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
• csinemergen_at_eecs.berkeley.edu
• 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

• Multi-path 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
• Multi-path delay spread effect
• Inter-Symbol Interference (ISI) when the delayed
  multipath signal overlaps with the symbols
  following it

6
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 high-speed serial information signal into
  multiple lower-speed sub-signals.
• Transmits simultaneously at different frequencies
  in parallel.
• Modulation ( BPSK PSKQPSK16QAM ).
• Pilot subcarriers used to prevent frequency and
  phase shift errors.

8
Benefits of OFDM

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

9
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
  time-invariant over the symbol interval.
• Rectangular Windowing of the transmitted pulses
• Perfect Synchronization of transmitter and
  receiver
• Additive white Gaussian channel noise

10
System Architecture-1
11
System Architecture-2
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
12
Pilot Arrangement

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

13
Channel Estimation _at_Block-Type
LS estimate
MMSE estimate
14
Channel Estimation _at_ Block-Type
15
Channel Estimation _at_ Comb-Type Pilot

• Np pilot signals uniformly inserted in X(k)
• LNumber of Carriers/Np
• Hp(k) k01Np channel at pilot
  sub-carriers
• Xp input at the kth pilot sub-carrier
• Yp output at the kth pilot sub-carrier

LMS Estimate
LS Estimate
16
Interpolation _at_ Comb-Type

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

17
Interpolation _at_ Comb-Type
Linear Interpolation
Second Order Interpolation
Low Pass Interpolation (interp in MATLAB)

• Insert zeros into the original sequence
• Low-pass filter while passing original data
  unchanged
• Interpolation such that mean-square error between
  ideal and interpolated values min.

Time Domain Interpolation
Spline Cubic Interpolation (spline in MATLAB)
18
OFDM Setup
19
Channels
ATTC (Advanced Television Technology Center)
and the Grande Alliance DTV laboratorys
ensemble E model
Simplified version of (Digital Video
Broadcasting) DVB-T channel model
Channel 2
Channel 1
Time Varying Channel (AR Model)
20
Simulation-1
21
Simulation-2
22
Simulation-3
23
Simulation-4
24
Simulation-5
25
Simulation-6
26
Simulation-7
27
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.
• Low-pass interpolation performs better since mean
  square error between the interpolated points and
  their ideal values is minimized.