2. search procedures are described, and several timing and
LONG 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 PROJECT
TECHNOLOGY COMPETITIVE.
System Description and Design Considerations
The diagram of the downlink OFDMA air interface is
shown in Figure 1. In the OFDMA system, modulated bits
multiple access (OFDMA) provides several advantages, are converted from serial to parallel first, and then
such as high spectral efficiency, simple receiver resign, mapped to different subcarriers. After IFFT, the output
and robustness in a multi-path environment. Due to signals are converted back to serial signals called an
these advantages, OFDMA was chosen as the downlink OFDM symbol. Cyclic prefix (CP) is attached to the begin-
air interface of 3GPP LTE systems [1]. 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 symbol
symbol timing with the (best) cell, and also to identify the timing, frequency offset and the cell ID. However, cell
cell ID. In order to obtain good cell search performance, search in 3GPP LTE systems has to consider multiple
an appropriate synchronization channel structure needs transmission bandwidths (UMTS has a fixed bandwidth of
to 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 low
synchronization channel are then discussed, and several processing complexity at the terminal and within a much
potential synchronization channel design solutions (syn- shorter time than that in UMTS systems. All of these
chronization symbol structures and corresponding requirements are expected to be fulfilled with system
sequences) 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 [1]. 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 hardware
FIGURE 2 Downlink frame structure of 3GPP LTE systems. implementation. In order to provide good timing
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3. detection performance, the synchronization sequence in
UMTS 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, SIMPLE
the synchronization sequence for UMTS systems. For RECEIVER RESIGN, AND ROBUSTNESS IN A
3GPP LTE systems, the synchronization sequence is MULTI-PATH ENVIRONMENT, AND SO WAS
mapped to the central band of entire bandwidth due to
CHOSEN AS THE DOWNLINK AIR INTERFACE OF
the 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, frequency
domain 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 domain
timing 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 to
timing at the terminal via simple time domain process- the central subcarriers in an equidistant manner. This is
ing. 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 syn-
chronization channel is coverage. One primary fac-
tor that affects coverage is the peak-to-average
power ratio (PAPR) of the synchronization
(a)
sequence, since this limits the maximum transmit
power of the cell. Hence, a synchronization
sequence 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 synchronization
sequence design. (b)
FIGURE 3 P-SCH symbol structure with repetitive pattern: (a) 2
P-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: repetitive
pattern, 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 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
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 structure
with a symmetrical-and-periodic pattern was pro-
posed 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 5 P-SCH symbol structure with non-repetitive pattern.
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4. pattern in the time domain. According to the property of
THE 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, a
REPETITIVE 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, in
any complex frequency domain synchronization which sequences AN/4 and B N/4 , and an appropriate
sequence 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 [8] 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-SCH
FIGURE 6 Generation of P-SCH symbols in the frequency domain needs to support 340 (i.e., 2 × 510/3) hypotheses.
approach [5], [6].
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 [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
FIGURE 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 [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
FIGURE 8 Generation of S-SCH symbols. sequence is defined as:
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5. 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 ARE
where u is the sequence index, N G is the sequence length, CONSIDERED: GOLD, GOLAY, AND GENERALIZED
k = 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 shown
in [4] 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 be
filter 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 synchronization
sequence index can be detected using one common dif- sequences.
ferential encoding based correlator. First, the frequency Auto-correlation based detection: This method can be
domain 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 by
formed by IDFT, which in turn becomes the Kronecker its conjugate after a delay of one repetition block and
delta function. In this way, the GCL sequence index can summed over one repetition block. The search window
be detected using one common correlator, instead of a slides along in time as the receiver searches for a P-SCH
bank of correlators. symbol. MMSE-type detection is used to obtain the
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 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) PSR
and its variations were widely adopted in many P-SCH
Gold 31 5.4 1.04
and S-SCH proposals. For example, the GCL sequence Golay 32 2.8 2.91
was applied to a P-SCH symbol with a repetitive pattern GCL 31 0 2.98
generated by the frequency domain method in [5], [6]. †: PAPR before pulse shaping filter.
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 sym-
bol with a non-repetitive pattern in [9]. For a P-SCH
symbol with a symmetrical-and-periodic pattern gener-
ated 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 gen-
erated 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.
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 summa-
rized in Figure 9. FIGURE 9 Hierarchical cell search procedure.
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6. 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 [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 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 (such
FIGURE 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 [5]–[9] 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 [5]–[9],
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 a
symbol structures. peak plateau of the same length as the cyclic prefix.
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7. 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 SEQUENCE
The P-SCH symbol with symmetrical-and-periodic struc- MEETS BOTH OF THE DESIGN CRITERIA OF BEST
ture yields an impulse-shaped timing metric but with PEAK-TO-AVERAGE POWER RATIO AND HIGH PEAK
two 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 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 For the P-SCH symbols with either a non-repetitive or
frequency 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 is
cross-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 Access
detection has a 3–6 dB performance degradation com- Network, Physical Layer Aspects for Evolved UTRA (Release 7), 3GPP TR25.814
pared 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 for
results 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 Hybrid
structure 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.
![search procedures are described, and several timing and
LONG 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 PROJECT
TECHNOLOGY COMPETITIVE.
System Description and Design Considerations
The diagram of the downlink OFDMA air interface is
shown in Figure 1. In the OFDMA system, modulated bits
multiple access (OFDMA) provides several advantages, are converted from serial to parallel first, and then
such as high spectral efficiency, simple receiver resign, mapped to different subcarriers. After IFFT, the output
and robustness in a multi-path environment. Due to signals are converted back to serial signals called an
these advantages, OFDMA was chosen as the downlink OFDM symbol. Cyclic prefix (CP) is attached to the begin-
air interface of 3GPP LTE systems [1]. 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 symbol
symbol timing with the (best) cell, and also to identify the timing, frequency offset and the cell ID. However, cell
cell ID. In order to obtain good cell search performance, search in 3GPP LTE systems has to consider multiple
an appropriate synchronization channel structure needs transmission bandwidths (UMTS has a fixed bandwidth of
to 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 low
synchronization channel are then discussed, and several processing complexity at the terminal and within a much
potential synchronization channel design solutions (syn- shorter time than that in UMTS systems. All of these
chronization symbol structures and corresponding requirements are expected to be fulfilled with system
sequences) 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 [1]. 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 hardware
FIGURE 2 Downlink frame structure of 3GPP LTE systems. implementation. In order to provide good timing
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