Title: A Survey on Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems
1A Survey on Channel Estimation Techniques Based
on Pilot Arrangement in OFDM Systems
- by
- Mustafa Ergen
- Authors Sinem Coleri Mustafa Ergen
- csinemergen@eecs.berkeley.edu
- Berkeley Web Over Wireless Group
- University of California Berkeley
2Outline
- Motivation for OFDM
- OFDM System Architecture
- Channel Estimation Techniques
- Performance Analysis
- Conclusion
3Motivation for OFDM
- Disadvantages of FDMA
- Bad Spectrum Usage
- Disadvantages of TDMA
- Multipath Delay spread problem
4OFDM 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
5Multipath 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
6OFDM 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.
7OFDM 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.
8Benefits of OFDM
- Higher data rates
- Overlap of subcarriers
- Lower bandwidth than spread spectrum.
- High spectral efficiency
- Lower multipath distortion
- Usage of cyclic prefix
9Our 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
10System Architecture1
11System 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
12Pilot Arrangement
- Block Type
- All subcarriers reserved for pilots wit a
specific period - Comb Type
- Some subcarriers are
- reserved for pilots for each symbol
13Channel Estimation @BlockType
LS estimate
MMSE estimate
14Channel Estimation @ BlockType
15Channel 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
16Interpolation @ CombType
- Linear Interpolation
- Second Order Interpolation
- Low pass Interpolation
- Spline Cubic Interpolation
- Time Domain Interpolation
17Interpolation @ 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
18OFDM Setup
19Channels
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
20Simulation1
21Simulation2
22Simulation3
23Simulation4
24Simulation5
25Simulation6
26Simulation7
27Conclusion
- 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.