Decision Feedback Equalization in OFDM with Long Delay Spreads

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Decision Feedback Equalization in OFDM with Long
Delay Spreads

• Zeeshan Qureshi

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Digital Video Broadcasting (DVB)

• Rapidly becoming world-wide standard for digital
  TV
• Being implemented in Europe, Australia, South
  Africa and India
• Popularity due to
• Bandwidth efficiency
• Improved picture/sound quality
• Additional programming options
• Utilizes OFDM to transmit over a Single Frequency
  Network (SFN) Model

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SFN Major Issues

• Simultaneous transmission of same OFDM data from
  multiple transmitters
• Receiver interprets a channel with a long Delay
  Spread
• Delay spread longer than Cyclic Prefix causes
  increase in interference components (ISI ICI)
• Performance Degradation Symbol recovery suffers
  due to interference

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Conventional Solutions

• Increase Cyclic Prefix length to Channel Delay
  Spread
• Increases Symbol Overhead
• Degrades OFDM system efficiency
• Equalization in time-domain
• Complex Receiver Design
• Impractical to implement

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Thesis Contribution

• Implementation of a DFE-PIC based receiver
  configuration over OFDM channels with long delay
  spreads
• Investigation of performance gain over
  conventional OFDM receiver
• Evaluation done via simulations
• Allows performance improvement
• Maintains efficiency of OFDM
• Receiver implementation is simple

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Decision Feedback Equalizer (DFE)

• DFE is a non-linear equalizer
• Feedback filter ISI cancellation using previous
  receiver decisions
• Feed-forward filter ICI cancellation on the
  transmitted symbols
• Advantages
• Performance comparable to the optimum demodulator
  but with much lower computational complexity
• Low noise enhancement

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DFE Block Diagram
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Parallel Interference Canceller (PIC)

• PIC detector estimates and subtracts interference
  for each channel in parallel
• Stage-wise implementation
• Stage 0 uses matched filter to estimate symbol
  without removal of interference
• Later stages use the symbol estimates of the
  previous stage to estimate and remove
  interference components
• Advantages
• Fast convergence
• Low complexity

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PIC Block Diagram
Initial Observation
Symbol Estimation (Stages 1)
?0(t)
SN(k)
Z0(k)

_
Z0(k)
ICI(N-1)(k)
Symbol Estimation (Stage 0 Only)
Interference Estimation
S0(k)
S(N-1)(k)
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Simulation Environment

• QPSK Modulation
• OFDM Symbol Transmission
• Rayleigh Channel Model
• Additive White Gaussian Noise (AWGN)
• Channel Delay Spread as long as the OFDM Symbol
  length
• Perfect Channel Estimation in Receiver
• Single-Tap DFE implementation

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Implemented Scenarios

• Response to System Parameter changes
• No. of ISI iterations
• No. of PIC stages
• Scaling of ICI components
• Length of Channel Delay Spread
• Performance in simulated SFN channels
• Inter-site Distance between Transmitters

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Performance Analysis

• Simulated SFN Channel
• ISD 20 Km
• Worst-case condition
• 0.5 CDF

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Final remarks

• Highlights of the DFE-PIC receiver
• Significant performance gain achieved over OFDM
  receiver
• Preserves OFDM system efficiency
• Compensation of interference effects due to long
  delay spread
• Simple to implement in the receiver
• Low computational complexity

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The End

• Questions?
• Thank You!?