search procedures are described, and several timing andLONG TERM EVOLUTION INVOLVES CHANGES frequency offset detection methods are presented. Perfor-TO BOTH RADIO INTERFACE AND NETWORK mance results of different primary synchronization chan-ARCHITECTURE IN ORDER TO KEEP 3RD nel design solutions are simulated and compared.GENERATION PARTNERSHIP PROJECTTECHNOLOGY COMPETITIVE. System Description and Design Considerations The diagram of the downlink OFDMA air interface is shown in Figure 1. In the OFDMA system, modulated bitsmultiple access (OFDMA) provides several advantages, are converted from serial to parallel first, and thensuch as high spectral efficiency, simple receiver resign, mapped to different subcarriers. After IFFT, the outputand robustness in a multi-path environment. Due to signals are converted back to serial signals called anthese advantages, OFDMA was chosen as the downlink OFDM symbol. Cyclic prefix (CP) is attached to the begin-air interface of 3GPP LTE systems . ning of the OFDM symbol before transmission. Subcarrier When a terminal powers on in a cellular system, it spacing of 15 kHz is used in the 3GPP LTE system.needs to perform cell search to acquire its frequency ref- As in UMTS systems, the cell search in 3GPP LTE sys-erence, frame timing, and the fast Fourier transform (FFT) tems will enable the terminal to obtain frame and symbolsymbol timing with the (best) cell, and also to identify the timing, frequency offset and the cell ID. However, cellcell ID. In order to obtain good cell search performance, search in 3GPP LTE systems has to consider multiplean appropriate synchronization channel structure needs transmission bandwidths (UMTS has a fixed bandwidth ofto be designed. 5MHz, while 3GPP LTE systems support 1.25, 2.5, 5, 10, 15 We start in this article by briefly describing the and 20 MHz bandwidths). Moreover, cell search proce-OFDMA air interface. The design considerations of the dure in 3GPP LTE systems should be completed with lowsynchronization channel are then discussed, and several processing complexity at the terminal and within a muchpotential synchronization channel design solutions (syn- shorter time than that in UMTS systems. All of thesechronization symbol structures and corresponding requirements are expected to be fulfilled with systemsequences) for 3GPP LTE system are presented. Cell overhead 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 central 1.25 MHz bandwidth regardless of the entire band- width of the system . In this way, the same syn- chronization channel is mapped to the central part of transmission bandwidth for all system band- widths. The central 1.25 MHz corresponds to 76 sub-FIGURE 1 OFDMA air interface in 3GPP LTE systems. carriers with subcarrier spacing of 15 kHz. The downlink frame structure of the 3GPP LTE system is shown in Figure 2. Each radio frame (10 ms) is divided into 10 sub-frame of 1 ms. Each sub- frame consists of 2 slots. There are 7 OFDM symbol per slot. There are two kinds of synchronization channels (SCH): primary SCH (P-SCH) and sec- ondary SCH (S-SCH). P-SCH and S-SCH symbols are time division multiplexed. Each radio frame con- tains two equal-spaced pairs of P-SCH and S-SCH symbols. For coherent detection of S-SCH symbols, P-SCH and S-SCH symbols are placed adjacent to each other in the last two OFDM symbols of the first slot within a sub-frame. Cell search in the WCDMA based UMTS system relies mainly on time domain processing to achieve low receiver complexity and efficient hardwareFIGURE 2 Downlink frame structure of 3GPP LTE systems. implementation. In order to provide good timing24 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.
detection performance, the synchronization sequence inUMTS systems should have very good auto-correlation. OFDMA PROVIDES SEVERAL ADVANTAGES,Due to this property, the Golay sequence was chosen as SUCH AS HIGH SPECTRAL EFFICIENCY, SIMPLEthe synchronization sequence for UMTS systems. For RECEIVER RESIGN, AND ROBUSTNESS IN A3GPP LTE systems, the synchronization sequence is MULTI-PATH ENVIRONMENT, AND SO WASmapped to the central band of entire bandwidth due to CHOSEN AS THE DOWNLINK AIR INTERFACE OFthe OFDMA based downlink air interface. However, the 3GPP LTE SYSTEMS.terminal 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 at non-repetitive pattern can be generated using consecu-each sample will make the cell search processing com- tive subcarriers in the frequency domain.plexity too high for the terminal. In order to obtain good There are two methods to generate the time domaintiming detection performance with low complexity, the repetitive and symmetrical-and-periodic P-SCH symbols:synchronization symbol structure should therefore be frequency domain and time domain. In the former, a fre-designed to allow the robust detection of the symbol quency domain synchronization sequence is mapped totiming at the terminal via simple time domain process- the central subcarriers in an equidistant manner. This ising. 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 synchronization (a)sequence, since this limits the maximum transmitpower of the cell. Hence, a synchronizationsequence that yields low PAPR is desirable.Design of Synchronization ChannelIn this section, we first describe P-SCH and S-SCH sym-bol structures, and then discuss the synchronizationsequence design. (b) FIGURE 3 P-SCH symbol structure with repetitive pattern: (a) 2P-SCH Symbol Structures repetitions; (b) 4 repetitions.The 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 , . 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 the FIGURE 4 P-SCH symbol structure with symmetrical-and-periodic pattern.beginning of the P-SCH symbol. As shown in Figure 4, a P-SCH symbol structurewith a symmetrical-and-periodic pattern was pro-posed in  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 .Unlike the P-SCH symbol with a repetitive patternwhich is discussed above, the P-SCH symbol with a FIGURE 5 P-SCH symbol structure with non-repetitive pattern.JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 25 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.
pattern in the time domain. According to the property ofTHE GOAL OF P-SCH IS TO FACILITATE THE DFT, the symmetrical-and-periodic pattern can be gener-TIMING AND FREQUENCY OFFSET DETECTION. ated when a real synchronization sequence is used.THERE ARE THREE P-SCH SYMBOL STRUCTURES: In the time domain method, on the other hand, aREPETITIVE PATTERN, SYMMETRICAL-AND-PERIODIC time domain synchronization sequence is precoded by a DFT and then mapped to localized (consecutive) sub-PATTERN, AND NONREPETITIVE PATTERN. carriers of the same symbol. Finally, a P-SCH symbol is generated after IDFT.shown in Figure 6. Using the frequency domain mapping, The example in Figure 7 illustrates the method, inany complex frequency domain synchronization which sequences AN/4 and B N/4 , and an appropriatesequence can be used to generate the K repetition blocks training pattern vector a = [1 −1 1 1] are used to generate symmetrical-and-periodic P-SCH symbol [ AN/4 − B N/4 AN/4 B N/4 ] , as proposed in  and shown in Figure 4. In the frequency domain implemen- tation, only a real number sequence can be used for the P-SCH symbol structure with a symmetrical-and- periodic pattern. With time domain implementation, a complex number sequence can be used. S-SCH Symbol Structure The design of S-SCH needs to supports a sufficient number of hypotheses to carry the following informa- tion: 510 cell IDs (jointly with P-SCH symbols) and the number of transmit antennas used for broadcast channel (1 bit). Suppose that three different P-SCH sequences are used in the system, hence the S-SCHFIGURE 6 Generation of P-SCH symbols in the frequency domain needs to support 340 (i.e., 2 × 510/3) hypotheses.approach , . Since there are at most 76 subcarriers can be used for S-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, say SG (1) and SG (2), as shown in Figure 8 . With this structure, the number of supported hypotheses is the product of numbers of different SG (1) and SG (2), which approximately equals to G 2 . Since there are more than one P-SCH symbols in a radio frame as shown in Figure 2, P-SCH symbols can only provide symbol timing but not frame timing (due to ambiguity of multiple same P-SCH symbols). Two different S-SCH symbols can be generated byFIGURE 7 Generation of P-SCH symbols in the time domain approach. swapping the frequency locations of SG (1) and SG (2). Upon detection of an S-SCH symbol, the terminal can obtain the frame timing as well. Synchronization Sequence Design In order to meet the synchronization sequence design considerations discussed above, we examine the PAPR and PSR of several candidate sequences. The candidate sequences include Gold, Golay , and generalized chirp like (GCL)  sequences. The Gold and Golay sequences and their PAPR and sequence detection properties are well known; On the other hand, the GCL sequence and its properties are less known. Therefore, we provide details of the GCL sequence here. A GCLFIGURE 8 Generation of S-SCH symbols. sequence is defined as:26 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.
exp −j 2πu k(k+1) , 2N G N G is odd, su (k) = k 2 (1) IN ORDER TO MEET THE SYNCHRONIZATION exp −j 2πu 2N G , N G is even, SEQUENCE DESIGN CONSIDERATIONS, THE PAPR AND PSR OF THREE CANDIDATE SEQUENCES AREwhere u is the sequence index, N G is the sequence length, CONSIDERED: GOLD, GOLAY, AND GENERALIZEDk = 0, 1, …, N G − 1, and u = 1, …, N G − 1. Furthermore, CHIRP LIKE (GCL) SEQUENCES.the GCL sequence has constant amplitude zero auto-cor-relation (CAZAC) property when N G is prime. It is shownin  that the DFT/IDFT output of a CAZAC sequence is Step 1: By processing the P-SCH symbols, OFDM sym-still a CAZAC sequence. Therefore, the IDFT output of the bol timing and the carrier frequency offset are detected.frequency domain GCL sequence has a constant envelope Depending on the P-SCH symbol structure, one of three(i.e., PAPR of 0 dB) as well. In practice, a pulse shaping methods of timing and frequency offset detection can befilter will be applied to the transmitted signals and will used: auto-correlation, cross-correlation, or hybrid detec-increase the PAPR of GCL sequence to about 4 dB. tion. Note that these detection methods can be applied to One key property of the GCL sequence is that the both time and frequency domain synchronizationsequence index can be detected using one common dif- sequences.ferential encoding based correlator. First, the frequency Auto-correlation based detection: This method can bedomain GCL sequence is differentially encoded, and applied to P-SCH symbols with repetitive or symmetrical-then the output of the differential encoder is trans- and-periodic pattern. The received signal is multiplied byformed by IDFT, which in turn becomes the Kronecker its conjugate after a delay of one repetition block anddelta function. In this way, the GCL sequence index can summed over one repetition block. The search windowbe detected using one common correlator, instead of a slides along in time as the receiver searches for a P-SCHbank of correlators. symbol. MMSE-type detection is used to obtain the 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 criteria TABLE 1 PAPR and PSR properties for different sequences.discussed above: best PAPR and high PSR. Therefore, in the 3GPP LTE study the GCL sequence Sequence Length PAPR† (dB) PSRand its variations were widely adopted in many P-SCH Gold 31 5.4 1.04and S-SCH proposals. For example, the GCL sequence Golay 32 2.8 2.91was applied to a P-SCH symbol with a repetitive pattern GCL 31 0 2.98generated by the frequency domain method in , . †: PAPR before pulse shaping filter.The Frank sequence, which is a special case of the GCLsequence as established in , was used for a P-SCHsymbol with a repetitive pattern generated by the timedomain method in . It was also used for a P-SCH sym-bol with a non-repetitive pattern in . For a P-SCHsymbol with a symmetrical-and-periodic pattern gener-ated in the frequency domain , 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 , which is a special case of GCL sequence,was used in  to generate S-SCH symbols.Cell Search ProcedureIn 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. FIGURE 9 Hierarchical cell search procedure.JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 27 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.
downlink P-SCH symbol timing. The sample timing TABLE 2 Simulation parameters. with the largest peak in the block-wise auto-correlator output is selected as the P-SCH symbol timing. The fre- Synchronization channel BW 1.25 MHz quency offset can be estimated easily from the output Carrier frequency 2 GHz of the auto-correlation as well. The advantages of this FFT Size 128 Total number of used subcarrier 64 method are its low complexity and reliable estimation Frank sequence length 16 of frequency offset. However, its main drawback is its Length of cyclic prefix (samples) 9 large timing estimation error at low SNR. Number of sync symbols per frame 2 Channel Model 6-path Typical Cross-correlation based detection: This method Urban can be applied to any P-SCH symbol structure. In Vehicle speed 120 km/hr this method, the transmitted P-SCH sequence is used Carrier frequency offset ±5 ppm to correlate the received P-SCH signals. The cross- correlation metric is used to obtain the timing and frequency offset. It is known that cross-correlation detection suffers in the presence of frequency offset. To mitigate this problem, the cross-correlation can be partitioned into M parts . The advantage of the method is its reliable estimation of timing. However, its main drawbacks are higher complexity compared to 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 frequency offset are estimated by using auto-correlation detec- tion. The received signal is then compensated with the estimated phase, and cross-correlation is performed to obtain a refined timing offset estimate. Hybrid detection 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 of 170), frame timing and cell-specific information (suchFIGURE 10 Correlated peaks for timing detection. as number of antennas used by BCH). Step 3: A one-to-one mapping between 3 P-SCH sequences (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, the cell ID (one out of 3) is derived within the cell ID group obtained in the step 2. Performance Results The performance of the different P-SCH structures pro- posed in – for 3GPP LTE systems is simulated and compared. The simulation parameters are summa- rized in Table 2. We assumed that the accumulation length for the first and second steps of the cell search is two radio frames (20 ms). For different P-SCH symbols proposed in –, the correlated peaks for timing detection using corre- sponding detection methods (e.g., auto-correlation or cross-correlation based detection) are shown and compared in Figure 10.FIGURE 11 Detection probabilities for different methods and P-SCH The P-SCH symbol with 2 repetitions generates asymbol structures. peak plateau of the same length as the cyclic prefix.28 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.
In contrast, the P-SCH symbol with 4 repetitions gener-ates a peak with steep roll off. No plateau is observed. ONLY THE GENERALIZED CHIRP LIKE SEQUENCEThe P-SCH symbol with symmetrical-and-periodic struc- MEETS BOTH OF THE DESIGN CRITERIA OF BESTture yields an impulse-shaped timing metric but with PEAK-TO-AVERAGE POWER RATIO AND HIGH PEAKtwo side-lobes. The non-repetition pattern yields the TO SIDE-LOBE RATIO.same 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, the For the P-SCH symbols with either a non-repetitive orfrequency offset is corrected, and the cell ID is identi- a symmetrical-and-periodic pattern, low cell miss detec-fied. If any of these conditions is not met, a miss detec- tion probability can be achieved with a short accumula-tion has occurred. tion length of two radio frames at low SNR (e.g., around The miss detection probabilities of the auto-correlation, −3 dB). Therefore, the synchronization channel design iscross-correlation, and hybrid detection methods are plot- considered to be sufficient to support the proper opera-ted and compared in Figure 11. In this section, we com- tion of 3GPP LTE systems.pare the performance of the following detection methods: ■ Cross-correlation detection with M-parts (M = 2) using Conclusions non-repetitive P-SCH symbols, denoted as “CC M = 2 [A]”; In this article, we have reviewed the cell search issue in ■ Auto-correlation detection using P-SCH symbols with 4 the 3GPP LTE systems. Design considerations for both repetitions, denoted as “AC [A − A A A]”; primary and secondary synchronization channels are dis- ■ Auto-correlation detection using P-SCH symbols with a cussed. We discussed and evaluated synchronization symmetrical-and-periodic structure, denoted as channel solutions proposed in 3GPP LTE standardization. “AC [A − B A B]”; The performance of these solutions is simulated and pre- ■ Hybrid detection using P-SCH symbols with 4 repeti- sented. The proposed synchronization channel design is tions, denoted as “HD [A − A A A]”; shown to be sufficient to support the proper operation of ■ Hybrid detection using P-SCH symbols with a sym- 3GPP LTE systems. metrical-and-periodic structure, denoted as “HD [A − B A B]”; As shown in Figure 11, the auto-correlation based References 1. 3rd Generation Partnership Project, Technical Specification Group Radio Accessdetection has a 3–6 dB performance degradation com- Network, Physical Layer Aspects for Evolved UTRA (Release 7), 3GPP TR25.814pared to cross-correlation based detection at 10% miss V1.0.1 (2005–11).detection probability. This is because the auto-correlation 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.based timing detection is very sensitive to the noise. The 3. T.M. Schmidl and D.C. Cox, “Robust frequency and timing synchronization forresults show that auto-correlation based detection with a OFDM,” IEEE Trans. Commun., vol. 45, pp. 1613–1621, Dec. 1997. 4. H. Minn et al., “A robust timing and frequency synchronization for OFDM sys-symmetrical-and-periodic structure is about 2 dB better tems,” IEEE Trans. Wireless Commun., vol. 2, no. 4, pp. 822–839, July 2003.than that with a 4-repetition structure. The reason is that 5. 3rd Generation Partnership Project, R1-051329, Cell Search and Initial Acquisi- tion for OFDM Downlink, Motorola.the former yields a more accurate timing detection metric 6. 3rd Generation Partnership Project, R1-061065, E-UTRA Cell Search, Ericsson.than the latter, as shown in Figure 10. 7. 3rd Generation Partnership Project, R1-062129, Non-hierarchical Cell Search Hybrid detection using a 4-repetition P-SCH symbol with Symmetric and Periodic SCH Signals, Huawei. 8. 3rd Generation Partnership Project, R1-062164, Channel Structure and Hybridstructure still underperforms cross-correlation based Detection for Evolved UTRA Cell Search, InterDigital.detection by 1.5 dB at 10% miss detection probability. 9. 3rd Generation Partnership Project, R1-061662, SCH Structure and Cell Search Method for E-UTRA Downlink, NTT DoCoMo.However, the hybrid detection using a P-SCH symbol with 10. M.J.E. Golay, “Complementary series,” IRE Trans., vol. IT-11, pp. 207–214, 1961.a symmetrical-and-periodic pattern outperforms cross- 11. C. Tellambura, “Upper bound on peak factor of N-multiple carriers,” IEEE. Elec-correlation detection by 0.6 dB. The reason for this is tronic Letters, vol. 33, no. 19, pp. 1608–1609, Sept. 1997. 12. B.M. Popovic, “GCL polyphase sequences with minimum alphabets_00265319,”that, once an accurate frequency offset is estimated via IEE Electronics Letter, vol. 30, no. 2, pp. 106–107, Jan. 1994.auto-correlation detection, the cross-correlation step in 13. D.C. Chu, “Polyphase codes with good periodic correlation properties,” IEEE Trans. Inform. Theory, vol. 18, pp. 531–532, July 1972.the hybrid method then needs to estimate one parameter 14. 3rd Generation Partnership Project, R1-071027, S-SCH sequence design,(timing) only. Ericsson.JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 29 Authorized licensed use limited to: Bambang Samajudin. Downloaded on March 29,2010 at 04:00:22 EDT from IEEE Xplore. Restrictions apply.