nfoec.2013.jth2a.43

JTh2A.43.pdf
OFC/NFOEC Technical Digest © 2013 OSA
An Ultralow Complexity Algorithm for Frame
Synchronization and IQ Alignment in CO-OFDM Systems
1
K. Puntsri1, O. Jan1, A. Al-Bermani1, C. Wördehoff2, D. Sandel1, S. Hussin1, M.F. Panhwar1,
R. Noé1 and U. Rückert2
University of Paderborn, EIM-E, ONT , Warburger Str. 100, D-33098 Paderborn, Germany , Email: puntsri@mail.uni-paderborn.de
2
Cognitronics and Sensor Systems, CITEC, Bielefeld University, Bielefeld, Germany
Abstract: We present a simple and efficient method for CO-OFDM frame synchronization and IQ
component aligning. A training sequence is used. Simulations and experimental results confirm
that our proposed method outperforms the widely-used method of Schmidl&Cox.
OCIS codes: 060.1660; 060.2330.
1. Introduction
Coherent optical orthogonal frequency division multiplexing (CO-OFDM) is highly tolerant against channel
impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), due to the added cyclic
prefix (CP) [1,3]. However, one of the disadvantages of OFDM is the need to align the starting point of the OFDM
symbols and the FFT window. In case of mismatches intersymbol interference (ISI) and intercarrier interference
(ICI) would occur, because the received data fed into the fast Fourier transform (FFT) unit contains faulty data from
the following OFDM symbol. Therefore it is essential to align the incoming symbol to the FFT window before any
further signal processing.
The conventional widely-used technique for OFDM frame synchronization of Schmidl&Cox [2] has been
applied to CO-OFDM communication by many researchers, such as in [3,4]. This approach uses one whole OFDM
symbol which is split it into two parts, both of which are identical in the time domain. At the receiver the starting
point of the OFDM frame is obtained by detecting the correlation peak between these two parts. If the correlation
result reaches a defined threshold, then the starting point is detected. However, this method results in a low OFDM
frame efficiency due to the necessary size of the training sequence (TS). One complete OFDM symbol is needed for
synchronization and the correlation peaks become plateaus because of Amplified Spontaneous Emission (ASE)
noise and channel effects [2-4]. Additionally, this method cannot detect I and Q component swapping and inversion
due to the random phase rotation which occurs in a real-time system environment.
Therefore we propose a very simple and powerful method to detect the starting point of an OFDM frame exactly,
as well as to compensate for IQ swapping and inversion. To the best of our knowledge, this is the first method for a
simultaneous IQ component alignment and OFDM frame synchronization in CO-OFDM.
2. Proposed OFDM frame synchronization and IQ aligning method
To overcome the problems described above we propose a simple
and powerful synchronization scheme based on TS in both IQ
components. Two different Gold codes [5] are, used for I and Q,
inserted at the beginning of each OFDM frame. One frame contains
many OFDM symbols, as shown in Fig. 1.
At the receiver, the sampled IQ data are correlated with the
known TS pattern. The correlation is given by
PIQ (d ) 
I component
TSI
OFDM frame
OFDM symbol1 … OFDM symbolN
Q component
TSQ
OFDM symbol1 … OFDM symbolN
Fig. 1. Adding TSI and TSQ to the head of OFDM frame
d
∑ C IQ,k rIQ (d  k ) ,
(1)
k d -L
where PIQ (d ) is the correlation result, C IQ , k denotes the known TS, rIQ (d + k ) is the I and Q component of
received data sample, and L the length of the TS. The proposed algorithm searches for the maximum or minimum
peak of the correlation for I and Q to detect phase-noise-based distortions, given by
Mˆ max IQ  max( PIQ (d )3 )  thmax or Mˆ min IQ  min( PIQ (d )3 )  thmin .
(2)
Here Thmax is the maximum threshold and Thmin is the minimum threshold. To detect the starting point as well as to
correct for swapped I and Q data channels both input frames have to be correlated with TSI and TSQ. Due to noise
distortions the correlation peak can become positive or negative, so the algorithm has to search for the minimum as
well as for the maximum. Hence, the swapped IQ components can be defined from the swapping between I and Q
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JTh2A.43.pdf
OFC/NFOEC Technical Digest © 2013 OSA
correlation peaks, as shown in Fig. 2A. Obviously, the proposed method can detect all the situations: 0°, 90°, 180°,
270° phase rotation. In terms of hardware efficiency this method is very simple compared with the conventional
approach. The Gold code based TS contain only +1 and -1, so only XOR-Gates and adders will be needed for
correlation. Additionally, this concept can be applied to PolMux-Optical OFDM as well [6].
3. Simulation Results
The performance of the proposed algorithm for OFDM frame synchronization and IQ alignment was confirmed by
simulations using the setup described in [6]. In this simulation one OFDM symbol was defined as 288 samples
(=FFT size of 256 + 32 of CP length). Each subcarrier was modulated by 4-QAM. 128 subcarriers around the centre
of the spectrum were modulated with zeros for oversampling. A pilot for one-tap channel equalization and carrier
recovery was inserted at every 8th subcarrier location, by Comb-type [7]. The sampling rate was set to 28 Gs/s. A
fiber length of 1,000 km of standard single mode (SSM) fiber with chromatic dispersion (CD) of 17 ps/ns/km was
assumed.
C
C
No peak
TSI
C
TSQ
C
TSQ
C
No peak
Q
TSI
Q
IQ swapped
TSQ
I
C
TSI
C
TSQ
No peak
C
No peak
A: Proposed technique; (C is the correlator as shown in Eq. (2) and (3))
Correlation peak
1
0.8
0.6
0.4
0.2
0
OSNR 14 dB
-600
-400
-200
0
200
Timing metric
400
600
Correlation peak
TSI
Correlation peak
I
No IQ swapped
1
OSNR 14 dB
0.5
0
-0.5
I component
-1
-200 -150 -100 -50
0
50 100 150 200
Timing metric
1
OSNR 14 dB
0.5
0
-0.5
-1 Q component
-200 -150 -100 -50
0
50 100 150 200
Timing metric
B: Conventional technique result
C: Proposed technique results
Fig. 2. Proposed algorithm(left) and proposed result(right) for CO-OFDM frame synchronization and IQ components aligning.
Fig. 2B shows the result of the conventional scheme [2-4] compared to the results of the proposed scheme, Fig.
2C. The new method shows clear correlation peaks even for a low OSNR of 14 dB, for both I and Q components.
Moreover, the proposed method also detects inverted IQ components.
4. Experimental Results
QAM
Mapping
Add frame
sync
DAC
LPF
Copy
DAC
LPF
Generated data by Matlab; then save to FPGA
LPF
LPF
Cyclic
prefix and
Sync
Sequence
Romoval
F
F
T
One Tap
Equali
zation
QAMDemapping
P
/
S
BER test
DPMZM
Oscilloscope
Bit
Stream
I
F
F
T
Data
Zeros and pilot
insertion
CP
Fig. 3 shows the experimental setup. At the
F
5 Giga samples
P
transmitter, the OFDM data are preprocessed in
data stream
G
two synchronized Virtex-4 FPGAs for I and Q.
A
I
Q
The OFDM frame is generated as described in
FPGAVirtex4 ; Stored OFDM LUT
section 2. Then the digital samples are converted to
COUPLER
ECL
DPMZM
the analog domain by two Micram digital-toSignal EDFA
Opt. Power
VOA
laser
meter
analog converters at 5 Gs/s with a resolution of 6
bits. For O/E conversion a Dual-Parallel Mach100 km
EDFA
COUPLER
SMF
OFDM Offline Processing
Zehnder Modulator (DPMZM) is used to modulate
Optical90 º hybrid
the 1550 nm light of an external cavity laser (ECL)
S
Y
with a specified linewidth of 150 kHz and -3 dBm
n
c
I
launch power. After transmission over 100 km of
standard single mode fiber (SSMF) the signal is
Q
optical signals
fed to a variable optical attenuator (VOA) followed
PD
electrical signals
by an EDFA. Polarization is controlled manually.
Fig. 3. Experimental setup for OFDM synchronization and IQ aligning
At the receiver the signal is demodulated by a
polarization-diverse 90° optical hybrid. Then, two differential photodiode pairs convert the optical signals to
electrical ones (O/E). After photodetection and linear amplification, I and Q signals are sampled and stored in an
oscilloscope (TDS6804B) for offline processing.
This work focuses on the synchronization units depicted as red blocks in Fig. 3. After that OFDM is processed
for carrier recovery and channel compensation, by the following steps: removal of CP and TS, then an FFT to
Matlab
JTh2A.43.pdf
OFC/NFOEC Technical Digest © 2013 OSA
transform the received data to frequency domain, cancelation of the channel effects by one-tap equalization and, as
the final step, 4-QAM de-mapping and bit error ratio (BER) calculation. There are two transmitter operation modes.
The first is the training sequence mode in which the TS of 32 samples is transmitted. The second mode is
transmission of a sequence of 149 OFDM symbols. Therefore, the OFDM frame efficiency is 0.9993
(=288*149/(288*149+32)) . Fig. 4A and 4B show the correlation peaks for the IQ components of one OFDM frame
at received powers of -14dBm and -29 dBm.
1
Correlation peak
0.5
0
-0.5
0.5
-150
-100
-50
0
50
Timing metric
100
150
0
Received power -29 dB
-1
-200
200
-150
-100
-50
0
50
Timing metric
100
150
200
1
Q
Received power -14dB
0
Q
Received power -29 dB
0.5
0
-0.5
-0.5
-1
-200
I
0.5
-0.5
Received power -14dB
-1
-200
1
Correlation peak
I
Correlation peak
Correlation peak
1
-150
-100
-50
0
50
Timing metric
100
150
200
-1
-200
-150
-100
-50
0
50
Timing metric
100
150
200
A: Correlation peak of IQ components at OSNR of -14 dB
B: Correlation peak of IQ components at OSNR of -29 dB
Fig. 4. The correlation peak of IQ component for one OFDM frame at the received power of -14dB(left) and -29 dB(right).
10 -3
BER
As is seen from Figs. 4A and 4B, the proposed method
performs well under practical conditions even at the low
received input power of -29 dBm. The calculated peak is sharp
and clear. After frame synchronization BER performance was
measured for different pilot durations, which means a pilot
symbol is inserted in every 4th, 8th and 10th subcarrier,
respectively. Fig. 5 shows an optimal performance for a pilot
duration of 8 for this experimental setup. A BER of 9.610-5 at a
received power level of -14 dB and 3.210-4 at a received power
level of -29 dB was achieved. For pilot durations of 4 and 10
the BER performance was worse due to the slop of the phase
fluctuation on each pilot duration can not be tracked. In this
experiment the linewidth times sample duration product was 6
10-5.
pilot duration = 4
pilot duration = 8
pilot duration = 10
10 -4
-30
-28
-26
-24
-22 -20
-18
Rx input power (dBm)
-16
-14
-12
Fig. 5. BER versus received power
5. Conclusion
We have been proposed a simple and efficient algorithm for CO-OFDM frame synchronization and simultaneous IQ
component alignment swapping and inversion. The TS is used for both purposes. The method has been evaluated by
simulation and experiment. It shows that the correlation peak is clear and sharp; the base of the peak is narrow. This
confirms that our proposed method outperforms the conventional technique in terms of accuracy and hardware
efficiency.
6. References
[1] J. Armstrong C. van Trigt, “OFDM for Optical Communications,” IEEE Journal of Lightwave Technology., 27, 189-204 (2009).
[2] T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Communication. 45, 1613–1621
(1997).
[3] Q. Yang, et. al., “Real-Time Coherent Optical OFDM Receiver at 2.5-GS/s for Receiving a 54-Gb/s Multi-band Signal,” OFC’09, paper
PDPC5.
[4] T. Liang, et. al., “An adaptive algorithm of fine synchronization for CO-OFDM system,” ACP’2011, pp.1-8.
[5] D. Esmael, et. al., “Spreading codes for direct sequence CDMA and wideband CDMA cellular networks,” IEEE Commu. Magazine., 36, 4854, (1988).
[6] K. Puntsri, et. al., “A low complexity and high accuracy frame synchronization for PolMux-Optical OFDM,”IPC’12, pp.181-182.
[7] S. Coleri, et. al., “Channel estimation techniques based on pilot arrangement in OFDM Systems,” IEEE Trans. Broadcasting. 48, 223–229
(2002).