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

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OFDMA Synch. Techniques

Introduction

AIMS:

High-speed data transmission in mobile

communications with multiple users

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OFDMA Synch. Techniques

Introduction: Context

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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

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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

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Orthogonal Frequency Division Multiplexing

Frequency Division Multiplexing AccessOrthogonal Frequency Division Multiple Access

Orthogonal Frequency Division Multiple Access:

OFDMA Synch. Techniques

Introduction: OFDMA (II)

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OFDMA Synch. Techniques

Introduction: OFDMA (III)

Advantages:

Disadvantages

Sub-carriers attenuated

Frequency Selectivity Fading

=

• Frequency Offset

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• 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

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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

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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)

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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) 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

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OFDMA Synch. Techniques

Linear Least Squares

Criteria

Definition AWGN

Solution

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OFDMA Synch. Techniques

Least Squares2 identical and successive Frames

constant

linearisation

nonlinearity Taylor Series

Mean Squared Error

MSE

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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

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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

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

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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

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OFDMA Synch. Techniques2007 - 2008

Project S5:

Any question

?

OFDMA Synchronisation Techniques

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