A Survey on Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems


1
A Survey on Channel Estimation Techniques Based
on Pilot Arrangement in OFDM Systems

• by
• Mustafa Ergen
• Authors Sinem Coleri Mustafa Ergen
• [e-mail]
• Berkeley Web Over Wireless Group
• University of California Berkeley

2
Outline

• Motivation for OFDM
• OFDM System Architecture
• Channel Estimation Techniques
• Performance Analysis
• Conclusion

3
Motivation for OFDM

• Disadvantages of FDMA
• Bad Spectrum Usage
• Disadvantages of TDMA
• Multipath Delay spread problem

4
OFDM Use of Frequency Spectrum
Frequency spectrum of the subcarriers

• Efficient use of spectrum
• Overlap in frequency spectrum of subcarriers
• Null point of all other subcarriers at the center
  frequency of any particular subcarrier

5
Multipath Delay Spread

• Multipath delay spread definition
• Time spread between the arrival of the first and
  last multipath signal seen by the receiver.
• Received radio signal consisting of a direct
  signal plus reflections from objects
• Multipath delay spread effect
• InterSymbol Interference ISI when the delayed
  multipath signal overlaps with the symbols
  following it

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OFDM Eliminating ISI

• Cyclic Prefix
• Prepend the last part of the signal to the
  beginning of the signal
• Duration of the CP larger than multipath delay
  spread
• Orthogonality of the carriers not affected.

7
OFDM Overview

• Divides highspeed serial information signal into
  multiple lowerspeed subsignals.
• Transmits simultaneously at different frequencies
  in parallel.
• Modulation BPSK PSKQPSK16QAM .
• Pilot subcarriers used to prevent frequency and
  phase shift errors.

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Benefits of OFDM

• Higher data rates
• Overlap of subcarriers
• Lower bandwidth than spread spectrum.
• High spectral efficiency
• Lower multipath distortion
• Usage of cyclic prefix

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Our OFDM System Assumptions

• Usage of cyclic Prefix
• Impulse response of the channel shorter than
  Cyclic Prefix.
• Slow fading effects so that the channel is
  timeinvariant over the symbol interval.
• Rectangular Windowing of the transmitted pulses
• Perfect Synchronization of transmitter and
  receiver
• Additive white Gaussian channel noise

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System Architecture1
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System Architecture2
Input to Time Domain
1
3
2
Guard Interval
Channel
5
4
Guard Removal
Output to Frequency Domain
7
6
Output
Channel Estimation
ICI
AWGN
Channel
Estimated Channel
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Pilot Arrangement

• Block Type
• All subcarriers reserved for pilots wit a
  specific period
• Comb Type
• Some subcarriers are
• reserved for pilots for each symbol

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Channel Estimation @BlockType
LS estimate
MMSE estimate
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Channel Estimation @ BlockType
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Channel Estimation @ CombType Pilot

• Np pilot signals uniformly inserted in Xk
• LNumber of Carriers/Np
• Hpk k01Np channel at pilot
  subcarriers
• Xp input at the kth pilot subcarrier
• Yp output at the kth pilot subcarrier

LMS Estimate
LS Estimate
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Interpolation @ CombType

• Linear Interpolation
• Second Order Interpolation
• Low pass Interpolation
• Spline Cubic Interpolation
• Time Domain Interpolation

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Interpolation @ CombType
Linear Interpolation
Second Order Interpolation
Low Pass Interpolation interp in MATLAB

• Insert zeros into the original sequence
• Lowpass filter while passing original data
  unchanged
• Interpolation such that meansquare error between
  ideal and interpolated values min.

Time Domain Interpolation
Spline Cubic Interpolation spline in MATLAB
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OFDM Setup
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Channels
ATTC Advanced Television Technology Center
and the Grande Alliance DTV laboratorys
ensemble E model
Simplified version of Digital Video
Broadcasting DVBT channel model
Channel 2
Channel 1
Time Varying Channel AR Model
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Simulation1
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Simulation2
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Simulation3
23
Simulation4
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Simulation5
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Simulation6
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Simulation7
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Conclusion

• OFDM System
• Block Type
• Direct or Decision Feedback
• Comb Type
• LS or LMS estimation at pilot frequencies
• Interpolation Techniques
• Linear
• Second Order
• Low Pass
• Spline
• Time Domain
• Modulation
• BPSKQPSK16QAMDQPSK
• Results
• Comb Type performs better since it tracks fast
  fading channels.
• Lowpass interpolation performs better since mean
  square error between the interpolated points and
  their ideal values is minimized.