Cell Search Procedure in LTE

JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE 1556-6072/07/$25.00©2007IEEE ||| 23

Cell Search in 3GPP LongTerm Evolution Systems

Yingming Tsai, Guodong Zhang, Donald Grieco, Fatih Ozluturk,

InterDigital Communications Corporation, and Xiaodong Wang, Columbia University

Introduction

In order to keep its technology com-petitive, 3rd Generation PartnershipProject (3GPP) is considering longterm evolution (LTE), in which evolu-

tion of both radio interface and networkarchitecture is necessary. 3GPP LTE sys-tems will provide higher data rate ser-vices with better QoS than the current3G systems. This will require reliable andhigh-rate communications over time-dis-persive (frequency-selective) channelswith limited spectrum and inter-symbolinterference (ISI) caused by multi-pathfading. Orthogonal frequency division

Abstract: This paper reviews and pre-sents the latest results in the cell searchissue of the 3GPP Long Term Evolution(LTE) systems. Cell Search is a basicfunction of any cellular system, duringwhich process time and frequency syn-chronization between the mobile termi-nal and the network is achieved. Suchsynchronization is especially importantfor 3GPP Long Term Evolution systems,which rely heavily on the orthogonalityof the uplink and downlink transmis-sion and reception to optimize theradio link performance. As in conven-tional cellular systems, the mobile ter-minal in an LTE system acquires timeand frequency synchronization by pro-cessing the synchronization channel.Design of the synchronization channelis being developed within the standard-ization activities of 3GPP Long TermEvolution and is still evolving. In thispaper, we present the design considera-tions and various new and promisingdesign concepts for the synchronizationchannel. We evaluate some specificsolutions and provide numerical perfor-mance results.

© DYNAMIC GRAPHICSDigital Object Identifier 10.1109/MVT.2007.912929

Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.

multiple access (OFDMA) provides several advantages,such as high spectral efficiency, simple receiver resign,and robustness in a multi-path environment. Due tothese advantages, OFDMA was chosen as the downlinkair interface of 3GPP LTE systems [1].

When a terminal powers on in a cellular system, itneeds to perform cell search to acquire its frequency ref-erence, frame timing, and the fast Fourier transform (FFT)symbol timing with the (best) cell, and also to identify thecell ID. In order to obtain good cell search performance,an appropriate synchronization channel structure needsto be designed.

We start in this article by briefly describing theOFDMA air interface. The design considerations of thesynchronization channel are then discussed, and severalpotential synchronization channel design solutions (syn-chronization symbol structures and correspondingsequences) for 3GPP LTE system are presented. Cell

search procedures are described, and several timing andfrequency offset detection methods are presented. Perfor-mance results of different primary synchronization chan-nel design solutions are simulated and compared.

System Description and Design Considerations

The diagram of the downlink OFDMA air interface isshown in Figure 1. In the OFDMA system, modulated bitsare converted from serial to parallel first, and thenmapped to different subcarriers. After IFFT, the outputsignals are converted back to serial signals called anOFDM symbol. Cyclic prefix (CP) is attached to the begin-ning of the OFDM symbol before transmission. Subcarrierspacing of 15 kHz is used in the 3GPP LTE system.

As in UMTS systems, the cell search in 3GPP LTE sys-tems will enable the terminal to obtain frame and symboltiming, frequency offset and the cell ID. However, cellsearch in 3GPP LTE systems has to consider multipletransmission bandwidths (UMTS has a fixed bandwidth of5MHz, while 3GPP LTE systems support 1.25, 2.5, 5, 10, 15and 20 MHz bandwidths). Moreover, cell search proce-dure in 3GPP LTE systems should be completed with lowprocessing complexity at the terminal and within a muchshorter time than that in UMTS systems. All of theserequirements are expected to be fulfilled with systemoverhead on par with UMTS systems.

It is desirable to define a synchronization chan-nel that is common to all cells in the system irre-spective of the bandwidth being used in the cell,since this will yield faster cell search and lower com-plexity. Therefore, it is agreed that the synchroniza-tion channel should be transmitted using the central1.25 MHz bandwidth regardless of the entire band-width of the system [1]. In this way, the same syn-chronization channel is mapped to the central partof transmission bandwidth for all system band-widths. The central 1.25 MHz corresponds to 76 sub-carriers with subcarrier spacing of 15 kHz.

The downlink frame structure of the 3GPP LTEsystem is shown in Figure 2. Each radio frame (10ms) is divided into 10 sub-frame of 1 ms. Each sub-frame consists of 2 slots. There are 7 OFDM symbolper slot. There are two kinds of synchronizationchannels (SCH): primary SCH (P-SCH) and sec-ondary SCH (S-SCH). P-SCH and S-SCH symbols aretime division multiplexed. Each radio frame con-tains two equal-spaced pairs of P-SCH and S-SCHsymbols. For coherent detection of S-SCH symbols,P-SCH and S-SCH symbols are placed adjacent toeach other in the last two OFDM symbols of thefirst slot within a sub-frame.

Cell search in the WCDMA based UMTS systemrelies mainly on time domain processing to achievelow receiver complexity and efficient hardwareimplementation. In order to provide good timing

FIGURE 1 OFDMA air interface in 3GPP LTE systems.

FIGURE 2 Downlink frame structure of 3GPP LTE systems.

24 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007

LONG TERM EVOLUTION INVOLVES CHANGESTO BOTH RADIO INTERFACE AND NETWORKARCHITECTURE IN ORDER TO KEEP 3RDGENERATION PARTNERSHIP PROJECTTECHNOLOGY COMPETITIVE.


detection performance, the synchronization sequence inUMTS systems should have very good auto-correlation.Due to this property, the Golay sequence was chosen asthe synchronization sequence for UMTS systems. For3GPP LTE systems, the synchronization sequence ismapped to the central band of entire bandwidth due tothe OFDMA based downlink air interface. However, theterminal does not know the downlink timing of the sys-tem at the beginning of the cell search; hence, frequencydomain processing (e.g., DFT) based timing detection ateach sample will make the cell search processing com-plexity too high for the terminal. In order to obtain goodtiming detection performance with low complexity, thesynchronization symbol structure should therefore bedesigned to allow the robust detection of the symboltiming at the terminal via simple time domain process-ing. To facilitate the detection, the synchronizationsequence should have large peak to side-lobe ratio(PSR). The PSR of a sequence is defined as the ratiobetween the peak to the side-lobes of its aperiodicautocorrelation function.

An important design consideration for the syn-chronization channel is coverage. One primary fac-tor that affects coverage is the peak-to-averagepower ratio (PAPR) of the synchronizationsequence, since this limits the maximum transmitpower of the cell. Hence, a synchronizationsequence that yields low PAPR is desirable.

Design of Synchronization Channel

In this section, we first describe P-SCH and S-SCH sym-bol structures, and then discuss the synchronizationsequence design.

P-SCH Symbol StructuresThe goal of P-SCH is to facilitate the timing and fre-quency offset detection. To achieve the goal, three P-SCH symbol structures have been proposed: repetitivepattern, symmetrical-and-periodic pattern, and non-repetitive pattern.

A P-SCH symbol structure with time domain repeti-tive blocks was proposed in [5], [6]. In the exampleshown in Figure 3, the P-SCH symbol in the timedomain contains K (K = 2 or 4) blocks of equallength, and the cyclic prefix (CP) is attached at thebeginning of the P-SCH symbol.

As shown in Figure 4, a P-SCH symbol structurewith a symmetrical-and-periodic pattern was pro-posed in [7] as an alternative to the P-SCH symbolstructure with a repetitive pattern. Block B in Figure 3is symmetrical (reverse) to block A.

A P-SCH symbol structure with a non-repetitivepattern, as shown in Figure 5, was proposed in [9].Unlike the P-SCH symbol with a repetitive patternwhich is discussed above, the P-SCH symbol with a

non-repetitive pattern can be generated using consecu-tive subcarriers in the frequency domain.

There are two methods to generate the time domainrepetitive and symmetrical-and-periodic P-SCH symbols:frequency domain and time domain. In the former, a fre-quency domain synchronization sequence is mapped tothe central subcarriers in an equidistant manner. This is

FIGURE 5 P-SCH symbol structure with non-repetitive pattern.

FIGURE 4 P-SCH symbol structure with symmetrical-and-periodic pattern.

FIGURE 3 P-SCH symbol structure with repetitive pattern: (a) 2repetitions; (b) 4 repetitions.

(a)

(b)

JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 25

OFDMA PROVIDES SEVERAL ADVANTAGES,SUCH AS HIGH SPECTRAL EFFICIENCY, SIMPLERECEIVER RESIGN, AND ROBUSTNESS IN AMULTI-PATH ENVIRONMENT, AND SO WASCHOSEN AS THE DOWNLINK AIR INTERFACE OF3GPP LTE SYSTEMS.


shown in Figure 6. Using the frequency domain mapping,any complex frequency domain synchronizationsequence can be used to generate the K repetition blocks

pattern in the time domain. According to the property ofDFT, the symmetrical-and-periodic pattern can be gener-ated when a real synchronization sequence is used.

In the time domain method, on the other hand, atime domain synchronization sequence is precoded bya DFT and then mapped to localized (consecutive) sub-carriers of the same symbol. Finally, a P-SCH symbol isgenerated after IDFT.

The example in Figure 7 illustrates the method, inwhich sequences AN/4 and BN/4 , and an appropriatetraining pattern vector a = [1 −1 1 1] are used to

generate symmetrical-and-periodic P-SCH symbol[AN/4 − BN/4 AN/4 BN/4] , as proposed in [8] andshown in Figure 4. In the frequency domain implemen-tation, only a real number sequence can be used forthe P-SCH symbol structure with a symmetrical-and-periodic pattern. With time domain implementation, acomplex number sequence can be used.

S-SCH Symbol StructureThe design of S-SCH needs to supports a sufficientnumber of hypotheses to carry the following informa-tion: 510 cell IDs (jointly with P-SCH symbols) and thenumber of transmit antennas used for broadcastchannel (1 bit). Suppose that three different P-SCHsequences are used in the system, hence the S-SCHneeds to support 340 (i.e., 2 × 510/3) hypotheses.Since there are at most 76 subcarriers can be used forS-SCH, the only solution to support such a large num-ber of hypotheses is to use a fixed equal-distant inter-leaving of two short sequences with length G , saySG(1) and SG(2), as shown in Figure 8 [13]. With thisstructure, the number of supported hypotheses is theproduct of numbers of different SG(1) and SG(2),which approximately equals to G2.

Since there are more than one P-SCH symbols in aradio frame as shown in Figure 2, P-SCH symbols canonly provide symbol timing but not frame timing(due to ambiguity of multiple same P-SCH symbols).Two different S-SCH symbols can be generated byswapping the frequency locations of SG(1) and SG(2).Upon detection of an S-SCH symbol, the terminal canobtain the frame timing as well.

Synchronization Sequence DesignIn order to meet the synchronization sequence designconsiderations discussed above, we examine the PAPRand PSR of several candidate sequences. The candidatesequences include Gold, Golay [10], and generalizedchirp like (GCL) [2] sequences. The Gold and Golaysequences and their PAPR and sequence detectionproperties are well known; On the other hand, the GCLsequence and its properties are less known. Therefore,we provide details of the GCL sequence here. A GCLsequence is defined as:

FIGURE 7 Generation of P-SCH symbols in the time domain approach.

FIGURE 8 Generation of S-SCH symbols.


THE GOAL OF P-SCH IS TO FACILITATE THETIMING AND FREQUENCY OFFSET DETECTION.THERE ARE THREE P-SCH SYMBOL STRUCTURES:REPETITIVE PATTERN, SYMMETRICAL-AND-PERIODICPATTERN, AND NONREPETITIVE PATTERN.

FIGURE 6 Generation of P-SCH symbols in the frequency domainapproach [5], [6].


su(k) ={

exp(− j 2πuk(k+1)

2NG

), NG is odd,

exp(− j 2πu k2

2NG

), NG is even,

(1)

where u is the sequence index, NG is the sequence length,k = 0, 1, …, NG − 1, and u = 1, …, NG − 1. Furthermore,the GCL sequence has constant amplitude zero auto-cor-relation (CAZAC) property when NG is prime. It is shownin [4] that the DFT/IDFT output of a CAZAC sequence isstill a CAZAC sequence. Therefore, the IDFT output of thefrequency domain GCL sequence has a constant envelope(i.e., PAPR of 0 dB) as well. In practice, a pulse shapingfilter will be applied to the transmitted signals and willincrease the PAPR of GCL sequence to about 4 dB.

One key property of the GCL sequence is that thesequence index can be detected using one common dif-ferential encoding based correlator. First, the frequencydomain GCL sequence is differentially encoded, andthen the output of the differential encoder is trans-formed by IDFT, which in turn becomes the Kroneckerdelta function. In this way, the GCL sequence index canbe detected using one common correlator, instead of abank of correlators.

The PAPR and PSR properties of all three candidatesequences are summarized in Table 1. Among the three,only the GCL sequence meets both of the design criteriadiscussed above: best PAPR and high PSR.

Therefore, in the 3GPP LTE study the GCL sequenceand its variations were widely adopted in many P-SCHand S-SCH proposals. For example, the GCL sequencewas applied to a P-SCH symbol with a repetitive patterngenerated by the frequency domain method in [5], [6].The Frank sequence, which is a special case of the GCLsequence as established in [12], was used for a P-SCHsymbol with a repetitive pattern generated by the timedomain method in [8]. It was also used for a P-SCH sym-bol with a non-repetitive pattern in [9]. For a P-SCHsymbol with a symmetrical-and-periodic pattern gener-ated in the frequency domain [7], the Golay sequencewas used. The Frank sequence can be used if a P-SCHsymbol with a symmetrical-and-periodic pattern is gen-erated by the time domain method. The Zadoff-Chusequence [13], which is a special case of GCL sequence,was used in [13] to generate S-SCH symbols.

Cell Search Procedure

In the WCDMA UMTS system, a common P-SCH is used forthe terminal to obtain the timing. Cell group ID isobtained from processing of the S-SCH. Then, the terminalfurther processes a cell-specific scrambling code via thecommon pilot channel to detect the cell ID within thegroup. This is called hierarchical cell search. Cell searchin the 3GPP LTE systems follows a similar hierarchicalprocedure as well, performed in the three steps summa-rized in Figure 9.

Step 1: By processing the P-SCH symbols, OFDM sym-bol timing and the carrier frequency offset are detected.Depending on the P-SCH symbol structure, one of threemethods of timing and frequency offset detection can beused: auto-correlation, cross-correlation, or hybrid detec-tion. Note that these detection methods can be applied toboth time and frequency domain synchronizationsequences.

Auto-correlation based detection: This method can beapplied to P-SCH symbols with repetitive or symmetrical-and-periodic pattern. The received signal is multiplied byits conjugate after a delay of one repetition block andsummed over one repetition block. The search windowslides along in time as the receiver searches for a P-SCHsymbol. MMSE-type detection is used to obtain the

Sequence Length PAPR† (dB) PSR

Gold 31 5.4 1.04Golay 32 2.8 2.91GCL 31 0 2.98

†: PAPR before pulse shaping filter.

TABLE 1 PAPR and PSR properties for different sequences.

FIGURE 9 Hierarchical cell search procedure.


IN ORDER TO MEET THE SYNCHRONIZATIONSEQUENCE DESIGN CONSIDERATIONS, THE PAPRAND PSR OF THREE CANDIDATE SEQUENCES ARECONSIDERED: GOLD, GOLAY, AND GENERALIZEDCHIRP LIKE (GCL) SEQUENCES.


downlink P-SCH symbol timing. The sample timingwith the largest peak in the block-wise auto-correlatoroutput is selected as the P-SCH symbol timing. The fre-quency offset can be estimated easily from the outputof the auto-correlation as well. The advantages of thismethod are its low complexity and reliable estimationof frequency offset. However, its main drawback is itslarge timing estimation error at low SNR.

Cross-correlation based detection: This methodcan be applied to any P-SCH symbol structure. Inthis method, the transmitted P-SCH sequence is usedto correlate the received P-SCH signals. The cross-correlation metric is used to obtain the timing andfrequency offset. It is known that cross-correlationdetection suffers in the presence of frequency offset.To mitigate this problem, the cross-correlation canbe partitioned into M parts [9]. The advantage of themethod is its reliable estimation of timing. However,its main drawbacks are higher complexity comparedto auto-correlation based detection.

Hybrid detection: This method can be applied to P-SCH symbols with repetitive or symmetrical-and-peri-odic pattern. First, the coarse timing and frequencyoffset are estimated by using auto-correlation detec-tion. The received signal is then compensated with theestimated phase, and cross-correlation is performedto obtain a refined timing offset estimate. Hybriddetection combines the advantages of auto- and cross-correlation based detection and has a lower complexi-ty compared to cross-correlation based detection.

Step 2: The S-SCH symbols are processed in the fre-quency domain to detect the cell ID group (one out of170), frame timing and cell-specific information (suchas number of antennas used by BCH).

Step 3: A one-to-one mapping between 3 P-SCHsequences (one of the 3 Cell IDs in each Cell ID group)and downlink reference signals are applied in the sys-tem. By processing the downlink reference signals, thecell ID (one out of 3) is derived within the cell ID groupobtained in the step 2.

Performance Results

The performance of the different P-SCH structures pro-posed in [5]–[9] for 3GPP LTE systems is simulatedand compared. The simulation parameters are summa-rized in Table 2. We assumed that the accumulationlength for the first and second steps of the cell searchis two radio frames (20 ms).

For different P-SCH symbols proposed in [5]–[9],the correlated peaks for timing detection using corre-sponding detection methods (e.g., auto-correlation orcross-correlation based detection) are shown andcompared in Figure 10.

The P-SCH symbol with 2 repetitions generates apeak plateau of the same length as the cyclic prefix.

Synchronization channel BW 1.25 MHzCarrier frequency 2 GHzFFT Size 128Total number of used subcarrier 64Frank sequence length 16Length of cyclic prefix (samples) 9Number of sync symbols per frame 2Channel Model 6-path Typical

UrbanVehicle speed 120 km/hrCarrier frequency offset ±5 ppm

TABLE 2 Simulation parameters.

FIGURE 10 Correlated peaks for timing detection.

FIGURE 11 Detection probabilities for different methods and P-SCHsymbol structures.




In contrast, the P-SCH symbol with 4 repetitions gener-ates a peak with steep roll off. No plateau is observed.The P-SCH symbol with symmetrical-and-periodic struc-ture yields an impulse-shaped timing metric but withtwo side-lobes. The non-repetition pattern yields thesame impulse-shaped timing metric without side-lobes.

The overall performance metric of cell search is thecell miss detection probability, which combines theresults of all steps of the cell search procedure. A cellsearch is considered to be successful if the acquiredtiming falls within the duration of the cyclic prefix, thefrequency offset is corrected, and the cell ID is identi-fied. If any of these conditions is not met, a miss detec-tion has occurred.

The miss detection probabilities of the auto-correlation,cross-correlation, and hybrid detection methods are plot-ted and compared in Figure 11. In this section, we com-pare the performance of the following detection methods:■ Cross-correlation detection with M-parts (M = 2) using

non-repetitive P-SCH symbols, denoted as “CC M = 2 [A]”; ■ Auto-correlation detection using P-SCH symbols with 4

repetitions, denoted as “AC [A − A A A]”;■ Auto-correlation detection using P-SCH symbols with a

symmetrical-and-periodic structure, denoted as“AC [A − B A B]”;

■ Hybrid detection using P-SCH symbols with 4 repeti-tions, denoted as “HD [A − A A A]”;

■ Hybrid detection using P-SCH symbols with a sym-metrical-and-periodic structure, denoted as“HD [A − B A B]”;As shown in Figure 11, the auto-correlation based

detection has a 3–6 dB performance degradation com-pared to cross-correlation based detection at 10% missdetection probability. This is because the auto-correlationbased timing detection is very sensitive to the noise. Theresults show that auto-correlation based detection with asymmetrical-and-periodic structure is about 2 dB betterthan that with a 4-repetition structure. The reason is thatthe former yields a more accurate timing detection metricthan the latter, as shown in Figure 10.

Hybrid detection using a 4-repetition P-SCH symbolstructure still underperforms cross-correlation baseddetection by 1.5 dB at 10% miss detection probability.However, the hybrid detection using a P-SCH symbol witha symmetrical-and-periodic pattern outperforms cross-correlation detection by 0.6 dB. The reason for this isthat, once an accurate frequency offset is estimated viaauto-correlation detection, the cross-correlation step inthe hybrid method then needs to estimate one parameter(timing) only.

For the P-SCH symbols with either a non-repetitive ora symmetrical-and-periodic pattern, low cell miss detec-tion probability can be achieved with a short accumula-tion length of two radio frames at low SNR (e.g., around−3 dB). Therefore, the synchronization channel design isconsidered to be sufficient to support the proper opera-tion of 3GPP LTE systems.

Conclusions

In this article, we have reviewed the cell search issue inthe 3GPP LTE systems. Design considerations for bothprimary and secondary synchronization channels are dis-cussed. We discussed and evaluated synchronizationchannel solutions proposed in 3GPP LTE standardization.The performance of these solutions is simulated and pre-sented. The proposed synchronization channel design isshown to be sufficient to support the proper operation of3GPP LTE systems.

References1. 3rd Generation Partnership Project, Technical Specification Group Radio Access

Network, Physical Layer Aspects for Evolved UTRA (Release 7), 3GPP TR25.814V1.0.1 (2005–11).

2. B.M. Popovic, “Generalized chirp-like polyphase sequences with optimal correla-tion properties,” IEEE Trans. Inform. Theory, vol. 38, pp. 1406–1409, July 1992.

3. T.M. Schmidl and D.C. Cox, “Robust frequency and timing synchronization forOFDM,” IEEE Trans. Commun., vol. 45, pp. 1613–1621, Dec. 1997.

4. H. Minn et al., “A robust timing and frequency synchronization for OFDM sys-tems,” IEEE Trans. Wireless Commun., vol. 2, no. 4, pp. 822–839, July 2003.

5. 3rd Generation Partnership Project, R1-051329, Cell Search and Initial Acquisi-tion for OFDM Downlink, Motorola.

6. 3rd Generation Partnership Project, R1-061065, E-UTRA Cell Search, Ericsson. 7. 3rd Generation Partnership Project, R1-062129, Non-hierarchical Cell Search

with Symmetric and Periodic SCH Signals, Huawei. 8. 3rd Generation Partnership Project, R1-062164, Channel Structure and Hybrid

Detection for Evolved UTRA Cell Search, InterDigital. 9. 3rd Generation Partnership Project, R1-061662, SCH Structure and Cell Search

Method for E-UTRA Downlink, NTT DoCoMo. 10. M.J.E. Golay, “Complementary series,” IRE Trans., vol. IT-11, pp. 207–214, 1961.11. C. Tellambura, “Upper bound on peak factor of N-multiple carriers,” IEEE. Elec-

tronic Letters, vol. 33, no. 19, pp. 1608–1609, Sept. 1997.12. B.M. Popovic, “GCL polyphase sequences with minimum alphabets_00265319,”

IEE Electronics Letter, vol. 30, no. 2, pp. 106–107, Jan. 1994.13. D.C. Chu, “Polyphase codes with good periodic correlation properties,” IEEE

Trans. Inform. Theory, vol. 18, pp. 531–532, July 1972.14. 3rd Generation Partnership Project, R1-071027, S-SCH sequence design,

Ericsson.

ONLY THE GENERALIZED CHIRP LIKE SEQUENCEMEETS BOTH OF THE DESIGN CRITERIA OF BESTPEAK-TO-AVERAGE POWER RATIO AND HIGH PEAKTO SIDE-LOBE RATIO.


Cell Search Procedure in LTE

Documents

Transcript of Cell Search Procedure in LTE