Timing synchronization for mc-cdma systems using a time-multiplexed synchronization channel

AJSTD Vol. 23 Issue 3 pp. 231-237 (2006) 
TIMING SYNCHRONIZATION FOR MC-CDMA 
SYSTEMS USING A TIME-MULTIPLEXED 
SYNCHRONIZATION CHANNEL 
Pham Hong Ky* and Nguyen Duc Long 
Research Institute of Post and Telecom 
122 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam 
Received 25 November 2005 
ABSTRACT 
In this paper, we present a synchronization algorithm for Multi-Carrier CDMA (MC-CDMA) 
systems using a time-multiplexed synchronization channel. The simulations results show that a 
system using proposed algorithm has much higher timing detection probability than using other 
algorithms, especially in exponential decay multipath fading. 
1. INTRODUCTION 
MC-CDMA (Multi Carrier Code Division Multiple Access) is one of principle multiple access 
methods proposed for next generation mobile communication system. However, like other 
multi-carrier systems, MC-CDMA system is limited by synchronization errors more than one 
carrier system. Synchronization errors reduce received signals strength and make its phase 
rotated, and cause ICI (Inter Carrier Interference) also. Due to ICI, the orthogonality between 
sub-carriers will be broken. This causes the performance reduction of the whole system. 
In MC-CDMA systems, the signals can be spread in frequency or time domain by spreading 
code [1 - 3]. The synchronization is acquired by using information on the pilot and 
synchronization (SCH) channel. SCH channel structure is shown in Fig. 1. There are some 
algorithms proposed to achieve the timing synchronization [4]: Detecting the correlation 
between received signals before FFT (Fast Fourier Transform) processing and SCH signals at 
the receiver, detecting the correlation between two consecutive received SCH in the time-
multiplexed structure and detecting the Guard Interval correlation in frequency-multiplexed 
SCH. 
We will review the above-mentioned algorithms [4] along with their advantages, disadvantages 
and in the next section; we propose one new algorithm to improve the performance of the whole 
system. 
We first present the correlation detection algorithm between the received signals before FFT 
processing and SCH signals at the receiver. In this algorithm, the SCH with one- or two-symbol 
duration is time-multiplexed into the coded data symbols at each sub-carrier at the beginning of 
each frame. The synchronization at the receiver is carried out by using one SCH-replica 
generated by performing IFFT (Inverse FFT) on the SCH. After calculating the correlation 
between the SCH-replica and the received signal sequence before FFT, the receiver averages the 
correlation values in squared form over several frame interval in order to decrease the impact of 
∗Corresponding author e-mail: phamhongky@hn.vnn.vn 
Pham Hong Ky and Nguyen Duc Long Timing synchronization for MC-CDMA systems using 
the interference and noise. Then, it finds the highest averaged correlation value and defines the 
timing. 
N
c 
su
b-
ca
rr
ie
rs
C
od
e
Fig. 1: Time-multiplexed SCH (above) and frequency-multiplexed SCH (below) 
Fig. 2: Detecting the correlation between received signals before FFT processing and SCH 
signals at the receiver 
The second algorithm is time-multiplexed synchronization with one-symbol-differentiated 
correlation method. This algorithm uses two successive SCH symbols multiplexed at the 
beginning of each frame. At the receiver, the correlation between the received signal and one-
symbol delayed version of the signal per FFT sampling duration is calculated. After coherently 
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AJSTD Vol. 23 Issue 3 
accumulating the correlation values over several frames, searching the highest correlation 
output, the receiver can detect the FFT window and frame timing. 
In two time-multiplexed SCH methods mentioned above, the results in [4] show that the 
algorithm using SCH-replica has a better performance and is widely used. However, its 
disadvantage is that the receiver must know the structure of the transmitted SCH. Beside these 
two methods, the synchronization can be achieved by using the Guard Interval in the frequency-
multiplexed SCH. Here, the SCH is frequency-multiplexed into the sub-carriers every n sub-
carriers. The synchronization is taken into two steps: FFT window timing and frame timing. 
FFT window timing is detected by using guard interval. The detector calculates the correlation 
between the received sequence and the version of that delayed by FFT window interval, then 
accumulates over all symbols of one frame. The timing is detected by searching the maximum 
value. 
Frame timing is detected by using the frequency-multiplexed SCH. Assuming that SCH is 
multiplexed into k sub-carriers, the receiver can use the FFT window timing detected from the 
first step for FFT processing. Firstly, it takes the correlation between SCH-replica and FFT 
output of each sub-carrier in which the SCH is transmitted, then takes the sum of k-calculated 
correlations. Finally, taking the average over several frames and finding the maximum value. 
Fig. 3: Detecting the correlation between two received consecutive time-multiplexed SCH 
symbols 
Fig 4: FFT window timing in frequency-multiplexed synchronization 
Among these three algorithms, the algorithm that detects the correlation between two 
consecutive received time-multiplexed SCH symbols has the advantages of simplicity and the 
structure of transmitted SCH is not required at the receiver. However, its disadvantage is the low 
synchronization performance in multi-path fading channel [5]. In the next section, we will 
present this algorithm in detail and propose one modification to improve the performance. 
 233
Pham Hong Ky and Nguyen Duc Long Timing synchronization for MC-CDMA systems using 
2. THE SYNCHRONIZATION USING TWO CONSECUTIVE TIME-MULTIPLEXED 
SCH SYMBOLS AT THE BEGINNING OF EACH FRAME 
Fig. 5: Synchronization using 2 consecutive time-multiplexed SCH symbols at the beginning of 
each frame 
In this synchronization method [4], the frame and the FFT window timing are found by 
searching the maximum value of the correlation between two consecutive symbols 
( ) ( ) ( )*
1 symbol
k r k r k NΘ = + +∑ GI
i
 (1) 
where N is the FFT window length. 
The disadvantage of this algorithm is that the plateau occurs in the correlation window, as 
shown in Fig. 6, so that it is difficult to define the peak. As consequence, the timing is not 
correct. 
In order to eliminate the plateau, in [6], two symmetrical SCH symbols, as shown in Fig. 7, are 
used instead of the two identical ones . 
The correlation is calculated as follow: 
( ) ( ) ( )*
1
N
i
k r k i r k
=
Θ = − +∑ (2) 
0 50 100 150 200 250
1.8
1.85
1.9
1.95
2
2.05
2.1
2.15
2.2
2.25
Chip
C
or
re
la
tio
n 
m
et
ric
Fig. 6: Plateau in the timing metric 
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a1 a2 a3 a4 a5GI a5 a4 a3 a2 a1GI
symmetric
Fig. 7: SCH structure in modified algorithm 
The correlation will reach the maximum value at correct timing. 
150 155 160 165 170 175 180 185 190 195 200
100
150
200
250
Fig. 8: Timing metric in modified algorithm 
The modified algorithm shows good performance in AWGN with well-defined timing metric 
peak, but it is not the case in fading channels. Same as in other algorithms mentioned above, the 
main reason is that the received power is concentrated not only in the direct ray but also in other 
reflected rays with different delays. As result, the maximum timing peak corresponds to the 
position that is delayed several samples comparing to exact timing. 
In case of multi-path fading channel, there are several peaks occurring in the timing metric 
window, corresponding to the delayed rays. The peak corresponding to correct timing must be 
the one that has much differences compared to the remaining ones. Therefore, to define the 
exact timing, we propose a different coefficient α. Then, the timing is found by searching the 
peak that is greater than α time of the previous maximum value (α is some preset value at the 
receiver). 
170 175 180 185 190 195 200
950
1000
1050
1100
1150
1200
1250
Fig. 9: Several peaks occur in fading channels 
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Pham Hong Ky and Nguyen Duc Long Timing synchronization for MC-CDMA systems using 
3. SIMULATION RESULTS 
In this section, we present the simulation results to confirm the performance of the modified 
algorithm. The MC-CDMA system has number of sub-carriers of 512cN = with the frame 
length of 7, the frames used for averaging are 3, spreading factor is 32SF = , guard interval is 
128 chip, modulation method is QPSK, spreading code is Walsh-Hadamard, the bandwidth is 
80MHz with the channel spacing between sub-carries of 156.25Khz, and the Doppler frequency 
is 80Hz. The fading channel is exponential decay with number of paths of 24, SNR of 6dB. 
4. CONCLUSION 
In this paper, we presented some timing synchronization algorithms proposed for the MC-
CDMA systems using time-multiplexed and frequency-multiplexed synchronization channels, 
and proposed one modified algorithm to improve the performance of the whole system. The 
modified algorithm uses the different coefficient to find the peak of correlation metric with high 
correction. The simulation results show that the timing detection probability is much higher 
compared to previous algorithms. 
-20 0 20 40 60 80 100 120 140
0 
0.01
0.02
0.03
0.04
0.05
0.06
0.07
detected timing
pr
ob
ab
ili
ty
Fig. 10: Timing detection probability in original algorithm 
-60 -40 -20 0 20 40 60
0 
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
detected timing
pr
ob
ab
ili
ty
Fig. 11: Timing detection probability in modified algorithm 
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