemes

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• References
• Eric Phillip Lawrey, Adaptive Techniques for
  multiuser OFDM Phd Thesis. Online
  http//www.skydsp.com/resources/OFDM_thesis_lawrey
  .PDF
• Marina Mondi, Orthogonal Frequency Division
  Multiplexing/Modulation OFDM. Online.
• Keithley Corporation, An Introduction to
  Orthogonal Frequency Division Multiplex
  Technology. Online http//www.ieee.li/pdf/viewgr
  aphs/introduction_orthogonal_frequency_division_mu
  ltiplex.pdf
• Muhammad Saad Akram, Pilot-based Channel
  Estimation in OFDM Systems . Online
  http//www2.imm.dtu.dk/pubdb/views/edoc_download.p
  hp/5405/pdf/imm5405.pdf
• Sung-Yoon Jung and Sunghwan Kim, Channel
  Estimation and LDPC Code Puncturing Schemes
  Based on Incremental Pilots for OFDM. ETRI
  Journal, Volume 32, Number 4, August 2010

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Thank you!