ZTE Communications

S pecial Topic
Exploiting the Faster-Than-Nyquist Concept in Wavelength-Division Multiplexing Systems Using Duobinary Shaping
Jianqiang Li, Ekawit Tipsuwannakul, Magnus Karlsson, and Peter A. Andrekson
shaping and filtering allows the PM-QPSK signals to be
accommodated on a 25 GHz grid with acceptable linear
crosstalk. The two WaveShapers and 3 dB coupler act as an
optical interleaver with the ability to tune the optical-filtering
bandwidth. This tunability allows us to determine the tolerance
of the proposed duobinary shaping scheme to the
optical-filtering bandwidth. Spectral shaping can also be
done by electrical filters such as those in [23] and [24]. Here,
we take an optical approach to spectral shaping because
WDM components are used in practice to combine WDM
channels. Then, the entire WDM signal is
polarization-multiplexed with a differential delay of
approximately 180 symbols between the two polarizations.
The fiber link was built up in a straight line using eight 80 km
standard single-mode fiber (SSMF) spans with
erbium-doped fiber amplifiers (EDFAs) only. The optical
power, PL, launched into the SSMF spool in each span was
the same. At the receiver, the WDM signal was pre-amplified
and then filtered by a 0.8 nm optical bandpass filter (BPF) to
suppress wideband noise. Intradyne detection of the central
channel was implemented using a commercial
integrated-coherent receiver in a conventional
polarization- and phase-diverse configuration. Finally, the
four detected tributaries were captured, each with 3×10 6
samples, by a 50 GSa/s digital sampling oscilloscope with
16 GHz analog bandwidth for offline processing.
The conventional DSP blocks for a PM-QPSK coherent
receiver are preserved without modification, which is a tenet
of the proposed DSP structure. The proposed structure allows
DSP reconfiguration because all the DSP blocks work in a
feed-forward fashion, and the additional post-filter and
MLSD can be easily switched to hard decision. After I/Q
imbalance is compensated for using the Gram-Schmidt
algorithm [31], electronic dispersion compensation (EDC)
based on static time-domain equalization is performed to
compensate for all the accumulated dispersion. The sample
streams are then resampled to two samples per symbol. A
blind adaptive equalizer with four 15-tap T/2-spaced
butterfly FIR filters adapted using the classic CMA follows.
Carrier recovery is then performed, which includes frequency
offset estimation based on fast Fourier transform (FFT) [34]
and carrier phase estimation based on the fourth-power
Viterbi-Viterbi algorithm [31]. After these conventional DSP
blocks, the digital post-filter performs duobinary shaping and
the MLSD performs suboptimum detection on each signal
quadrature of each polarization, that is, each electrical lane in
a practical PM-QPSK transponder. (Fig. 2, highlighted
boxes). On each signal quadrature in each polarization, the
signal can be considered to have a 2-ary pulse-amplitude
modulation (PAM) format. Therefore, the MLSD based on
Viterbi algorithm only has two states for each quadrature of
each polarization. Differential coding is used for all cases to
overcome cycle slipping and phase ambiguity.
We first investigated performance at different symbol rates.
Fig. 3 shows B2B performance at a 25 Gbaud symbol rate,
which is equal to the channel spacing. First, the performance
of a single-channel PM-QPSK signal was measured by
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ZTE COMMUNICATIONS
March 2012 Vol.10 No.1
Log10 (BER)
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-3
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Theoretical Limit
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13 14 15 16
OSNR (dB) in 0.1 nm Reference Bandwidth
: 1 ch without WaveShaper
: 1 ch with 22 GHz WaveShaper
: 3 ch with 22 GHz WaveShaper
BER: bit error rate OSNR: optical signal-to-noise ratio
▲Figure 3. BER as a function of OSNR in B2B at 25 Gbaud.
configuring the WaveShaper in all-pass mode. The post-filter
and MLSD were turned off in the DSP because there was no
strong ISI. Compared with the theoretical limit and taking into
account the differential coding, there is approximately 1 dB
typical optical SNR (OSNR) penalty at a bit error rate (BER) of
10 -3 . Second, aggressive spectral shaping was performed on
the single-channel PM-QPSK signal by configuring the
WaveShaper with a 22 GHz 3 dB bandwidth. The post-filter
and MLSD were activated to perform duobinary shaping and
suboptimum detection. By virtue of the post-filter and MLSD,
a single-channel PM-QPSK signal only suffers approximately
0.5 dB required OSNR penalty. Third, we turned on the two
channels that were closest to each other in order to determine
the effect of inter-channel linear crosstalk. Another 0.4 dB
required OSNR penalty appears (Fig. 3). There is less than 1
dB OSNR penalty in WDM systems with a channel spacing
equal to symbol rate. Next, we pushed the symbol rate to 28
Gbaud while maintaining the 25 GHz channel spacing. The
symbol rate was faster than Nyquist, and the raw spectral
efficiency was above 4 b/s/Hz. Fig. 4 shows B2B BER as a
function of OSNR at 28 Gbaud. There is only 0.6 dB
implementation penalty for single channel, similar to the
penalty at 25 Gbaud. However, the penalty is larger when
there is linear crosstalk due to the boosted symbol rate. This
implies that the proposed duobinary shaping technique works
well, and performance loss is mainly due to interchannel linear
crosstalk not the technique itself. The BER of 10 -3 cannot be
reached in the three-channel setup if the proposed technique
is disabled at 25 Gbaud or 28 Gbaud. In sum, we have shown
that there is an implementation penalty of approximately
0.9 dB at 25 Gbaud and an implementation penalty of
approximately 1.7 dB at 28 Gbaud on a 25 GHz WDM grid. By
comparison, the systems in [13], [15], and [20] had greater
than 2 dB OSNR penalty at B2B. To the best of our
knowledge, the implementation penalties described in this
paper are the smallest for PM-QPSK WDM systems with such
high spectral efficiency. This small implementation penalty is
achieved by only using one-symbol memory in the MLSD.
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1 dB 0.5 dB 0.4 dB
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Experiments
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