High-Level Review of New
Modulation Formats for
High-Capacity Optical Networking
This technical tutorial provides a high-level description of
various modulation formats used to address high-capacity
needs. This paper covers coherent detection approach,
from 50G to 400G+ channel rates, and describes an
example of direct detection scheme as well.
Insatiable Need for Higher
Capacity
The exponential demand for bandwidth is
fueled not only by increases in the number of
users, the access methods and rates, but also
the increased importance of cloud, cloud
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services, and data centers and the number of
services (such as mobile, social media, and
video in general). On any given night Netflix
and YouTube make up greater than 50% of
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traffic in the US; 62% of consumers watched
online videos in 2015. A great influence on
network evolution is likely to be the bandwidth
requirements for 4K video, which is hitting the
market, to be followed by 8K video. This
evolution is illustrated by the advent of new
players whose bandwidth needs are strongly
increasing and add to those from traditional
telecoms service providers. E.g. traffic
delivered through content delivery network
players progressively increased, and now
accounts for about 50% of global Internet
traffic.
In a traditional telecom network traffic flows
from the user to the network central office and
back – this is commonly referred to as northsouth traffic. The proliferation of data centers
built by OTT providers and communication
service providers is having a drastic effect on
traffic. Most traffic in a data center network is
generated by machine-to-machine traffic
between data centers – termed east-west
traffic. This east-west DCI traffic represents
now a very predominant part of all traffic
compared with the north-south traffic. The
requirements of the optical backbone networks
supporting east-west traffic significantly differ
from those for traditional telecom operators
with higher capacity per fiber, longer optical
data paths and fewer sites where access to the
traffic is required.
Introduction of coherent
approach was driven by higher
spectral efficiency needs.
And at the Beginning Was
Direct Detection
Optical data transport started with the simplest
(and therefore cheapest) digital coding
schemes: On/Off-Keying (OOK), where the
physical parameter that is modulated is
amplitude (power) of the optical wave. The
detection is carried out by a simple
photodetector that detects power level hitting
the receiver and converts it into a photocurrent
reflecting the optical amplitude modulation in
the so-called direct detection implementation.
Each transmitted symbol takes two values: low
optical power level (digital 0) or high optical
power level (digital 1). Therefore each
transmitted symbol can be encoded with one
bit (binary digit):


0 for low optical power level
1 for high optical power level
Figure 1: Traditional two-level power modulation for direct detection, with one symbol corresponding to one bit.
The basic role of the receiver is to make a
decision for each received symbol, which may
be corrupted by optical noise and symbol
distortion following fiber propagation.
In the case of a two-level power modulation
like OOK format, the symbol rate is equal to
the bit rate. For a 10G carrier modulated by an
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OOK signal, 10 giga symbols are transmitted
per second, corresponding to the same
number of bits transmitted per second, i.e. a bit
rate of 10 Gbit/s. In telecommunication theory,
baud is the unit of the symbol rate; 10 giga
symbols per second is then equivalent to
10 Gbaud. Current commercially-available
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optoelectronics technologies are limited to
about 35 Gbaud (in this document, we will
assume a figure of 30 Gbaud for the sake of
simplicity). In order to transmit, e.g.,
100 Gbit/s per wavelength, more bits per
symbol are required. As this has been done
for many decades in wireless communications,
this can be simply achieved by using multilevel modulation formats where symbols can
take more than two values (levels) of one or
several physical parameters.
Two simple examples of multi-level modulation
formats are provided here:

Four-level modulation format
o Symbols can take 4 different
values/levels (e.g. 0, 1, 2 and 3)
o Two bits are encoded in one
symbol to enable four different
values/levels (00, 01, 10 and 11)

Sixteen-level modulation format
o Symbols can take 16 different
values/levels (e.g. 0 to 15).
o Four bits are encoded in one
symbol to enable 16 different
values/levels (0000 to 1111).
Here Comes (Optical)
Coherent Detection
Continuous capacity growth and lower cost per
transported bit are key challenges any longhaul optical transmission infrastructure needs
to cope with. The advent at the beginning of
this decade of 100G channel rate in
commercial networks, associated with digital
coherent detection (again, not a new thing in
radio communications), offered a 10-fold
capacity increase compared to the previous
transmission technology based on 10G waves.
Unlike the 40G developments where multiple
formats were assessed and proposed (NRZ,
ODB/PSBT, CSRZ, ADPSK, DBPSK,
DQPSK...), 100G development avoided the
multiple modulation formats “mistake” of 40G.
A general consensus was quickly reached
around PM (or DP)-QPSK modulation format
with coherent detection (PM: Polarization
Multiplexing, DP: Dual Polarization, QPSK:
Quadrature Phase Shift Keying). Although the
net channel rate is close to 100 Gbit/s, the
gross line rate is in the range of 120 Gbit/s, or
even larger, to account for overhead (e.g. for
forward error correction).
The basic principle of PM-QPSK modulation
format is to trade speed for parallelism at the
expense of added complexity, as illustrated in
Figure 2. The first parallelism (or multiplexing)
step is to encode the data to be transported
not into two states like in any binary
modulation format (e.g. OOK format), but into
four phase states. Consequently one phase
symbol transports two bits of data. The
quadrature phase shift keying halves the
symbol rate compared to the binary phase shift
keying (or, equivalently, doubles the number of
bits per second, from 30 Gbaud to 60 Gbit/s).
QPSK modulation format also halves the
spectral width (the symbol duration is longer
than the “equivalent” bit duration) but doubles
the count of components (e.g. two Mach
Zehnder modulators in parallel are required in
today’s practical implementation).
Figure 2: PM-QPSK modulation format leading to a gross line rate of 120 Gbit/s, assuming 30 Gbaud symbol rate (net channel
rate of 100G). Each yellow circle in the constellation plot represents one of the four possible phase states/levels/symbols.
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The next parallelism step is to use the optical
polarization dimension. Instead of modulating
a single optical carrier at 100G using
100 Gbit/s binary modulation format, two
independent orthogonal states of optical
polarization, at the very same optical
frequency (because delivered by the same
single laser source), are modulated. Each
state of optical polarization carries a 60 Gbit/s
stream, leading to an optical signal carrying a
gross rate of 120 Gbit/s.
can be achieved using the same technologies
and techniques (30 Gbaud optoelectronics and
two multiplexing dimensions – modulation level
and polarization) as for 100G PM-QPSK
solution.
Going Above (and Below)
100G
The second step of multiplexing (multiplexing
two orthogonal states of optical polarization)
doubles the number of bits per second,
bringing the gross PM-16QAM carrier rate from
120 to 240 Gbit/s. This 200G solution,
depicted in Figure 3, is the one that has been
deployed in the field since 2015, starting with
terrestrial networks where high capacities are
required (typically for interconnecting data
centers).
Continuous capacity growth forecast shows a
clear need for increasing the capacity per
channel beyond 100G for further maximizing
the line capacity and lowering the cost of
optical transport. Increase in the channel rate
For the sake of practicality, the number of
states of optical polarization is kept to two,
while the number of levels in the modulation
format can be increased from 4 to 16, using
16-level Quadrature Amplitude Modulation
(16QAM) format. Four bits are encoded in one
of the 16 symbols as shown in Figure 3.
Compared to the symbol rate, this quadruples
the number of bits per second from 30 Gbaud
to 120 Gbit/s.
Figure 3: PM-16QAM modulation format leading to a gross line rate of 240 Gbit/s (net channel rate of 200G).
The 16QAM modulation format requires higher
Optical Signal-to-Noise Ratio (OSNR) than
QPSK due to the shortest distance between
the dots in the symbol constellation plot. If we
normalize the average signal energy by the
QPSK constellation corner to 1, the normalized
average signal energy is reduced to 5/9 for
16QAM constellation. Since signal vectors no
longer have the same amplitude over the
constellation, 16QAM signals are also less
robust to fiber nonlinearities than their QPSK
counterparts. Both OSNR and fiber
nonlinearities significantly reduce the reach of
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200G carriers in comparison with 100G
channel performance (typically by a factor of
four).
Some applications may require fiber capacity
higher than what could be enabled by 100G
carriers (typically 88 x 100G in the C band of
erbium-doped fiber amplifier) together with
reach longer than what is achieved with 200G
carriers. To respond to this need, optical
PM-8QAM modulation was developed
(Figure 4).
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Figure 4: PM-8QAM modulation format leading to a gross line rate of 180 Gbit/s (net channel rate of 150G).
In the other direction, carrier rate lower than
100G may be required for very specific
applications, like upgrades of old-generation
repeatered subsea cable systems where
coherent 100G carriers do not work properly
due to the line fiber specifications and
chromatic dispersion management scheme.
For such demanding applications from an
optical transport perspective, two-level Binary
Phase Shift Keying (BPSK) modulation format
has been implemented in conjunction with
polarization multiplexing as shown in Figure 5,
for building 50G net channel rate.
Figure 5: PM-BPSK modulation format leading to a gross line rate of 60 Gbit/s (net channel rate of 50G).
In this case, one symbol is equivalent to one
bit. The distance between the two dots of the
constellation plot is larger than what is found in
the QPSK, 8QAM and 16QAM plots, leading to
higher OSNR tolerance and higher robustness
to fiber nonlinearities.
Coherent 100G channel is the
new 10G with excellent
performance – cost tradeoff for
both terrestrial and subsea
applications.
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Beyond 200G with Today’s
Technologies
Using the currently-available 30 Gbaud
optoelectronics components, 400G channel
rate can be achieved by introducing a new
multiplexing dimension: multiplexing two 200G
carriers in the wavelength domain in order to
form a Dual-Carrier (DC) 400G channel. This
approach is depicted in Figure 6 where the bit
rate figures correspond to gross rates
accounting for overhead on the top of the
service-carrying traffic.
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Figure 6: DC-PM-16QAM modulation format leading to a gross line rate of 480 Gbit/s (net channel rate of 400G).
In the practical implementation enabled by the
technology available and deployable in 2016,
400G channels are actually mini super
channels made of two 200G carriers (hence
the full name of today’s solution for 400G
channels: DC-PM-16QAM). Many field trials
with, e.g., 1 Tbit/s channels claimed in 20152016 were actually carried out by using super
channels made of multiple 200G Carriers (like
5 x 200G for a so-called 1 Tbit/s channel).
The spacing between the two 200G carriers is
an adjustable parameter allowing to increase
the spectral efficiency and the resulting fiber
capacity. Effective signal processing and
spectral shaping optimization at the transmit
end can enable carrier spacing as low as
37.5 GHz with acceptable transmission
penalty.
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The figure on next page summarizes the
different channel rates that can be obtained
with current 30 Gbaud optoelectronics
components.
The left-hand column indicates the net rate
(available for the optical layer payload) while
the three right-hand columns give the gross
line rate taking into account overhead for
forward error correction.
The top row includes the three multiplexing
dimensions that can be combined to get a net
channel rate ranging from 50G to 400G, being
understood that the wavelength multiplexing
step can be extended to combine more than
two 200G carriers in order to build a channel
rate larger than 400G.
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Figure 7: Table showing different ways to use the various multiplexing dimensions in order to get a net channel rate ranging
from 50G to 400G, assuming 30 Gbaud optoelectronics (enabling a symbol rate of 30 Gbaud).
Evolutions of Coherent
Modulation Formats
60 Gbaud optoelectronics components are
expected to become available in the midterm.
This will change the game with the possibility,
e.g., to build a single-carrier 400G channel
based on PM-16QAM modulation format as
shown in Figure 8. 60 Gbaud technology will
also enable to build 200G carriers with PMQPSK modulation format instead of the PM16QAM that is used today: this means that
tomorrow’s 200G PM-QPSK carriers will offer
better reach performance than today’s 200G
PM-16QAM carriers.
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Another way to increase the channel rate is to
play with the number of levels in the
modulation format. Some experimental works
have been already reported with 32QAM,
64QAM and even 128QAM modulation format,
but they have not turned into actual products
yet.
In addition to the number of modulation levels
and the speed of the optoelectronics
components, a lot of activity at component and
equipment levels is spent to improve the
spectral shaping of the optical signals,
increase the spectral efficiency and make
more powerful digital signal processing at both
transmit and receive ends.
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Figure 8: New [Modulation format – Channel rate] combinations enabled by 60 Gbaud optoelectronics (100G PM-QPSK with
30 Gbaud optoelectronics is shown for reference). The carrier multiplexing dimension is available for higher channel rate.
Return of Direct Detection:
PAM4 Modulation Format
Multi-level modulation formats and coherent
detection were introduced into optical
backbone networks with the primary objectives
to increase the fiber capacity and lower the
cost per transported bit over fiber lengths
(1,000+ km). One clear beneficiary is longhaul optical transmission infrastructure
transporting huge amount of date over long
distances, as found in content delivery
networks. Over transport distances shorter
than 200 km, there is a gap between lower-
cost optical interfaces supporting up to 10 km
reach and higher-cost coherent channel cards.
Short optical links result in large output OSNR,
making solutions with noise tolerance lower
than that of coherent detection suitable and
attractive for these applications.
Several advanced direct detection solutions,
offering different channel capacities, have
been examined to fill that optical reach gap,
while offering cost, footprint and power
consumption advantages over coherent
detection approach. This section focuses on
the 4-level Pulse Amplitude Modulation
(PAM4) format, which is one of the direct
detection options identified for achieving
optical reach in the range of 100 km.
Figure 9: Pulse pattern and eye diagram of a PAM4 signal (Source: Tektronix).
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Since PAM4 is a four-level modulation format
(Figure 9), each symbol can take 4 different
values (e.g. 0, 1, 2 and 3), and two bits are
encoded in one symbol (00, 01, 10 and 11).
Therefore a 100G carrier (supporting actually a
line rate close to 120 Gbit/s to account for FEC
overhead) could be achieved using 60 Gbaud
optoelectronics components.
Due to current limitations in the component
bandwidth, the short-term practical approach is
to build PAM4 50G signals at 30 Gbaud
symbol rate (for an actual line rate of
60 Gbit/s). The polarization multiplexing
dimension, as used in 100G PM-QPSK
modulation format, is not available here
because the game is to use simple direct
detection instead of a coherent approach. A
100G channel is then enabled by two PAM4
50G signals, while a 400G signal relies on
8 x PAM4 50G signals.
Direct detection is used at the receive end of
the link. The line fiber chromatic dispersion
must be compensated for by optical dispersion
compensator but feed forward equalizer can
be used for improving the chromatic dispersion
tolerance.
An example of feed forward equalization is
represented in Figure 10 where the voltage
levels of symbols prior to transitions are
boosted by a constant factor relative to the
voltage levels of non-transition bits. This
technique boosts the high frequency
components of the waveform relative to its low
frequency components in an effort to invert the
impairments caused by the transmission
channel (backplane or line fiber).
Figure 10: PAM4 eye diagrams prior to and after propagating through a backplane, top without transmitter Feed Forward
Equalization (FFE) and, bottom, with transmitter FFE (Source: Tektronix).
In addition to various types of equalization,
adjusting the levels of the signal to walk away
from equally-spaced levels is another
approach to increase the transmission
performance.
March 2016 was announced a reference
design for a DWDM QSFP28 transceiver that
uses PAM4 for 100 Gbit/s (2 x 50 Gbit/s)
transmission for data center interconnect
requirements at reaches up to 80 km.
In March 2016, 400 Gbit/s transmission over
100 km via PAM4 optical modulation format
was reported; the transmission comprised
eight channels of 50 Gbit/s apiece. Also in
Short-reach 400G PAM4 solutions are
currently under standardization at IEEE
802.3bs.
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Conclusion
The objective of this technical tutorial was not
to go in the details of the technology of how to
design and build high-speed optical
transceivers. Instead, the goal was to explain
in simple terms the past and future evolution of
the optical signals delivered by coherent line
interfaces for high-capacity optical transport
applications. After the successful introduction
of 100G channels delivering excellent
performance – cost tradeoff, additional
coherent channel rates were developed to
cope with different needs (either very long
optical reach over non-optimized line fiber, or
very high fiber capacity over shorter
distances).
Coherent approach responds to the
exponential rise in bandwidth requirements in
backbone networks but is often over
engineered from a cost, form factor and power
consumption perspectives for shorter optical
reach applications. Several players in the
industry have determined that a direct detect
approach would be a better fit for data center
interconnect over distances shorter than
100 km. There has been recently renewed
interest in direct detection scheme, with the
advent of new formats like PAM4, to meet
more modest optical reach requirements, while
offering cost, footprint and power consumption
advantages over coherent detection approach.
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Maximizing Network Capacity, Reach and Value
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Edition Date:
June 2016
Version:
1.0
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