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Synchronization and Cell SearchSynchronization and Cell SearchSeminar LTE: Der Mobilfunk der Zukunft Seminar LTE: Der Mobilfunk der Zukunft

Fabian SchuhUniversity Erlangen-Nrnberg

Chair of Mobile Communications

20. January 2010

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Outline Outline

1 Motivation

2 Theory of Zadoff Chu sequences

3 PSS: Primary Synchronization Signal

4 SSS: Secondary Synchronization Signal

5 RS: Cell-specic reference signals

6 Concept: Cell Search procedure

7 Summary

Fabian SchuhFabian Schuh Synchronization and Cell Search Synchronization and Cell Search1/401/40

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MotivationMotivation

Outline Outline

1 Motivation






7 Summary


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Why synchronizeSynchronous frame structure in time domainOFDM systems are sensitive to time and frequencysynchronization errors

What else to doFind the strongest signalIdentify the cell correctly

The full synchronization and cell identication (cell search)procedure needs to be done as fast as possible.


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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences

Outline Outline

1 Motivation






7 Summary


fff ff f

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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Denition of ZC sequencesDenition of ZC sequences

Denition of Zadoff Chu sequences Denition of Zadoff Chu sequences

Denition

ZC N,M [k] =exp j Mk

2

N for N integer even

exp j Mk (k +1)N for N integer odd

For k = 0 , 1, 2, . . . , N 1 and root index M relative prime to N (No common devider, i.e. M = N 1). M denotes the sequencefamily.Zadoff-Chu sequences are also known as generalized chirp-likesequences.


Th f Z d ff ChTh f Z d ff Ch P i f ZCP i f ZC

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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Properties of ZC sequencesProperties of ZC sequences

Properties of ZC sequences Properties of ZC sequences

Property 1 A ZC has a constant amplitude. ( limited PAR)Property 2 ZC sequences of any length have ideal cyclicAuto-Correlation.

kk ( ) =

N ZC 1

n =0a k (n )ak (n + ) = ( )

Property 3 Two Sequences with M 1 and M 2 have constant cyclicCross-Correlation if M 1 is relative prime to M 2. This

value (i.e. 1 N ZC ) achieves the minimum

Cross-Correlation for any two sequences with idealAuto-Correlation.


Th f Z d ff ChTheor of Zadoff Ch seq ences E lE ample seq ence

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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Example sequenceExample sequence

Example sequence Example sequence

0 10 20 30 40 50 60 1 , 5

1

0 , 5

0

0 , 5

1

1 , 5

L = 63 , M = 1



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Example sequence Example sequence

0 10 20 30 40 50 60 1 , 5

1

0 , 5

0

0 , 5

1

1 , 5

L = 63 , M = 1


Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Auto-CorrelationAuto-Correlation

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Auto-Correlation M = 29 (time offset)Auto-Correlation M = 29 (time offset)

0 10 20 30 40 50 60

1

0

1

L = 63 , M = 29

S e q u e n c e

Offset: 0

60 40 20 0 20 40 60

0

0 , 5

1

L = 63 , M = 29

A u t o - C o r r e

l a t i o n Offset: 0



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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Auto CorrelationAuto Correlation

Auto-Correlation M = 29 (time offset)Auto-Correlation M = 29 (time offset)

0 10 20 30 40 50 60

1

0

1

L = 63 , M = 29

S e q u e n c e

Offset: 0 . 5

60 40 20 0 20 40 60

0

0 , 5

1

L = 63 , M = 29

A u t o - C o r r e

l a t i o n Offset: 0 . 5


Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Fourier DualityFourier Duality

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Theory of Zadoff Chu sequencesTheory of Zadoff Chu sequences Fourier DualityFourier Duality

Fourier Duality Fourier Duality

Fourier DualityThe DFT of a ZC sequence xu [k] is a weigthed cyclicly-shifted ZC sequence X w [k] such that

w =

1u mod N ZC .

So, . . .ZF sequences can be generated directly in frequency domainwithout any DFT operation.

A synchronization by correlation may be done in frequencydomain and/or in time domain.


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal

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y y gy y g

Outline Outline

1 Motivation






7 Summary


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal IntroductionIntroduction

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y y g

IntroductionIntroduction

Time & Frequency Synchronization in LTETo be synchronized

1 OFDM symbol timing2 Coarse carrier frequency offset3 Fine carrier frequency offset4 LTE frame begin (BOF)

1 and 2 will be estimated in frequency domain using the PSS.

3 and 4 are retrieved from time domain using both PSS and SSS.


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal Primary Synchronization SignalPrimary Synchronization Signal

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Primary Synchronization Signal Primary Synchronization Signal

Zadoff Chu Sequences in LTEZC used for Primary Synchronization Signal (PSS) SequencesLength: 63 (no DC)ZC-Roots: M = {29, 34, 25}PSS Sequences mapped to subcarriers

...... ......

......

DC

c 31c 30c 29c 28c 27c 26c 3c 2c 1c 0 c32 c33 c34 c35 c36 c59 c60 c612 c62

-1-2-3-4-5 0 1 2 3 4 5 59 60-61 -60 -28-29-30-31-32 28 29 30 31 32

128 Subcarriers

ZC-Sequence:


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal Correlation resultsCorrelation results

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Auto-Correlation M = 29Auto-Correlation M = 29

60 40 20 0 20 40 60

0

0 , 5

1

L = 63 , M = 29

A u t o - C

o r r e l a t i o n

60 40 20 0 20 40 60

0

0 , 5

1

L = 63 , M = 29

A u t o - C

o r r e

l a t i o

n w

i t h o u t D C


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal Time shift and Frequency offsetTime shift and Frequency offset

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Sensitivity CCF against frequency offset I Sensitivity CCF against frequency offset I M = 29 , N ZC = 63


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal Time shift and Frequency offsetTime shift and Frequency offset

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Sensitivity CCF against frequency offset II Sensitivity CCF against frequency offset II M 1 = 25, M 2 = 29, N ZC = 63


PSS: Primary Synchronization SignalPSS: Primary Synchronization Signal Time & Frequency Synchronization procedureTime & Frequency Synchronization procedure

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Synchronization procedure I Synchronization procedure I

Synchronization methods

PSS Maximum Likelihood correlation based estimationCyclic prex correlation based tracking loop



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Synchronization procedure II Synchronization procedure II

A maximum likelihood (ML) detector nds the timing offset mM that corresponds to the maximum correlation:

mM = arg maxm

N 1

i=0

Y [i + m ]S M [i]2

i : time indexm : timing offsetN : PSS time domain signal length

Y [i]: received signalS M [i]: PSS signal with root M



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Synchronization procedure III Synchronization procedure III

A cyclic prex correlation based tracking loop autocorrelates thereceived signal giving a match on the cyclic prex position.The maximum likelihood estimates the timing offset.

mM = arg maxm {2| (m )| (m )}

(m ) =m + L +1

k = m

r (k)r(k + N ) (Correlation)

(m

) =

m + L +1

k = m |r(k

)|2

+ |r(k

+ N

)|2

(Energy)

The cyclic prex based method remains uneffected by the presenceof high CFO.


SSS: Secondary Synchronization SignalSSS: Secondary Synchronization Signal

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Outline Outline

1 Motivation






7 Summary


SSS: Secondary Synchronization SignalSSS: Secondary Synchronization Signal IntroductionIntroduction

d

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IntroductionIntroduction

Sector and Cell SearchWhen a terminal powers on in a cellular system, it needs to perform cell search to acquire its frequency reference,frame timing, and the fast Fourier transform (FFT)symbol timing with the (best) cell, and also to identify the cell-ID .

Cell Area

Omni Cell

Cell Area

Site (3 sectors)

3 sector Site


SSS: Secondary Synchronization SignalSSS: Secondary Synchronization Signal Individual Cell InformationIndividual Cell Information

d d l ll fI di id l C ll I f i

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Individual Cell InformationIndividual Cell Information

sector-ID N s Each sector can be identied using the PSS. Onephysical cell consists of three sectors.

group-ID N g A numerical classication. 168 are dened by 3GPP.cell-ID N c Identication of the best possible serving cell.

Required for extracting the reference signal. Isdened as N c = 3N g + N s and thus depends onsector-ID and group-ID.


SSS: Secondary Synchronization SignalSSS: Secondary Synchronization Signal Secondary Synchronization SignalSecondary Synchronization Signal

S d S h i i Si lIS d S h i i Si l I

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Secondary Synchronization Signal I Secondary Synchronization Signal I

Interleaved PN SequencesBased on two maximum-length sequences (pseudo number)Generated by linear feedback shift registerTwo sequences s0 and s1 with length 31Both sequences depend on m0 and m1m 0 and m1 identify the group-ID N g (from 0 to 167)Interleaved in frequency domain



S d S h i i Si lIIS d S h i ti Si l II

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Secondary Synchronization Signal II Secondary Synchronization Signal II Interleaving:

...

...

...

...

s 0

( n

)

s 1 (

n )

I n t e r l e a v e

d P N

s e q u e n c e



S d S h i ti Si lIIIS d S h i ti Sig l III

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Secondary Synchronization Signal III Secondary Synchronization Signal III

First scramblingInterleaved PN sequences are scrambledBase scramble code: c[n ]Construct c0[n ] and c1[n ] as cyclic shifted versions of c[n ]Shift value depends on the sector-ID N s

Second scramblingConstruct zm 01 [n ] and z

m 11 [n ] (cyclic shifts of c[n ])

Shifts depend on group-ID N gScrambling only in odd entries of the S-SCH to enabledetection of beginning of radio frame



S d S h i ti Sig lIVSecondary Synchronization Signal IV

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Secondary Synchronization Signal IV Secondary Synchronization Signal IV

Secondary Synchronization Signal

d[2n ] =s m 00 [n ]c0[n ] in subframe 0s m 11 [n ]c0[n ] in subframe 5

d[2n + 1] = sm 11 [n ]c1[n ]zm 01 [n ] in subframe 0

s m 00 [n ]c1[n ]zm 11 [n ] in subframe 5

NoteThe S-SCH is different for each subframe index (0 or 5).



Revision: Reference signalsRevision: Reference signals

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Revision: Reference signals Revision: Reference signals

0 1 2 3 4 5 6 7 8 9

radio frame = 10 ms

0 1 2 3 4 5 6 7 8 9 10 11 12 13

PSS (Primary Synchronization Signal)

SSS (Secondary Synchronization Signal)

RS (Reference Symbol)


RS: Cell-specic reference signalsRS: Cell-specic reference signals

OutlineOutline

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Outline Outline

1 Motivation






7 Summary


RS: Cell-specic reference signalsRS: Cell-specic reference signals

Cell specic reference signalsCell specic reference signals

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Cell-specic reference signals Cell-specic reference signals

Cell-specic reference signal P [n ]

Complex-valued sequence P [n ]use of a Gold sequence length 31Initial value cinit of the Gold generatedcinit depends on the cell-ID

P [n ] = 1 2 (1 2 c[2n ]) + j 1 2 (1 2 c[2n + 1])cinit = 2 10

(7

(n s + 1) + l + 1)

(2

N c + 1) + 2

N c + N CP

N c is the cell-IDn resource element indexn s to be the slot-indexN CP = 0 if extended CP, else 0


Concept: Cell Search procedureConcept: Cell Search procedure

OutlineOutline

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Outline Outline

1 Motivation






7 Summary




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Concept: Cell Search procedure Concept: Cell Search procedure

Synchronization

Primary Signaldetection

Secondary Signaldetection

CellConrmation

Synchronization

N o c e l l

- I D

c o n

r m a t i o n

a f t e r s e v e r a

l s u b f r a m e s

N o c e l l

- I D a f t e r

s e v e r a

l i t e r a t i o n s

Symbol timingCFO (fractional)

Sector-ID

Group-ID Cell-IDCFO (integer), BOF

Successfull Synchronizationand cell search

SynchronizationCross-Correlate received signal withthe three possible PSS sequencesChoose the strongest sector by

maximizing the correlation resultsThe time relative position of thecorrelation peak gives the timeoffset to the radio frame begin(with uncertainty between rst and sixthsubframe)




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Synchronization



CellConrmation


N o c e l l

- I D

c o n

r m a t i o n


l s u b f r a m e s

N o c e l l

- I D a f t e r

s e v e r a



Sector-ID



Primary Signal detectionThe knowledge of the right ZCsequences indicates the sector-ID:

N s = arg max

i[n ]




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Synchronization



CellConrmation


N o c e l l

- I D

c o n

r m a t i o n


l s u b f r a m e s

N o c e l l

- I D a f t e r

s e v e r a



Sector-ID



Secondary Signal detectionGroup-ID can be jointly estimatedwith the integer CFO and thesubframe index (0 or 5)

Exploit the shifts of s0[n ] and s1[n ](respectively m0 and m1)The group-ID N g can be identiedthrough m0 and m1

Cell-ID gets: N c = 3N g + N s




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Synchronization



CellConrmation

CellConrmation

N o c e l l

- I D

c o n

r m a t i o n


l s u b f r a m e s

N o c e l l

- I D a f t e r

s e v e r a



Sector-ID



Cell ConrmationCalculate the estimated ReferenceSignalVerify the cell-ID by a

Cross-Correlation with the receivedcell-specic Reference SignalCross-Corrleation in the frequencydomain

If peaks in correlation arise Synchronization and Cell searchsuccessful


SummarySummary

OutlineOutline

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Outline Outline

1 Motivation






7 Summary


SummarySummary

Summary Summary

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yy

SummaryZadoff-Chu Sequences with ideal cyclic auto-correlationFourier Duality of Zadoff-Chu Sequences

Primary Synchronization Signal and ZC-MappingSecondary Synchronization Signal with Interleaving andScramblingConcept procedure for LTE synchronization


LiteratureLiterature

Outline Outline

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1 Motivation






7 Summary



Literature I Literature I

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[FZH62] Frank, R. ; Zadoff, S. ; Heimiller, R.:Phase shift pulse codes with good periodic correlationproperties (Corresp.).In: IRE Transactions on Information Theory 8 (1962),

Oktober, Nr. 6, S. 381382.http://dx.doi.org/10.1109/TIT.1962.1057786 .DOI 10.1109/TIT.1962.1057786

[Koc] Koch, Prof. W.:LTE-Downlink Physical Layer for FDD



Literature II Literature II
http://dx.doi.org/10.1109/TIT.1962.1057786http://dx.doi.org/10.1109/TIT.1962.1057786

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[LH07] Li, C.-P. ; Huang, W.-C.:A Constructive Representation for the Fourier Dual of the ZadoffChu Sequences.53 (2007), November, Nr. 11, S. 42214224.http://dx.doi.org/10.1109/TIT.2007.907336 .

DOI 10.1109/TIT.2007.907336[MGEJ+ 09] Manolakis, K. ; Gutierrez Estevez, D. M. ; Jungnickel,

V. ; Xu, Wen ; Drewes, C.:A Closed Concept for Synchronization and Cell Search

in 3GPP LTE Systems.In: Proc. IEEE Wireless Communications and Networking Conference WCNC 2009 , 2009, S. 16



Literature III Literature III
http://dx.doi.org/10.1109/TIT.2007.907336http://dx.doi.org/10.1109/TIT.2007.907336http://dx.doi.org/10.1109/TIT.2007.907336

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[Myu08] Myung, Hyung G.:Technical Overview of 3GPP LTE .http://hgmyung.googlepages.com/3gppLTE.pdf .Version: May 2008

[Pop92] Popovic, B. M.:Generalized chirp-like polyphase sequences withoptimum correlation properties.38 (1992), Nr. 4, S. 14061409.http://dx.doi.org/10.1109/18.144727 . DOI 10.1109/18.144727. ISSN 00189448



Literature IV Literature IV
http://hgmyung.googlepages.com/3gppLTE.pdfhttp://dx.doi.org/10.1109/18.144727http://dx.doi.org/10.1109/18.144727http://hgmyung.googlepages.com/3gppLTE.pdf

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[SHRC09] Shim, Myung J. ; Han, Jung S. ; Roh, Hee J. ; Choi,Hyung J.:A frequency synchronization method for 3GPP LTEOFDMA system in TDD mode.In: Proc. 9th International Symposium on

Communications and Information Technology ISCIT 2009 , 2009, S. 864868

[STB09] Sesia, Stefania ; Touk, Issam ; Baker, Matthew:LTE, The UMTS Long Term Evolution: From Theory

to Practice .Wiley Publishing, 2009. ISBN 0470697164, 9780470697160



Literature V Literature V

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[TZ07] Tsai, Yingming ; Zhang, Guodong:Time and Frequency Synchronization for 3GPP LongTerm Evolution Systems.In: Proc. VTC2007-Spring Vehicular Technology Conference IEEE 65th, 2007. ISSN 15502252, S. 17271731

[TZGO07] Tsai, Yingming ; Zhang, Guodong ; Grieco, D. ;Ozluturk, F.:Cell search in 3GPP long term evolution systems.2 (2007), Nr. 2, S. 2329.http://dx.doi.org/10.1109/MVT.2007.912929 . DOI 10.1109/MVT.2007.912929. ISSN 15566072



Questions? Questions?
http://dx.doi.org/10.1109/MVT.2007.912929http://dx.doi.org/10.1109/MVT.2007.912929http://dx.doi.org/10.1109/MVT.2007.912929

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Thank you for your attention.

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