Single Carrier QPSK and 16 QAM System
Demonstration Using Frequency Domain
Equalization and Training Sequences
A. V. Tran*, C. Zhu*, C. C. Do*, S. Chen*, L. B. Du**, T. Anderson*,
D. Hewitt*, A. J. Lowery**, and E. Skafidas*
* Victoria Research Laboratory, NICTA Ltd., Level 2, Building 193,
Electrical and Electronic Engineering, University of Melbourne, Australia
** Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS),
Electrical and Computer Systems Engineering, Monash University, Australia
Abstract
We experimentally demonstrate coherent polarizationmultiplexed QPSK and 16QAM single-carrier system with
frequency-domain-equalization
over
800-km
in-line
transmission using binary training sequences. The
sequences are designed to be practical and achieve accurate
channel estimation in full-system demonstration.
I. INTRODUCTION
Recently, single carrier system with frequency domain
equalization (SC-FDE) has gained strong interest for high
bit rate optical communication system due to its similar
performance compared to orthogonal frequency division
multiplexing (OFDM) with essentially the same digital
signal processing (DSP) complexity, but offering extra
advantages such as no digital-to-analog converter (DAC)
at the transmitter, less sensitivity to nonlinear
impairments due to lower peak-to-average power ratio
(PAPR), and less sensitivity to phase noise and frequency
offset [1-3]. For SC-FDE systems, channel estimation
based on training sequences is usually employed for fast
convergence and reduced processing complexity [1-3].
Previously, all SC-FDE system demonstrations use
complex multi-level training codes for channel
equalization, which make it impractical for use in binary
systems such as commercial 100 Gb/s quadrature phaseshift keying (QPSK) systems. In this paper, we propose
the use of binary training sequences to achieve long-haul
transmission of QPSK and 16QAM coherent pol-mux
SC-FDE system. The proposed training sequences are in
binary form which enables easy integration into QPSK
and QAM systems and have accurate estimation of the
channel due to their clean signal spectra.
II. BINARY TRAINING SEQUENCES
A. Chu-based QPSK Sequence
The Chu sequence [4] is a class of training sequences
having flat frequency spectrum, and is ideal for channel
estimation. However, this sequence has multiple levels
and is not suitable for use in commercial binary QPSK
systems. We propose to use a Chu-based quadriphase
training sequence by passing the Chu sequence through a
QPSK slicer, to transform the positive amplitudes of the
real and imaginary parts of the Chu sequence into value
‘1’, and the null and negative amplitudes of the real and
imaginary components into value ‘-1’. Figs. 1 (a, b, c)
show the frequency spectrum of the Chu (flat spectrum),
Chu-based QPSK – denoted as Q-Chu (relatively flat
spectrum), and random QPSK sequences (noisy
spectrum), respectively.
B. Complementary Golay Sequences
Golay sequences are pairs of A and B sequences that have
the sum of autocorrelation of A and B being a Dirac delta
function [5]. Golay sequences have complementary
power spectra and can be used to estimate channel
transfer functions with high accuracy. For easy
integration with binary data channels, we propose a novel
design of Golay sequences that fit the QPSK alphabet by
using the following complex seed pairs: A = [1+j 1-j] and
B = [1+j -1+j]. The design of Golay sequences for polmux FDE is achieved by using Alamouti code [5] with
the frequency-domain training matrix scheduled as
ܵ1 ܵ2
‫ ܣ‬−‫∗ ܤ‬
ቀ
ቁ=ቀ
ቁ, where B* is complex conjugate
ܵ3 ܵ4
‫∗ܣ ܤ‬
of B. The frequency spectrum of the sum of
autocorrelation of A and B is shown in Fig. 1(d) and
illustrates a flat response similar to that of an ideal Chu
sequence.
(a)
(b)
(c)
(d)
Fig. 1. Frequency spectrum of (a) Chu; (b) Q-Chu; (c) Random
QPSK; (d) Sum of autocorrelation of Golay sequences.
III. EXPERIMENTAL DEMONSTRATION
Fig. 2 shows the experimental setup. Two 10-Gsymbols/s
arbitrary waveform generators (AWG1 and AWG2) are
used to generate two independent QPSK/16QAM SCFDE signals for the pol-mux system. The signal laser
with 100 kHz linewidth is modulated by two I/Q
modulators to represent X and Y polarizations. The two
signals are pol-muxed through the polarization beam
combiner (PBC). The signals are then transmitted through
10×80 km spans of standard single-mode fiber. An ASE
source is used to investigate the optical signal-to-noiseratio (OSNR) performance of the system. A polarization
mode dispersion (PMD) emulator is used to study system
performance under 1st order PMD. The inset of Fig. 2
shows the design of the preamble, which consists of 768
symbol training sequence and 16 pilot symbols for phase
noise correction. The training sequence consists of 2
QPSK - Q-Chu
QPSK - Golay
QPSK - Random
QPSK
15
10
5
0
0
400
800
Distance (km)
16QAM
(a)
Q Value (dB)
20
15
10
QPSK -B2B
QPSK -400km
16QAM - B2B
16QAM - 400km
5
0
Preamble Information Symbols
15
Training Sequence
16QAM - Q-Chu
16QAM - Golay
16QAM - Random
20
Q Value (dB)
sequences S1 and S2 for X-polarization (or S3 and S4 for
Y-polarization) of 256 symbols each, sandwiched
between two guard intervals (GI) of 64 symbols each.
The preamble is followed by 12288 information symbols.
Overlap FDE (O-FDE) [3] is adopted here to eliminate
the inter block interference (IBI) by discarding some
distorted samples at the head and end of each block after
FDE. Moreover, O-FDE helps the data symbols avoid
being truncated by the cyclic prefix, resulting in fewer
pilot symbols for phase correction. In the offline DSP
procedures at the receiver, timing synchronization is
achieved using the same training sequence. Channel
estimation is performed using 256-point FFT equalization
based on MMSE criterion with 64-symbol overlap cut
size. Phase correction is done using data-aided maximum
likelihood (DAML) algorithm [6].
20
25
Pilot
30
35
OSNR (dB)
GI
S1 GI GI
S2 GI
X pol.
GI
S3 GI GI
S4 GI
Y pol.
(b)
Q Value (dB)
20
15
10
5
QPSK - B2B
QPSK - 400km
16QAM - B2B
16QAM - 400km
0
0
10
20
30
DGD (ps)
(c)
Fig. 2. Experimental setup.
Fig. 3(a) shows the measured Q values for various
transmission distances using Q-Chu, Golay, and random
training sequences for both QPSK and 16QAM systems.
The results show that we can achieve Q value above 7 dB
for 16QAM after 800 km transmission for both Q-Chu
and Golay sequences, which is the 20% soft-decision
FEC limit to achieve bit-error-rate of 10-12. It is also clear
that the Q-Chu and Golay sequences have similar
performance over the measurement range, whereas the
random QPSK sequence has a poor performance due to
its noisy frequency spectrum. The insets next to Fig. 3(a)
show the measured constellation diagrams of the X
polarization for QPSK (top) and 16QAM (bottom)
system at 800 km transmission using Golay sequences.
Since the performance of Q-Chu and Golay sequences is
similar, for OSNR and PMD investigation, we only focus
on Golay sequences. Fig. 3(b) shows the measured Q
values versus OSNR for Golay sequences at back-to-back
and 400 km transmission for both QPSK and 16QAM
systems. The results prove that the proposed training
sequences have good performance in the presence of
strong ASE. Fig. 3(c) shows the measured Q values
versus first-order PMD for back-to-back and 400 km
transmission for both QPSK and 16QAM systems using
Golay training sequences. It is clear that the proposed
sequences achieve robust channel estimation against firstorder PMD up to 30 ps differential group delay (DGD).
Fig. 3. Measured Q factor for: (a) Different training sequences at
different distances; (b) Golay training sequences at different OSNR
values; (c) Golay training sequences at different DGD values.
IV. CONCLUSIONS
We have experimentally demonstrated coherent polmux QPSK and 16QAM SC-FDE system over 800 km
in-line transmission distance using binary training
sequences. The proposed training sequences have
accurate estimation of the channel due to their clean
signal spectra and are proven to be robust against high
level of ASE and first-order PMD. The proposed
sequences have the advantage of being in binary form
which enables easy integration into commercial QPSK
and QAM systems.
REFERENCES
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