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
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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.
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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.
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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.
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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.
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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.
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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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