40G/100G long-haul optical transmission system design using
coherent receivers
Citation for published version (APA):
Borne, van den, D., Alfiad, M. S., Jansen, S. L., & Wuth, T. (2009). 40G/100G long-haul optical transmission
system design using coherent receivers. In
Proceedings of the 14th OptoElectronics and Communications
Conference, OECC 2009, 13-17 July 2009, Hong Kong
(pp. FN-1-1/2). Institute of Electrical and Electronics
Engineers. https://doi.org/10.1109/OECC.2009.5214277
DOI:
10.1109/OECC.2009.5214277
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Published: 01/01/2009
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40G/100G long-haul optical transmission system design using
digital coherent receivers
Dirk van den Borne
1
, Mohammad Alfiad
2
, Sander L. Jansen
1
and Torsten Wuth
1
1
Nokia Siemens Networks GmbH & Co.KG, St-Martin Strasse 76, D-81549, Munich, Germany, E-mail:
dirk.vandenborne@nsn.com
2
Cobra institute, Eindhoven University of Technology, Eindhoven, 5000MB, The Netherlands
Abstract
The rise of coherent detection and digital signal
processing is drastically changing the design of optical
transmission systems. In this paper we review the
challenges and opportunities offered by such receivers in
the design of long-haul 40G/100G systems.
Introduction
The recent progress on high-speed digital signal
processing enables the use of digital coherent receivers
in long-haul optical transmission systems. Although this
is a new development in the field of optical
communications, it mirrors the technical evolution taken
years ago in radio and wireline communication. As such,
it is likely that digital coherent receivers will rapidly
become the technology of choice in long-haul optical
transmission systems.
The rapid shift away from direct-detection receivers and
towards digital coherent receivers is fuelled by a number
of technology drivers. Among others, digital coherent
receivers have spurred the use of higher-order
modulation formats (e.g. quadrature phase shift keying
[QPSK]), polarization-multiplexing, the compensation of
linear transmission impairments such as chromatic and
polarization-mode dispersion (PMD) [1-2] as well as
improved possibilities for optical performance
monitoring [3]. Although most of these technologies are
not exclusive to digital coherent receivers, it is the
combination that generally results in improved optical
performance and enables a more cost-effective solution.
In this paper we review some of the possibilities offered
by digital coherent receivers and detail how this will
enable the next generation of long-haul 40G/100G
optical transmission systems. We focus in this discussion
on 40G/100G transmission using polarization-
multiplexed quadrature phase shift keying (POLMUX-
QPSK) modulation, often also referred to as CP-QPSK,
PDM-QPSK or DP-QPSK, which is likely to become the
next standard for long-haul optical transmission systems.
Transmitter and receiver architecture
A POLMUX-QPSK transmitter consists of two
quadrature (e.g. QPSK) modulators and a polarization
beam splitter (PBS) to multiplex the two outputs on
orthogonal polarizations. At the receiver side, the optical
signal is split in two tributaries with arbitrarily, but
orthogonal, polarizations using a second PBS. Both
tributaries are subsequently mixed in a 90
0
hybrid
structure with the output of a local oscillator. The
outputs of the 90
o
hybrid (in-phase and quadrature
components of both polarizations) are then detected with
4 photodiodes (either balanced or single-ended) and
converted to the digital domain using high-speed analog-
to-digital converters (ADCs) [1,2].
Fig. 1 shows the constellation diagram of POLMUX-
QPSK modulation, represented as a 4-dimensional
hypercube (in-phase and quadrature on two
polarizations). The use of a 4 bits/symbol modulation
format results in a low symbol rate of 10.75 Gbaud
(43-Gb/s line rate) and 28 Gbaud (112-Gb/s line rate),
respectively. Note that the 43-Gb/s and 112-Gb/s line
rate result in 40-Gb/s resp. 100-Gb/s net data rate when
forward-error correction (FEC) and Ethernet overhead
are subtracted. The lower symbol rate improves the
tolerance to linear transmission impairments as well as
making it possible to use lower-frequency electrical
components. In addition, the combination of POLMUX-
QPSK modulation and coherent detection allows for an
OSNR requirement close to the theoretical optimum [1].
Fig.1: POLMUX-QPSK constellation diagram.
System design using digital coherent detection
At first sight the advantage of digital coherent receivers
might not be entirely straightforward, as the transmitter
and receiver complexity is generally much higher in
comparison to established direct-detection modulation
formats such as 43-Gb/s differential phase shift keying
(DPSK) or even 43-Gb/s differential quadrature phase
shift keying (DQPSK) [4]. However, when we look at
the overall system complexity, the advantages of
POLMUX-QPSK modulation and digital coherent
receivers are much more pronounced.
In order to upgrade existing transmission links to a
43-Gb/s or 112-Gb/s line rate, the modulation format
should be able to cope with all of the transmission
impairments incurred by the already installed equipment.
This includes transmission over high-PMD fiber,
installed dispersion compensation modules (e.g.
dispersion compensating fiber or fiber Bragg gratings
[FBGs]) [5] as well as a limited optical bandwidth
FN1
978-1-4244-4103-7/09/$25.0 © 2009 IEEE
through cascaded filtering in photonic cross-connects
(PXC) [1]. Transmission links with high-PMD fiber or
multiple cascaded PXCs are straight-out challenging for
43-Gb/s DPSK modulation, resulting in a reduced
transmission reach. For such links, 43-Gb/s DQPSK is a
promising alternative as the lower symbol rate
(21.5 Gbaud) significantly improves the tolerance to
both PMD and narrowband optical filtering. However, at
an 112-Gb/s line rate the optical spectrum of DQPSK
modulation is too broad to fit within a 50-GHz channel
grid making it incompatible to most field-deployed
transmission systems. Digital coherent receivers
combined with either 43-Gb/s or 112-Gb/s POLMUX-
QPSK modulation, on the other hand, can compensate
for dispersion map deviations, PMD, FBGs induced
phase ripples, as well as being more tolerant to the
optical filtering resulting from cascaded PXCs on a
50-GHz grid. Hence, this will enable field-deployed
transmission links to upgrade to 43-Gb/s and 112-Gb/s
line rates without an exchange in installed equipment.
On newly deployed transmission links, which generally
use high-quality fibers, the optical performance of
43-Gb/s DPSK is roughly equivalent to 43-Gb/s
POLMUX-QPSK modulation. In this case not the optical
performance, but simplicity of the transmission link is
the most important advantage of using digital coherent
receivers. It negates the need for dispersion management
along the transmission link, which in turn offers
advantages in transmission latency, sparing of dispersion
compensation modules, and allows for simpler amplifier
structures. In addition, the optical performance
monitoring capabilities that a digital coherent receiver
offers reduces the number of required measurements on
the installed fiber base and simplifies monitoring of
transmission performance.
Regretfully, coherent detection does not simplify all
aspects of transmission link design. In particular the
nonlinear tolerance of 43-Gb/s POLMUX-QPSK
modulation is significantly reduced compared to DPSK
modulation as it is very sensitive to XPM-induced
nonlinear phase shifts. This would have a significant
impact on the maximum transmission reach, where it not
for the fact that the improved OSNR requirement offsets
the reduced nonlinear tolerance. As a result both
modulation formats have a comparable transmission
reach. For 112-Gb/s POLMUX-QPSK modulation, the
nonlinear tolerance is significantly higher compared to a
43-Gb/s line rate as the higher symbol rate reduces the
impact of XPM. This helps to partially offset the ~4 dB
increase in required OSNR (and therefore lower reach)
when scaling from a 43-Gb/s to an 112-Gb/s line rate.
Still, careful system design is required when 43-Gb/s or
112-Gb/s POLMUX-QPSK modulated channels co-
propagate with other modulation formats (in particular
10-Gb/s on-off-keying) on the same fiber.
Receiver complexity of digital coherent detection
Compared to direct-detection receivers, coherent
detection and the associated digital signal processing
imply a significant shift in system complexity from the
optical to the electrical domain. In particular the ADCs
are a key component for any digital coherent receiver
implementation. Ideally the optical signal is converted to
the electrical domain using a factor of two over-
sampling, which implies that ~60-Gsample/s ADCs are
required to realize a 100G coherent receiver. The design
of a 60-Gsample/s ADC that allows for a >18-GHz
electrical bandwidth, effective vertical resolution of at
least 4 bits, and a power consumption of only a couple of
Watts is truly challenging and requires state-of-the-art
mixed signal design [6]. The same is true for the
receiver-side digital signal processing, which may
consist of as many as 40 to 100 million gates. It therefore
requires state-of-the-art 40nm or 65nm CMOS processes
in order to reduce power consumption. In addition, the
ADC and digital signal processing are preferably
integrated on a single-chip in order to limit the power
consumption associated with inter-chip communication.
The optical components in a POLMUX-QPSK
transmitter and receiver represent as well a higher
complexity compared to more conventional direct-
detection modulation formats (e.g. DPSK). Optical
integration might be one of the promising directions to
reduce footprint, power consumption and improve
optical specifications. For example, a single POLMUX-
QPSK Mach-Zehnder modulator at the transmitter and
an integrated quad photo-diode array combined with a
90
0
hybrid structure at the receiver are both promising
directions of optical integration.
Finally, an important consideration for 100G
transmission systems is the implementation of advanced
FEC coding and de-coding schemes. Due to the
25-Gbaud symbol rate it is possible to add up to 20%
FEC overhead without incurring significant optical
filtering penalties in cascaded PXCs, a problem which
prevents the use of high overhead FEC for 43-Gb/s
DPSK modulation. The use of low-density parity check
codes with ~20% overhead combined with soft-decision
decoding enables a 2-3 dB improvement in effective
coding gain over the class of FEC codes typically used at
43-Gb/s line rates (~11 dB coding gain at 10
-15
BER) [7].
Conclusions
Within the next few years, coherent detection and digital
signal processing will drastically change the way optical
communication systems are designed. The advantages
this technology offers in optical performance and
operation simplicity will truly enable the next-generation
of long-haul 40G/100G transmission systems.
References
[1] C.R.S Fludger, et al., JLT, Vol. 26, pp. 64-72, 2008.
[2] G. Charlet, et al., JLT, Vol. 27, pp. 153-157, 2009.
[3] J.C. Geyer, et al., PTL, Vol. 20, pp. 776-778, 2008.
[4] D. van den Borne, et al., OFC 2008, paper OMQ1.
[5] V. Veljanovski, et al., OFC 2009, paper JThA40.
[6] J. Sitch, ECOC 2008, paper Th1A1.
[7] T. Mizuochi, OFC 2008, paper OTueE5.
![Fig. 1 shows the constellation diagram of POLMUXQPSK modulation, represented as a 4-dimensional hypercube (in-phase and quadrature on two polarizations). The use of a 4 bits/symbol modulation format results in a low symbol rate of 10.75 Gbaud (43-Gb/s line rate) and 28 Gbaud (112-Gb/s line rate), respectively. Note that the 43-Gb/s and 112-Gb/s line rate result in 40-Gb/s resp. 100-Gb/s net data rate when forward-error correction (FEC) and Ethernet overhead are subtracted. The lower symbol rate improves the tolerance to linear transmission impairments as well as making it possible to use lower-frequency electrical components. In addition, the combination of POLMUXQPSK modulation and coherent detection allows for an OSNR requirement close to the theoretical optimum [1].](/figures/fig-1-shows-the-constellation-diagram-of-polmuxqpsk-23orc1so.webp)