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OFDMA Synch. Techniques
OFDMA Synchronisation Techniques
Students:AMPLIATO, MiguelBEAURIN, BastienGUILLEMIN, MatthieuPOVEDA, HectorSOULIER, Bruno
Supervisor:GROLLEAU, JulieGRIVEL, Eric
2007 - 2008
Project S5:
OFDMA Synch. Techniques
Contents
0) Introduction Context Mobile Telephony (EU) OFDMA
I) Working Principles Prefix Cyclic Frequency Synchronisation
Experimental Work
II) Frequency offset estimation Maximum Likelihood (ML) Linear Least Squares (LS) Extended Kalman Filter (EKF)
III) Conclusion & Further Work
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OFDMA Synch. Techniques
Contents 0) Introduction Context Mobile Telephony (EU) OFDMA
I) Working Principles Prefix Cyclic Frequency Synchronisation
Experimental Work
II) Frequency offset estimation Maximum Likelihood (ML) Least Squares (LS) Extended Kalman Filter (EKF)
III) Conclusion & Further Work
1/17
OFDMA Synch. Techniques
Introduction
AIMS:
High-speed data transmission in mobile
communications with multiple users
2/17
OFDMA Synch. Techniques
Introduction: Context
3/17
Original
Signal
Restrictions:
• Multipath Signal Fading
• Doopler effect Desynchronisation
OFDMA Synch. Techniques
Introduction: Mobile Telephony (EU)
1G - Analogue Technology [1981]: FDMA
2G - Digital Technology [1991]: TDMA
3G - Digital Technology [2001]: W-CDMA
4/17
OFDMA Synch. Techniques
Introduction: OFDMA
OFDM + FDMA = OFDMA
Orthogonal Frequency Division Multiplexing:
0 1 1 1 1 0 0 1
2/T1/T
1 1 1 0
T is the Symbol time
Property that keep a secure distance Orthogonality
5/17
Orthogonal Frequency Division Multiplexing
Frequency Division Multiplexing AccessOrthogonal Frequency Division Multiple Access
Orthogonal Frequency Division Multiple Access:
OFDMA Synch. Techniques
Introduction: OFDMA (II)
6/22
OFDMA Synch. Techniques
Introduction: OFDMA (III)
Advantages:
Disadvantages
Sub-carriers attenuated
Frequency Selectivity Fading
=
• Frequency Offset
7/17
• Synchronisation:
OFDMA Synch. Techniques
Contents
0) Introduction Context Mobile Telephony (EU) OFDMA
I) Working Principles Prefix Cyclic Frequency Synchronisation
Experimental Work
II) Frequency offset estimation Maximum Likelihood (ML) Least Squares (LS) Extended Kalman Filter (EKF)
III) Conclusion & Further Work
OFDMA Synch. Techniques
Why a cyclic prefixe is used?
Prefix Cyclic
Without timing offset With timing offset, IBI avoided
8/17
The canal has 2 effects:
◦ Time offset◦ Frequency offset
The frequency synchronisation has been performed for downlink transmission using:
◦ Maximum of Likelihood ◦ Linear Least Square ◦ Extended Kalman Filter
Frequency synchronisation
OFDMA Synch. Techniques 9/17
OFDMA Synch. Techniques
Experimental Work
Y(1) Y(2) Y(3) Y(4) Y(5)… …Y(N)
Y(1) Y(2) Y(3) Y(4) Y(5)… …Y(N)
Frequency offset +Noise w
ML and Linear LS estimation methods -> 2 identical symbol blocks transmitted
r(1) r(2) r(3) r(4) r(5)… …r(N)
r(1) r(2) r(3) r(4) r(5)… …r(N)
10/17
OFDMA Synch. Techniques
Contents 0) Introduction Context Mobile Telephony (EU) OFDMA
I) Working Principles Prefix Cyclic Frequency Synchronisation
Experimental Work
II) Frequency offset estimation Maximum Likelihood (ML) Linear Least Squares (LS) Extended Kalman Filter (EKF)
III) Conclusion & Further Work
OFDMA Synch. Techniques
r= yH(ℇ)+w AWGN
w=r-yH(ℇ) white gaussian noise
ML maxpw(r/ℇ)
Maximum Likelihood
* Based on Mr. Moose paper
11/17
OFDMA Synch. Techniques
Linear Least Squares
Criteria
Definition AWGN
Solution
12/17
OFDMA Synch. Techniques
Least Squares2 identical and successive Frames
constant
linearisation
nonlinearity Taylor Series
Mean Squared Error
MSE
13/17
Carrier Frequency Offset (CFO) estimation :
Criteria : Minimum Mean Squared Error,
Hypothesis on state equation : e is constant (e n) = (e n-1).
Non linear measurement equation Extended Kalman Filter (EKF) We linearize around e
Advantage 1 data block (length : 32 bits) used for CFO
estimation.
Extended Kalman FilteringMean Squared
Error
MSE
* Based on Mr. P.Zaho, Mr. L.Kuang and J.Lu paper
14/17
OFDMA Synch. Techniques
Contents 0) Introduction Context Mobile Telephony (EU) OFDMA
I) Working Principles Prefix Cyclic Frequency Synchronisation
Experimental Work
II) Frequency offset estimation Maximum Likelihood (ML) Least Squares (LS) Extended Kalman Filter (EKF)
III) Conclusion & Further Work
Achievements : Downlink Synchronization ◦ Carrier Frequency Offset : Maximum likelihood (ML) method, Least Squares (LS), Extended Kalman Filter (EKF).
Results : ML & LS estimation methods : 2 data blocks required, EKF : only 1 data block needed. lower performance in terms of MSE.
We are currently working on this method to remedy this issue
Conclusion
15/17
Channel modelling Timing offset estimation LMS filter RLS filter
EKF improvement Model on e higher MSE expected.
Implement the complete transmission chain : Receiver
Further work
16/17
OFDMA Synch. Techniques2007 - 2008
Project S5:
Any question
?
OFDMA Synchronisation Techniques
17/17