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ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
A Low Complex Time Synchronization
Technique Suitable For both OFDM and LTE
Systems
Shoba B, P.Gouthame
Assistant Professor, Department of Electronics and Communication Engineering, Rajiv Gandhi College of Engineering
and Technology, Puducherry, India
Department of Electronics and Communication Engineering, Coimbatore Institute of Technology, Tamilnadu, India
ABSTRACT: Orthogonal Frequency Division Multiplexing (OFDM) systems are widely popular for high data rate
services. An evolution of UMTS standard namely Long Term Evolution (LTE) is based on Orthogonal Frequency
Division multiple Access (OFDMA) in down-link and Single-Carrier Frequency Division Multiple Access (SC-FDMA)
in up-link. Synchronization plays a vital role in enhancing the overall performance of both OFDM and LTE systems.
In this paper, a robust training symbol based timing synchronization technique proposed initially for OFDM systems
has been further extended to Primary Synchronization Signal (PSS) for LTE systems. The proposed signal structure
gives low complexity as compared to previous techniques. Simulation results are shown for both OFDM and LTE
systems.
KEYWORDS: Orthogonal Frequency Division Multiplexing (OFDM), Long Term Evolution (LTE), Time
Synchronization, Primary Synchronization Signal (PSS).
I. INTRODUCTION
Orthogonal Frequency Division Multiplexing (OFDM) has become widely used for its high spectral efficiency and
robustness to multipath fading and delay. However OFDM systems are highly sensitive to timing and frequency
synchronization errors. Improper synchronization leads to Inter Symbol Interference (ISI) and Inter Carrier Interference
(ICI) which will drastically degrade the overall system performance [1]. It also leads to Symbol Timing Offset (STO)
and Carrier Frequency Offset (CFO) [2]. LTE with its advanced version serves as the key technology of Fourth
Generation (4G) systems. There are two types of synchronization signals adopted in LTE namely Primary
Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) [3].
Several time synchronization methods have been proposed for OFDM systems to find an estimate of the starting of the
symbol. A method using correlation with the cyclic prefix to find the symbol timing has been proposed in [4]. Later a
method to find both timing and carrier frequency offset has been suggested in [5]. Schmidl and Cox have proposed a
training symbol consisting of two identical halves for timing synchronization but the plateau of the timing metric
produces uncertainty about the start of the frame [6]. In [7], two timing offset estimation methods namely sliding
window method and training symbol method have been proposed. There are several timing offset estimation methods
suggested for OFDM systems [8-12] . Similarly several synchronization techniques for PSS have been proposed [1315]. Two new detectors namely Almost Half Complexity (AHC) and Central Self Correlation (CSC) have been
proposed to achieve reliable PSS detection with lower complexity by exploiting symmetric property of ZC sequence.
CSC has been proposed to combat large frequency offset [13]. In [14], an architecture where the PSS can be detected
efficiently and accurately at lower cost and power has been proposed. A new frequency domain PS & SS for step 1 and
2 in cell search which halves the computational complexity and lowers the hardware searcher complexity leading to
higher efficiency has been proposed [15].
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57
ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
Fig. 1 Block diagram of OFDM system
In this paper, a coarse timing offset is obtained using the cross-correlation function outputs based on the proposed
training symbol consisting of eight and sixteen identical segments. Using the proposed training symbol, lower
complexity and better bit error rate performance of timing synchronization is obtained even at very low SNR values.
The same idea of segmented symbol pattern is applied for PSS in LTE systems which in turn gives reduced complexity.
Simulation results are presented for our proposed algorithm and compared with existing time synchronization
algorithms. Section I gives the introduction. Section II gives the system description, existing techniques are discussed
in section III and proposed method is explained in detail in section IV. Section V discusses about the simulation results
and section VI gives the conclusion.
II. SYSTEM DESCRIPTION
An OFDM system block diagram is shown in fig.1.Here input bits are applied to serial to parallel converter and the
parallel bits undergo Inverse Fast Fourier Transform (IFFT). Since the OFDM signal is in frequency domain, IFFT is
the appropriate choice to be used in the transmitter, which can be thought of as converting frequency domain samples
to time domain samples at a faster rate. Then the parallel bits are converted to serial bits by using parallel to serial
converter. Cyclic prefix is then added and applied to Digital to Analog Converter (DAC). Analog bits travel through the
channels which are converted to digital after applying to an Analog to Digital converter (ADC). Added cyclic prefix is
now discarded and sent to serial to parallel converter. Parallel bits are applied to Fast Fourier Transform (FFT) and are
converted to serial bits by using parallel to serial block.
In LTE, an User Equipment (UE) must undergo a cell search procedure to access an LTE cell. When the UE is
switched on, it searches for a strongest cell in the downlink band usually in the central part of the bandwidth. Cell
search is a pre requisite function of any cellular system, during which timing and frequency synchronization is obtained
between the UE and the network. The UE must compute the cell search and selection procedure and acquire initial
system information prior to communicate with the network. In LTE, a radio cell is identified by a cell identity. It
consists of 504 unique physical-layer cell identities which are packed into 168 unique physical-layer cell-identity
groups [3]. Each group contains three unique identities such that each physical-layer cell identity is part of one and only
one physical-layer cell-identity group which is given by
NIDCell = 3 NID(1 )+ NID(2)
(1)
where NID(1) denotes the physical-layer cell-identity group lying in the range (0 to 167) and NID (2) denotes the
physical-layer identity within the physical-layer cell-identity group lying in the range ( 0 to 2). The sequence du(n) used
for the PSS is generated from a frequency-domain Zadoff-Chu sequence represented as
(2)
Where,
and
is the sequence length. Zadoff-Chu root sequence index u takes the values 25,29
and 34 corresponding to NID(2) values 0,1 and 2 respectively.
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ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
III. EXISTING TECHNIQUES
In OFDM system, the timing metric is generally defined as
M(d)=
(3)
Where P(d) is the correlation sequence and R(d) is the energy of the received symbol at dth sample. The strongest point
of correlation, i.e., the index of max {M(d)} is considered as the coarse timing point. The six popular techniques that
have been proposed in the literature for coarse timing synchronization are Schmidl & Cox (S&C) Method[6], Minn and
Bhargava method [7,8], Park and Cheon method[9], Ren et al method[10], Kang et al method[11] and Yi et a method
[12]. In [6], PN sequence is used as a training data which consists of two identical segments by transmitting a PN
sequence on the even frequencies while zeros are sent on the odd frequencies. Then the timing estimator takes as the
start of the symbol, the maximum point of the timing metric. Minn and Bhargava suggested the use of four identical
segments to reduce the uncertainty that has occurred in finding the correct timing [7, 8]. The constant envelope
preamble from the FFT of a CAZAC (constant amplitude zero autocorrelation) sequence is used for the training symbol
[10]. Kang et al. designed a preamble pattern independent technique, which uses the principle behind the preamble
structures employed to make the impulsive timing metric characteristic [11]. In [12], the technique uses conjugate
symmetric halves in the training symbol by interposing zeros as the guard band in the frequency domain form of the
preamble [12].
Zadoff –Chu (ZC) sequence is an ultimate option for PSS in LTE systems [13]. A ZC sequence has constant amplitude
which in turn limits the Peak-to-Average Power Ratio (PAPR) and generates bounded and time-ﬂat interference to
other users. It also simpliﬁes the implementation. ZC sequences of any length have „ideal‟ cyclic autocorrelation. The
main beneﬁt of this CAZAC property is that it allows multiple orthogonal sequences to be generated from the same ZC
sequence. The absolute value of the cyclic cross-correlation function between any two ZC Sequences is a constant.
IV. PROPOSED METHOD
The proposed training symbol for OFDM is composed of eight and sixteen parts as shown in fig,2. Eight and sixteen
portions of the training sequence are created by repeating the FFT of quarter length CAZAC sequence in all the
portions. Eight and sixteen segments help to provide steeper fall off of the timing metric from the strongest correlation
point.
In PSS, the FDD transmission scheme is assumed. The PSS signals are Zadoff-Chu sequences with their centre made
zero to represent DC. Each one has their amplitudes on the unit circle at different cyclically shifted phases and each has
constant amplitude, but are located at different phases. The memory of UE has a copy of the three possible PSS.
Determination of the symbol start is a first step to be performed by UE which is done by using a sliding window
method with a delay length of symbol length.
Fig. 2 Structure of the proposed training symbol
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59
ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
In this method, the ratio of the aggregated cross correlation to the aggregated auto correlation at the output of the delay
over a set of samples helps in detecting the symbol start. It is determined by checking the peak triangle in the ratio of
the cross and auto correlation. Now the UE has to match the received signal to one of the three sequences it knows.
The correlation amplitude maximum is obtained only when the received sequence matches with one of the three
sequences. Hence UE can now determine which PSS the radio cell is transmitting. Also, the UE knows the position of
the amplitude maximum peak when there is no offset and depending on the position of the peak it finds when it detects
the PSS it is able to calculate the offset. At the end of this step
the UE knows symbol boundary, Cell ID index (NID (2)) and sub frame timing. So, with the detection of PSS, the UE
knows whether it is synchronized with either sub frame 0 or sub frame 5. The proposed synchronization scheme adopts
the same TDD frame structure as LTE. Here the ZC sequence is divided into four segments instead of two as in [15]. It
is given by
(K)
exp
P(K) ,
(4)
(L)
,
(5)
(M)
(N)
,
(6)
,
(7)
V. SIMULATION RESULTS
The simulation was done in MATLAB and the parameters used for OFDM system are 1152 subcarriers, guard interval
128, 64 QAM modulation and Rayleigh fading. Fig.3 shows the timing metric for Schmidl and Cox methoe under 4
multipath for different SNR values. It shows a plateau and henceforth there is uncertainty about the starting point of the
window. As the SNR value increases better timing metric is obtained. Fig.4 shows the timing metric for Yi method
under outdoor scenario. Fig. 5 shows the timing metric for Park method for 4 multipath under outdoor scenario. Park‟s
method has impulse shaped timing metric, but large side lobes occur. Fig. 6 shows the timing metric for proposed
method with 8 portions.
Fig. 3 Timing metric for S & C method
under 4 multipath.
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Fig. 4 Timing Metric for Yi method under
Outdoor scenario with 4 multipath
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60
ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
Fig. 5 Timing metricof Park method
for outdoor scenario with 4 multipath.
Fig. 6 Proposed method for outdoor scenario
The proposed method shows better performance since the side lodes are suppressed and perfect timing is achieved.
Also the proposed method alone with suppressed side lobes is shown clearly for varying SNR values for outdoor
environment. It is obvious that the proposed algorithm is robust even under low SNR values for outdoor scenario. Fig.
7. gives the estimation of timing for 16 segments.
The magnitude of the CAZAC sequence is represented by Fig.8 . It can be seen that the amplitude is constant for all
the points except DC. The constant amplitude property limits the Peak-to-Average Power Ratio (PAPR) and generates
bounded and time-ﬂat interference to other users It also simpliﬁes the implementation as only phases need to be
computed and stored, not amplitudes. The cross correlation figures for the PSS sequences are represented in fig.9- 10.
The best way to check out this is to examine the correlation graph. If the correlation curve do not possess a peak in the
beginning of the slot, it is said to suffer from a fixed offset which can be estimated and rectified during the SSS
detection process.
Magnitude of PSS - It is a CAZAC sequence with DC punctured
2
1.8
Absolute Magnitude--->
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Fig. 7 Estimation of timing.
0
10
20
30
40
Sample--->
50
60
70
Fig. 8 Constant magnitude of PSS sequence
.
PSS - Cross Correlation of transmitted Cazac sequence with root 25 and root 29
70
Cross correlation of CAZAC (PSS) Sequence with root 25 and root 25 with no offset
70
60
Correlation Magnitude(Absolute)--->
Correlation Magnitude(Absolute)--->
60
50
40
30
20
40
30
20
10
10
0
50
0
0
10
20
30
40
50
60
70
0
Delay--->
Fig. 9 cross correlation of PSS sequence
with root 25 and 25 with no offset
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10
20
30
Delay--->
40
50
60
Fig. 10 Cross correlation of PSS sequence with root
25 and 29
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61
ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
.For simulation, it was assumed that the PSS with root 25 is the transmitted signal from the radio cell. Fig.9 shows the
correlation results of PSS at the receiver when there is an exact match of the sequence with root 25 without offset. Fig.
10 shows the correlation results of PSS at the receiver when there is no exact match with root 29.Similar result is
obtained for match with root 34. It is evident from fig. 9 that the correlation amplitude maximum is obtained only when
the received sequence matches with one of the sequences. The UE has determined which PSS the radio cell is
transmitting (in this case, N(2)ID is 0 ). The symbol start is determined by checking the peak triangle in the ratio of the
cross and auto correlation.
-5
6
x 10
5
Time s
4
3
2
1
0
64 Sequence
32 Sequence
16 Sequence
Fig. 11 Computation time comparison of three sequences
The Time taken for generation of the 64, 32 and 16 sequences are compared in fig. 11. The graph shows that the
amount of time saved in each cycle of generation of the codes for synchronization.
VI. CONCLUSION
The results of the timing methods help to determine which method performs better under different environments. The
Schmidl and Cox method and the Minn and Bhargava method have significant implementation drawbacks that hinder
their usage for reliable timing synchronization purposes. The Park et al. method and the proposed method performed
well for outdoor channels. The proposed method, a training symbol based method, showed a robust and satisfactory
performance for both indoor and outdoor channels even for low SNR. The proposed method with 8 and 16 portions of
the training symbol shows robust performance. The extended idea of four segments used in PSS of LTE systems also
results in low computation time and complexity which leads to power and component saving as time and computation
are less respectively.
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ISSN(Online): 2320-9801
ISSN (Print): 2320-9798
International Journal of Innovative Research in Computer and Communication Engineering
An ISO 3297: 2007 Certified Organization
Vol.3, Special Issue 3, April 2015
2nd National Conference On Emerging Trends In Electronics And Communication Engineering (NCETECE’15)
Organized by
Dept. of ECE, New Prince Shri Bhavani College Of Engineering & Technology, Chennai-600073, India during 6th & 7th April 2015
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