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OFDM Synchronization Using PN Sequence and ItsPerformance

Chunlii i Yan, Jiayi Fang, Youxi Tang, a n d Shaoqian LiNational key Lab of CommunicationsUniversity of Electronic Science and Technology of ChinaChengdu, 610054 , China

Email: ycl@uestc.edu.cn

Yingtao LiWireless R&D Center

Huawei Co.Shenzhen, 518057 , ChinaEmail: Liyingtao@huawei.com

,4b st rr ~- ln th is paper, we propose a novel method fororthogonal frequency-division multiplexing (OFD M) time andfrequency synchronization by using PN sequence and barkercode. Time synchronization performance is improved by barkercode, and we also propose a simple but effective method by settingthe threshold during time synchronization. Improved frequencys) nchronization algorithm is presented in this paper too. T he ideaof multipath utilizing is the core o f the frequency synchronizationalgorithm. The performance of the synchronization proposedhere is better than that of conventional methods. Estimationprecision is boosted from LO- to for moderate S N R . Therequirements of time and frequency synchronization for Beyond3G system based on OFDM can be well satisfied while using theproposed method.

I . I N T R O D U C T I O NThe trend of modern mobile communication is to achievefaster transmission rate, better transmission quality, higherspectrum efficiency. and larger system capacity. Orthogonalfrequency-division inultiplexing (OFDM) is known to m eet allthese challenges [ I ] and has been successfully used in D A B

[2], D VB [3] an d H I P E R L A N / 2 [4]. However, OFDM is verysensitive to time synchronization error and frequency offset[S I . The performances are degraded dramatically by inter-symbol interference (ISI) because of time synchronizationerror or inter-carriers-interference (ICI) for frequency offset.

Several approaches have been studied for time synchroniza-tion and frequency offset estimation [6]-[I 11 . The methodusing cyclic prefix can estimate time offset and frequencyoffset without bandwidth waste. but its time synchronizationproperty isnt so good and frequency estimation range doesno t exceed 0.5 subcarrier sp acing [ 6 ] .Another method usingrepeated data can accurately estimate the frequency offset, butit reduces data rate and frequency estimation range [5]. Short-ening the repeated data length, estimation range will extend atthe cost of estimation precision reducing. The frequency offsetestimation method proposed by Schmidl and Cox can realizecoarse frequency offset estimation and fine frequency offsetestimation [7], but performance of time synchronization is notgood enough yet and it requires two symbols fo r synchroniza-tion. Tufvesson proposed an approach for time synchronizationand frequency offset estimation by using PN sequence [8].This method can handle the frequency offset estimation withlarge cstimation range, and time synchronization result is

- .

0-7803-7822-9/03/$17.000 003 IEEE. 936

more accurate than that of other methods. In addition, othermethods using virtual subcarriers estimates frequency offsetwith MUSIC [9], [lo] or ESPRIT 1111 algorithm dont needextra data. Although they dont waste bandwidth, they needtime synchronization as prior.In this paper we study the OFDM time synchronization andfrequency offset estimation useing PN sequence. We presenta novel time synchronization method by combining barkercode with PN sequence . Lower false detection probability andlower probability of missing are the obvious advantage ofthe time synchronization method. A new algorithm to realizeOFD M frequency synchronization is also proposed by usingmultipath signals in Rayleigh multipath fadin g cha nnel . So ithas better performance of frequency synchronization than thatof conventional method proposed in [SI.

11 . SYSTEM E S C R I P T I O NWe consider an OFDM system implemented by the inverse

fast Fourier transform an d FFT. Nu subcarriers out of total Nsubcarriers are used to avoid aliasing effect at the edges of thetransmission spectrum. Xk is the modulated data on the kthsubcarrier. After IFFT operation the output i sN..1

n= o , 1 , 2 . . . : N - 1After adding cyclic prefix, an O F D M symbo l can be expressedas { ( c N - N ~ , ...,X N - ~ ,o,x1, ...,Z N - ~ } , the first Ng dada iscyclic prefix used for eliminating ISI.

We can realize OFDM frequency synchronization by usingtraining sequence c [ k ] .The training sequence c [ k ] s definedas

bl[k/] 7 7 2 [ k lllod Nr,],E [o , (Nb+1 )N~n 11m [ k mod N,,] k: E [(Nb+1)fvT,, N - 11(2 )ijk] =Where m[k] is a PN sequence with length N,, b [ k ] is aninverse barker code of length Nb (Nb _< L ) . Figure 1 depictsthe structure of the training sequence and OFDM symbol,

Figure 2 shows how to gen erate the transmitted signals. And

mailto:Liyingtao@huawei.commailto:Liyingtao@huawei.com

7/29/2019 OFDM Sync Using PN Seq

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Fig. 2 . St ruc t u re 04Transmitter

fi g I . l'lie slrticltitc of [raining seqtierice an d OFDM symbol

h1 [k:] is defined as1: : =0h l [ k- 1 .k E [ l , Nb]& b[k - 11 =1' [ 6 ; ] = (3)i b ' [k - ] , k e [ I , Nb] & b [k - ] = -1Adding the training sequence c [ k ] and O F D M symbol( 4 X . I with respective coefficient JiT an d G. or PNsequence and O F D M data which P N sequence is superim-posed on , their S N R becomes pa:/ ((1- ) 0," +0:) an d

(1 - 0 ) U: : / (( 1- ) 0: +0;) respectively. The advantage ofsuperimposing the training sequence on the OFDM data is thatit will not waste the system bandwidth. In AWGN channel, thei-eceived signal is

? j k ]=s [ k ]+1z[k]= (&+[k:] +f i a 3 q k . l ) ? j ( W + @ , + tZ[k] (4)

Wherc d [ k ] is the OFDM data sequence and n [ k ] s whiteGaussian noise with spectral density No. 8, is the carrier phaseand E , is the frequency offset normalized by the subcarrierspacing. The received si'gnal power and noise power areof = E,/T, = E [ l ~ s [ k ] l ' ]nd 0: = NO/Ts = E [ / n [ k ] 1 2 ]respectively.

111. O F D M SYNCIHRONI%AT'ION A L G O R I T H MA , f i n 1e q!mchl-onizrr [ion

The perfect autocorrelation property of P N sequence andgood no-shift-autocoi-l-elation property of barker code can beused for OFDM time synchronization. Performance of the im-proved mcthod is better than Tufvesson's for using the barkercodc. Figire 3 depicts the amplitude of time synchronizationobject function of using barker code or not.

Then 0 =k . Th and N, are the parameters used for setting thethreshold. It can be easy found that the threshold is adaptive.When only part of training sequenc e is synchronized with localPN sequence, formula (6) also is satisfied. A more accurateestimation of 8 becomes

8 =arg max Y [0 ,4 ( 7 )@E[k , k+N, , ,A J , > + l ) ]B. Frequency Synchronization(we call it method I)

The frequency synchronization method proposed in [8] isN,,,-

c, = T [ k+ 2 +P ) f y n ] c *I C ] ( 8 )B = C ~ C ; + ~ ( 9 )

k=OL-P-1

l= ON

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The 14thEEE 2003 InternationalSymposium on Persona1,lndoor and Mobile Radio Communication Proceedings

I / t. . . . . ... .. .... .

F I ~ . I h e amplitude of time synchronization object function. (a) is themethod using PN sequence an d barker code, (b) is the method only using PNseq11ence

16.2(N/(2N,,)=l 6.2). The simulations below were performedwith lo 4 runs. Some of the parameters are Nb=7, N, =127,T,,=40 an d N,,=lO. The Rayleigh channel is modelled by asimple three-path M.1225 model. 4 . Titne Synchronization Performance

The performance of time synchronization for Rayleighchannel is showed in table I while using the time synchro-nization method proposed in this paper. We consider the timesynchronization is right when one of the multipath sign als iscaptured by the estimator. The estimator can resolve multipathof the Rayleigh channel. Also it can capture the first pathsignal of the Rayleigh channel. If we operate the FFT fromthe first path signal but not the powerful path signal, we canget better OFDM performance.for we use most signals in theRayleigh channel (This is the advantage of OFDM systemwith cyclic prefix). This me thod can do this. This is one o f itsadvantages.B. f i e y u e n c , ~ ynchronization Pecformnnce

In A WG N channel, the performance of method I1 is worsethan method I in estimating the frequency offset. But whenS N R is larger than 15 dB, their performance is almost thesame. Figure 4 is the simulation curve in AWGN channel .

Figure 5 shows performance of method I and method I t inRayleigh channel. We can see that the performance of methodI is much worst than that of method [ I in Rayleigh channelbecause method I I uses multipath signals while method 1n o t . In method I only the most powerful signal is usedfor calculating the frequency estimation because of the PNsequences perfect autocorrelation property.Figure 6 shows the BER-SNR curve o f Beyond 3G systemfor Rayleigh channel. It depicts the BER-SNR curve when

, frequency offset is zero with no frequency synchronizationand BER-SNR curve when frequency offset is 11.78 withfrequency synchronization (Method l l ), and the vehicle speed

1 0 - 6L_Io0 15S NR ( d B )

Fig. 4.parameters: N =4096, Nm = 1 2 7 ; c=l0.2 and P=l4 Variance of frequency offset estimation in AWGN channel. the

Fig. 5.parameters: N=4096, N, =127, ~ 1 0 . 2 .U =60km./h an d P= lVariance of frequency offset estimation in Rayleigh channel, the3

is 5Okm/h. The channel estimation is not ideal. Tine systemuses QPSK signal constellation. Th e improved estimator usingPN sequence and barker code can satisfy t h e frequency syn-chronization requirements of Beyond 3G system in Rayleighchannel .

V. CONCLUSIONIn this paper, we propose an improved method for OFDMtime and frequency synchronization by using PN sequenceand barker code. After adding the barker code, the amplitudeof the time synchronization object function of the method is

sharper than before. Th e performance is better than that of theformer. Also we propose a new method setting the threshold.The threshold is adaptive, simple and effective. A simple buteffective frequency sync hronization algorithm is also proposedin this paper. I t can use the multipath signals in the Rayleighchannel, and its performance is much better than that of theconventional methods. By using the proposed method, therequirements of time and frequency syn chronization in Beyond3G system can be well satisfied.

R E F E R E N C E S[I] J.A.C. Bingham, Multicarrier modulation for data transmission: an ideawhose time has come, IEEE Commun. Mag., vol. 28. 110.5, May 1990,pp . 5-14[2] ETS 300 401 Second Edition, 1997. Radio broadcasting systems; DigitalAudio Broadcasting ( D A B ) to mobile, portable and fixed receivers.

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i +requencyOffset is zero, no Synchronizationf [-e Frequency Offset is 11.78, with Synchronization 1L J10 20 30 40SN R (dB)

Fig. 6. BEK-SN R Ierforiiiance o f QPSK system i n Rayleigh channel,frequency offset is zero with no synchronization and frequency offset is 11.78\r ill1 syiichi-onization (Metho d I I ) , zJ=jOkm/h.

I A B L E I~ r H E E R F O R M A N C E OF T I M E SYNCHRONIZATION IN RA Y LEI CHCHANNEL

[ 3 ] U. Keiniers DVB-T: the COFDM-based system for terrestrial television.Electronics & Comm unication Engineerig .loLirnal, Vol. 9 ( I 997), No. 1 ,[4J .IKhun-.lush. P. Schramm. U. Wachsmann, and F. Wenger. Structurea nd performance of the I-IIPERLANR physical layer, in Proc. IEEE

41th Vehiculai. leechnology Con f., vol. 5 , 1999. pp.2667-2671.[ S ] Moose, P.1-I, A technique for oi-thogotial frequency division multiplex-ing freqtiency offset correction. Com niun icati ons, IEEE Transactionson , Volume 42, Issue 10 , Oct. 1994. pp . 2908-2914[ 6 ) . ) - I van de Beek. M . Sandell. and P. 0. Brjesson. ML estimation oftiming and frequency offset it1 OFDM systems, IEEE Trans. SignalProcessing. vol. 45, July 1997, pp . 1800-1805.[7 ] 1 M. Schmidl and D. C. Cox, Robust Frequency and Timing Synchro-ni~a t ioi i or OFDM, I E EE Trans. Commun., Vol. 4 5 , No. 12, December,1997. pp.1613-1621.181 Fredrtk lufvess on, Mike Faulkner an d Ove Edfors, Time and frequencysynchronization foi-OFDM using PN-sequence preambles, Proceedingsof I EEE Vehicular Tcchnology Conference, Amsterdam, The Nether-lands. September 19-22, 1999, pp . 2203-2207191 Biao chen, Maximum likelihood estimation o f OFDM carrier frequencyoffset, IEEE Sihnal Processing Letters, Vol. 9, No 4, April 2002, pp .123-1 26

[ I O ] U. ltireli. D. Kivanc. and H. liu. Experimental and Analytical Studieson a High Resolutioii OFDM Carrier Frequency Estimator, I EE ETransactions 011 Vehicular Technology, Vol. 50 , No. 2. March 2001.p p . 629-643I I ] l l f i ik Tureli, H U i Liu and Michael D.Zoltowsk i, OFD M Blind Carrier1~ t m a t i o n :ESPRIT. IEEE Transactions on Comniunications Vol 48 ,N o . 9, September 2000. pp. 1459-1461

s . pp . 28-32

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