Abstract: This paper reviews and presents the latest results in the cell search issue of the 3GPP Long Term Evolution (LTE) systems. Cell Search is a basic function of any cellular system, during which process time and frequency synchronization between the mobile terminal and the network is achieved. Such synchronization is especially important for 3GPP Long Term Evolution systems, which rely heavily on the orthogonality of the uplink and downlink transmission and reception to optimize the radio link performance. As in conventional cellular systems, the mobile terminal in an LTE system acquires time and frequency synchronization by processing the synchronization channel. Design of the synchronization channel is being developed within the standardization activities of 3GPP Long Term Evolution and is still evolving. In this paper, we present the design considerations and various new and promising design concepts for the synchronization channel. We evaluate some specific solutions and provide numerical performance results. Cell Search in 3GPP Long Term Evolution Systems Yingming Tsai, Guodong Zhang, Donald Grieco, Fatih Ozluturk, InterDigital Communications Corporation, and Xiaodong Wang, Columbia University Introduction n order to keep its technology competitive, 3rd Generation Partnership Project (3GPP) is considering long term evolution (LTE), in which evolution of both radio interface and network architecture is necessary. 3GPP LTE systems will provide higher data rate services with better QoS than the current 3G systems. This will require reliable and high-rate communications over time-dispersive (frequency-selective) channels with limited spectrum and inter-symbol interference (ISI) caused by multi-path fading. Orthogonal frequency division I Digital Object Identifier 10.1109/MVT.2007.912929 © DYNAMIC GRAPHICS JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE 1556-6072/07/$25.00©2007IEEE ||| 23 LONG TERM EVOLUTION INVOLVES CHANGES TO BOTH RADIO INTERFACE AND NETWORK ARCHITECTURE IN ORDER TO KEEP 3RD GENERATION PARTNERSHIP PROJECT TECHNOLOGY COMPETITIVE. multiple access (OFDMA) provides several advantages, such as high spectral efficiency, simple receiver resign, and robustness in a multi-path environment. Due to these advantages, OFDMA was chosen as the downlink air interface of 3GPP LTE systems [1]. When 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. In order to obtain good cell search performance, an appropriate synchronization channel structure needs to be designed. We start in this article by briefly describing the OFDMA air interface. The design considerations of the synchronization channel are then discussed, and several potential synchronization channel design solutions (synchronization symbol structures and corresponding sequences) for 3GPP LTE system are presented. Cell FIGURE 1 OFDMA air interface in 3GPP LTE systems. FIGURE 2 Downlink frame structure of 3GPP LTE systems. 24 ||| search procedures are described, and several timing and frequency offset detection methods are presented. Performance results of different primary synchronization channel design solutions are simulated and compared. System Description and Design Considerations The diagram of the downlink OFDMA air interface is shown in Figure 1. In the OFDMA system, modulated bits are converted from serial to parallel first, and then mapped to different subcarriers. After IFFT, the output signals are converted back to serial signals called an OFDM symbol. Cyclic prefix (CP) is attached to the beginning of the OFDM symbol before transmission. Subcarrier spacing of 15 kHz is used in the 3GPP LTE system. As in UMTS systems, the cell search in 3GPP LTE systems will enable the terminal to obtain frame and symbol timing, frequency offset and the cell ID. However, cell search in 3GPP LTE systems has to consider multiple transmission bandwidths (UMTS has a fixed bandwidth of 5MHz, while 3GPP LTE systems support 1.25, 2.5, 5, 10, 15 and 20 MHz bandwidths). Moreover, cell search procedure in 3GPP LTE systems should be completed with low processing complexity at the terminal and within a much shorter time than that in UMTS systems. All of these requirements are expected to be fulfilled with system overhead on par with UMTS systems. It is desirable to define a synchronization channel that is common to all cells in the system irrespective of the bandwidth being used in the cell, since this will yield faster cell search and lower complexity. Therefore, it is agreed that the synchronization channel should be transmitted using the central 1.25 MHz bandwidth regardless of the entire bandwidth of the system [1]. In this way, the same synchronization channel is mapped to the central part of transmission bandwidth for all system bandwidths. The central 1.25 MHz corresponds to 76 subcarriers 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 subframe consists of 2 slots. There are 7 OFDM symbol per slot. There are two kinds of synchronization channels (SCH): primary SCH (P-SCH) and secondary SCH (S-SCH). P-SCH and S-SCH symbols are time division multiplexed. Each radio frame contains 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 hardware implementation. In order to provide good timing IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 detection performance, the synchronization sequence in UMTS systems should have very good auto-correlation. Due to this property, the Golay sequence was chosen as the synchronization sequence for UMTS systems. For 3GPP LTE systems, the synchronization sequence is mapped to the central band of entire bandwidth due to the OFDMA based downlink air interface. However, the terminal does not know the downlink timing of the system at the beginning of the cell search; hence, frequency domain processing (e.g., DFT) based timing detection at each sample will make the cell search processing complexity too high for the terminal. In order to obtain good timing detection performance with low complexity, the synchronization symbol structure should therefore be designed to allow the robust detection of the symbol timing at the terminal via simple time domain processing. To facilitate the detection, the synchronization sequence should have large peak to side-lobe ratio (PSR). The PSR of a sequence is defined as the ratio between the peak to the side-lobes of its aperiodic autocorrelation function. An important design consideration for the synchronization channel is coverage. One primary factor that affects coverage is the peak-to-average power ratio (PAPR) of the synchronization sequence, since this limits the maximum transmit power of the cell. Hence, a synchronization sequence that yields low PAPR is desirable. OFDMA PROVIDES SEVERAL ADVANTAGES, SUCH AS HIGH SPECTRAL EFFICIENCY, SIMPLE RECEIVER RESIGN, AND ROBUSTNESS IN A MULTI-PATH ENVIRONMENT, AND SO WAS CHOSEN AS THE DOWNLINK AIR INTERFACE OF 3GPP LTE SYSTEMS. non-repetitive pattern can be generated using consecutive subcarriers in the frequency domain. There are two methods to generate the time domain repetitive and symmetrical-and-periodic P-SCH symbols: frequency domain and time domain. In the former, a frequency domain synchronization sequence is mapped to the central subcarriers in an equidistant manner. This is (a) Design of Synchronization Channel In this section, we first describe P-SCH and S-SCH symbol structures, and then discuss the synchronization sequence design. (b) P-SCH Symbol Structures FIGURE 3 P-SCH symbol structure with repetitive pattern: (a) 2 repetitions; (b) 4 repetitions. The goal of P-SCH is to facilitate the timing and frequency offset detection. To achieve the goal, three PSCH symbol structures have been proposed: repetitive pattern, symmetrical-and-periodic pattern, and nonrepetitive pattern. A P-SCH symbol structure with time domain repetitive blocks was proposed in [5], [6]. In the example shown in Figure 3, the P-SCH symbol in the time domain contains K ( K = 2 or 4) blocks of equal length, and the cyclic prefix (CP) is attached at the beginning of the P-SCH symbol. As shown in Figure 4, a P-SCH symbol structure with a symmetrical-and-periodic pattern was proposed in [7] as an alternative to the P-SCH symbol structure with a repetitive pattern. Block B in Figure 3 is symmetrical (reverse) to block A. A P-SCH symbol structure with a non-repetitive pattern, as shown in Figure 5, was proposed in [9]. Unlike the P-SCH symbol with a repetitive pattern which is discussed above, the P-SCH symbol with a FIGURE 4 P-SCH symbol structure with symmetrical-and-periodic pattern. JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE FIGURE 5 P-SCH symbol structure with non-repetitive pattern. ||| 25 THE GOAL OF P-SCH IS TO FACILITATE THE TIMING AND FREQUENCY OFFSET DETECTION. THERE ARE THREE P-SCH SYMBOL STRUCTURES: REPETITIVE PATTERN, SYMMETRICAL-AND-PERIODIC PATTERN, AND NONREPETITIVE PATTERN. shown in Figure 6. Using the frequency domain mapping, any complex frequency domain synchronization sequence can be used to generate the K repetition blocks pattern in the time domain. According to the property of DFT, the symmetrical-and-periodic pattern can be generated when a real synchronization sequence is used. In the time domain method, on the other hand, a time domain synchronization sequence is precoded by a DFT and then mapped to localized (consecutive) subcarriers of the same symbol. Finally, a P-SCH symbol is generated after IDFT. The example in Figure 7 illustrates the method, in which sequences AN/4 and B N/4 , and an appropriate 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 [8] and shown in Figure 4. In the frequency domain implementation, only a real number sequence can be used for the P-SCH symbol structure with a symmetrical-andperiodic pattern. With time domain implementation, a complex number sequence can be used. S-SCH Symbol Structure FIGURE 6 Generation of P-SCH symbols in the frequency domain approach [5], [6]. FIGURE 7 Generation of P-SCH symbols in the time domain approach. The design of S-SCH needs to supports a sufficient number of hypotheses to carry the following information: 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-SCH needs to support 340 (i.e., 2 × 510/3) hypotheses. Since there are at most 76 subcarriers can be used for S-SCH, the only solution to support such a large number of hypotheses is to use a fixed equal-distant interleaving of two short sequences with length G, say SG (1) and SG (2), as shown in Figure 8 [13]. 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 by 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 FIGURE 8 Generation of S-SCH symbols. 26 ||| 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 [10], and generalized chirp like (GCL) [2] 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 GCL sequence is defined as: IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 su (k) = , exp −j 2πu k(k+1) 2N G 2 k exp −j 2πu 2N , G N G is odd, N G is even, (1) where u is the sequence index, N G is the sequence length, k = 0, 1, …, N G − 1, and u = 1, …, N G − 1. Furthermore, the GCL sequence has constant amplitude zero auto-correlation (CAZAC) property when N G is prime. It is shown in [4] that the DFT/IDFT output of a CAZAC sequence is still a CAZAC sequence. Therefore, the IDFT output of the frequency domain GCL sequence has a constant envelope (i.e., PAPR of 0 dB) as well. In practice, a pulse shaping filter will be applied to the transmitted signals and will increase the PAPR of GCL sequence to about 4 dB. One key property of the GCL sequence is that the sequence index can be detected using one common differential encoding based correlator. First, the frequency domain GCL sequence is differentially encoded, and then the output of the differential encoder is transformed by IDFT, which in turn becomes the Kronecker delta function. In this way, the GCL sequence index can be detected using one common correlator, instead of a bank of correlators. The PAPR and PSR properties of all three candidate sequences are summarized in Table 1. Among the three, only the GCL sequence meets both of the design criteria discussed above: best PAPR and high PSR. Therefore, in the 3GPP LTE study the GCL sequence and its variations were widely adopted in many P-SCH and S-SCH proposals. For example, the GCL sequence was applied to a P-SCH symbol with a repetitive pattern generated by the frequency domain method in [5], [6]. The Frank sequence, which is a special case of the GCL sequence as established in [12], was used for a P-SCH symbol with a repetitive pattern generated by the time domain method in [8]. It was also used for a P-SCH symbol with a non-repetitive pattern in [9]. For a P-SCH symbol with a symmetrical-and-periodic pattern generated in the frequency domain [7], the Golay sequence was used. The Frank sequence can be used if a P-SCH symbol with a symmetrical-and-periodic pattern is generated by the time domain method. The Zadoff-Chu sequence [13], which is a special case of GCL sequence, was used in [13] to generate S-SCH symbols. IN ORDER TO MEET THE SYNCHRONIZATION SEQUENCE DESIGN CONSIDERATIONS, THE PAPR AND PSR OF THREE CANDIDATE SEQUENCES ARE CONSIDERED: GOLD, GOLAY, AND GENERALIZED CHIRP LIKE (GCL) SEQUENCES. Step 1: By processing the P-SCH symbols, OFDM symbol timing and the carrier frequency offset are detected. Depending on the P-SCH symbol structure, one of three methods of timing and frequency offset detection can be used: auto-correlation, cross-correlation, or hybrid detection. Note that these detection methods can be applied to both time and frequency domain synchronization sequences. Auto-correlation based detection: This method can be applied to P-SCH symbols with repetitive or symmetricaland-periodic pattern. The received signal is multiplied by its conjugate after a delay of one repetition block and summed over one repetition block. The search window slides along in time as the receiver searches for a P-SCH symbol. MMSE-type detection is used to obtain the TABLE 1 PAPR and PSR properties for different sequences. Sequence Gold Golay GCL Length PAPR† (dB) PSR 31 32 31 5.4 2.8 0 1.04 2.91 2.98 †: PAPR before pulse shaping filter. Cell Search Procedure In the WCDMA UMTS system, a common P-SCH is used for the terminal to obtain the timing. Cell group ID is obtained from processing of the S-SCH. Then, the terminal further processes a cell-specific scrambling code via the common pilot channel to detect the cell ID within the group. This is called hierarchical cell search. Cell search in the 3GPP LTE systems follows a similar hierarchical procedure as well, performed in the three steps summarized in Figure 9. JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE FIGURE 9 Hierarchical cell search procedure. ||| 27 TABLE 2 Simulation parameters. Synchronization channel BW Carrier frequency FFT Size Total number of used subcarrier Frank sequence length Length of cyclic prefix (samples) Number of sync symbols per frame Channel Model Vehicle speed Carrier frequency offset 1.25 MHz 2 GHz 128 64 16 9 2 6-path Typical Urban 120 km/hr ±5 ppm FIGURE 10 Correlated peaks for timing detection. downlink P-SCH symbol timing. The sample timing with the largest peak in the block-wise auto-correlator output is selected as the P-SCH symbol timing. The frequency offset can be estimated easily from the output of the auto-correlation as well. The advantages of this method are its low complexity and reliable estimation of frequency offset. However, its main drawback is its large timing estimation error at low SNR. Cross-correlation based detection: This method can be applied to any P-SCH symbol structure. In this method, the transmitted P-SCH sequence is used to correlate the received P-SCH signals. The crosscorrelation 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 [9]. 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 PSCH symbols with repetitive or symmetrical-and-periodic pattern. First, the coarse timing and frequency offset are estimated by using auto-correlation detection. 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 crosscorrelation based detection and has a lower complexity compared to cross-correlation based detection. Step 2: The S-SCH symbols are processed in the frequency domain to detect the cell ID group (one out of 170), frame timing and cell-specific information (such 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 system. 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 FIGURE 11 Detection probabilities for different methods and P-SCH symbol structures. 28 ||| The performance of the different P-SCH structures proposed in [5]–[9] for 3GPP LTE systems is simulated and compared. The simulation parameters are summarized 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 [5]–[9], the correlated peaks for timing detection using corresponding detection methods (e.g., auto-correlation or cross-correlation based detection) are shown and compared in Figure 10. The P-SCH symbol with 2 repetitions generates a peak plateau of the same length as the cyclic prefix. IEEE VEHICULAR TECHNOLOGY MAGAZINE | JUNE 2007 In contrast, the P-SCH symbol with 4 repetitions generates a peak with steep roll off. No plateau is observed. The P-SCH symbol with symmetrical-and-periodic structure yields an impulse-shaped timing metric but with two side-lobes. The non-repetition pattern yields the same impulse-shaped timing metric without side-lobes. The overall performance metric of cell search is the cell miss detection probability, which combines the results of all steps of the cell search procedure. A cell search is considered to be successful if the acquired timing falls within the duration of the cyclic prefix, the frequency offset is corrected, and the cell ID is identified. If any of these conditions is not met, a miss detection has occurred. The miss detection probabilities of the auto-correlation, cross-correlation, and hybrid detection methods are plotted and compared in Figure 11. In this section, we compare 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 repetitions, denoted as “HD [A − A A A]”; ■ Hybrid detection using P-SCH symbols with a symmetrical-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 compared to cross-correlation based detection at 10% miss detection probability. This is because the auto-correlation based timing detection is very sensitive to the noise. The results show that auto-correlation based detection with a symmetrical-and-periodic structure is about 2 dB better than that with a 4-repetition structure. The reason is that the former yields a more accurate timing detection metric than the latter, as shown in Figure 10. Hybrid detection using a 4-repetition P-SCH symbol structure still underperforms cross-correlation based detection by 1.5 dB at 10% miss detection probability. However, the hybrid detection using a P-SCH symbol with a symmetrical-and-periodic pattern outperforms crosscorrelation detection by 0.6 dB. The reason for this is that, once an accurate frequency offset is estimated via auto-correlation detection, the cross-correlation step in the hybrid method then needs to estimate one parameter (timing) only. JUNE 2007 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ONLY THE GENERALIZED CHIRP LIKE SEQUENCE MEETS BOTH OF THE DESIGN CRITERIA OF BEST PEAK-TO-AVERAGE POWER RATIO AND HIGH PEAK TO SIDE-LOBE RATIO. For the P-SCH symbols with either a non-repetitive or a symmetrical-and-periodic pattern, low cell miss detection probability can be achieved with a short accumulation length of two radio frames at low SNR (e.g., around −3 dB). Therefore, the synchronization channel design is considered to be sufficient to support the proper operation of 3GPP LTE systems. Conclusions In this article, we have reviewed the cell search issue in the 3GPP LTE systems. Design considerations for both primary and secondary synchronization channels are discussed. We discussed and evaluated synchronization channel solutions proposed in 3GPP LTE standardization. The performance of these solutions is simulated and presented. The proposed synchronization channel design is shown to be sufficient to support the proper operation of 3GPP LTE systems. 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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. ||| 29
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