1. Month Year
doc.: IEEE yy/xxxxr0
January 2008
Midamble Aided OFDM Performance Analysis in High Mobility Vehicular Channel
Date:
Authors:
Hanbyeog Cho (ETRI)
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
January 2008
Abstract
We present the Mid-amble based approach for the channel estimation and its performance analysis of OFDM signal in high mobility vehicular radio channel. WAVE system experiences outdoor time-variant and frequency- selective fading channel. It means that channel coefficients will change with time in the vehicular environment due to relative motions between vehicles.
The training symbol is assigned to a particular OFDM symbol, which is sent periodically in time domain. Mid-amble simply reuses the long training sequence of preamble. The number of data symbols between two Mid-ambles is varied.
The preamble is used for packet detection and channel estimation. But time-variant channel due to the user mobility may cause some errors in the progress of compensation. If tracking of the time-varying channels and updating of the channel estimation are not provided, the performance can be degraded dramatically.
Proposed scheme utilizes the preamble to the initial channel estimation and the Mid-amble to update the channel coefficients for the next symbol until a new Mid-amble is received.
Hanbyeog Cho (ETRI)
John Doe, Some Company

3. Contents Objective and Approach Channel estimation Proposed Algorithm
January 2008
Contents
Objective and Approach
Channel estimation
Proposed Algorithm
Simulation Results
Conclusion
Hanbyeog Cho (ETRI)
Slide 3

4. Objectives Time-variant character of channel in WAVE(IEEE 802.11p).
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
January 2008
Objectives
Time-variant character of channel in WAVE(IEEE p).
Channel coherence time ,which is the maximum packet length duration without being distorted during the transmission.*
In IEEE p, the maximum information data size transferred from MAC layer is about 4000 bytes.
Current training sequences, at beginning of each packet, are not sufficient to estimate the channel correctly for a long packet.
**fd : Doppler spread
ex) velocity v = 200km/h, carrier frequency fc = 5.9GHz : fd = 1kHz => Tc = 423μs
* T.S.Rappaport, Wireless Communications: Principle and Practice, Chs.5, Upper Saddle River, NJ: Prentice-Hall, 1996.
Hanbyeog Cho (ETRI)
Slide 4
Page 4
John Doe, Some Company
John Doe, Some Company

5. Month Year
doc.: IEEE yy/xxxxr0
January 2008
Approaches
The time-varying channel creates the different channel condition between the preamble and the behind of the transmission packet.
In the case of long-sized packet transmission, preamble based if channel estimation may occur critical errors.
So, tracking the time-varying channel and updating the channel estimation are needed.
We propose midamble aided transmission scheme for WAVE.
In the case of long-sized packet transmission,
if receiver estimate a channel with only preamble,
it may occur the critical estimate errors.
Hanbyeog Cho (ETRI)
Slide 5
John Doe, Some Company

6. Channel estimation Recovers data which is corrupted by channel.
January 2008
Channel estimation
Recovers data which is corrupted by channel.
The channel frequency response (CFR) is calculated by using the known training sequences or pilot signals.
yn= xn * hn + wn
Y(k) = FFT(yn )
Y(k) = X(k)H(k) + W(k), 0≤ k ≤ N-1
yn : received signal
xn : transmit signal
hn : channel impulse response
wn : AWGN
N : FFT length
: Channel frequency response
Hanbyeog Cho (ETRI)
Slide 6

7. Channel estimation Conventional method(only preamble) January 2008
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
January 2008
Channel estimation
Conventional method(only preamble)
channel estimated by preamble at the beginning of the packet.
it compensates every data symbol in a packet.
OFDM Training Structure (WAVE)
Preamble
Data
···
Channel
estimation
Equalizer
Channel Estimation Method
Hanbyeog Cho (ETRI)
Page 7
John Doe, Some Company
John Doe, Some Company

8. Proposed Algorithm ∙∙∙ January 2008 OFDM Training Structure
Midamble
Data subcarrier
Pilot subcarrier
∙∙∙
OFDM symbol(time)
OFDM subcarrier(freq.)
Preamble(long training symbol)
Midamble
DATA
Hanbyeog Cho (ETRI)
Slide 8

9. Proposed Algorithm Proposed method (preamble + midamble) January 2008
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
January 2008
Proposed Algorithm
Proposed method (preamble + midamble)
Preamble :initial channel estimation
Midamble : channel estimation(tracking) & update the equalizer coefficients
Preamble
Data
···
Midamble
Channel
estimation
Equalizer
tracking
& Update
N symbol
Channel Estimation Method
Hanbyeog Cho (ETRI)
Slide 9
Page 9
John Doe, Some Company
John Doe, Some Company

10. Simulation Description
January 2008
Simulation Description
Used MATLAB
WAVE(802.11p) and specs
Differential modulation (QPSK, 16QAM)
Simulated 1000-byte packet length
Interval between two Midmables : 4(16QAM) and 10(QPSK)
No. of Paths = 4
Velocity : 50, 100, 150 and 200km/h
Result : BER vs. SNR
Hanbyeog Cho (ETRI)
Slide 10

11. Simulation Block Diagram
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
January 2008
Simulation Block Diagram
In this simulation model, channel coding is not considered and perfect synchronization is assumed.
Channel
coding
Modulation
IFFT
Add GI
decoding
De-modulation
FFT
Remove
Equalizer
Estimation
BER
calculation
Synchronization +
AWGN
Input Data
Hanbyeog Cho (ETRI)
Slide 11
Page 11
John Doe, Some Company
John Doe, Some Company

12. Simulation Results Performance analysis January 2008
varying the packet size
conventional method(only preamble) : packet length increasing  BER increasing
Hanbyeog Cho (ETRI)
Slide 12

13. Simulation Results Performance analysis January 2008
varying the velocity
differential tracking method : Pilot vs. Midamble
Hanbyeog Cho (ETRI)
Slide 13

14. January 2008
Conclusions
We proposed a new frame structure for long packet size transmission.
Conventional scheme is not sufficient to estimate the channel correctly for a long packet.
The proposed scheme improves the performance in comparison with conventional schemes under high mobility environment.
We concluded that the midamble aided transmission scheme suitable solution to support WAVE system.
Hanbyeog Cho (ETRI)
Slide 14