ZHOU LAYOUT_Layout 1 3/1/13 5:40 PM Page 41
NEW PARADIGMS IN
OPTICAL COMMUNICATIONS AND NETWORKS
Rate-Adaptable Optics for
Next Generation Long-Haul
Transport Networks
Xiang Zhou, Lynn E. Nelson, and Peter Magill, AT&T Labs-Research
ABSTRACT
We discuss the emerging rate-adaptable optical transmission technology and how this new
technology may be employed to further reduce
the transport network cost to meet ever growing
bandwidth demand in the core network. Two different types of transponders are considered:
those adjusting either the transported bit rate
(i.e., client data rate) or the symbol rate (with a
fixed bit rate). We propose a methodology for
calculating the (normalized) cost to build out an
entire long-haul transport network with several
options for bit-rate-adaptable transponders. By
using link-length demands from an exemplary
distance-diverse network, we demonstrate that
time-domain hybrid-QAM-enabled fine-grain
rate-adaptable transponders can reduce network
cost by more than 20 percent within a traditional, fixed-bandwidth, wavelength-division-multiplexed grid. We also argue that the total
transponder expense using symbol-rate-adaptable technology will be greater than when using
bit-rate-adaptable technology, as well as requiring more costly flex-grid ROADMs for channel
routing.
INTRODUCTION
Until now the industry has managed to continually drive down the transport network cost, in
terms of cost per transmitted bit, by simultaneously increasing per-channel data rates and spectral efficiency (SE), while maintaining a
reasonable optical reach. This capability has
mainly been achieved by continual advancement
in high-speed electronics, performance improvement of critical optical elements (e.g., lasers,
modulators, optical amplifiers, and optical
demultiplexers/filters), and the use of more
advanced optical modulation, coding, and detection technologies. However, as the transport
interface rate evolves from today’s 100 Gb/s to
beyond that, it has become impossible to continue on this path, in large part due to the nonlinear Shannon capacity for single-mode fibers
IEEE Communications Magazine • March 2013
[1–3]. To further increase transport spectral efficiency to 4 b/s/Hz or greater, higher-order
quadrature amplitude modulation (QAM) must
be used, which, however, requires a larger optical signal-to-noise ratio(OSNR), and thus
inevitably reduces the transmission reach [4, 5].
Shorter transmission reach mandates more frequent use of costly optical-to-electrical-to-optical
(O-E-O) regeneration, which increases transport
cost.
The recent introduction of digital signal processing (DSP) in both the receiver and transmitter has inspired a new optical transponder
concept, the rate-adaptable optical transponder
[6–8], which may open new possibilities for
improving the transport economics by exploiting
its unique capability to perform channel-bychannel capacity/SE optimization based on the
actual distance required for each link demanded
of the transport network. In a realistic optical
network where reconfigurable optical add/drop
multiplexers (ROADMs) route the optical channels, different optical channels often have different nodes as their destinations, and thus have
different reach requirements. When using traditional single-rate transponders in such a network, the capacity delivered by each optical
channel is identical, although the attainable
capacity for many optical channels could be
higher because the transmission distance is less
than the transponder’s maximum reach. On the
other hand, there may also be some channels
having a transmission distance greater than the
transmission reach for the single-rate transponder in use. For this case, costly O-E-O regeneration has to be deployed, resulting in increased
transport cost. Given this limitation of a singlerate transponder, the new concept of a rateadaptable transponder may be attractive if it can
reduce transport costs for single-mode fiber systems facing the nonlinear Shannon capacity
limit.
In this article we discuss two different types
of rate-adaptable transponders: those adjusting
either the transported bit rate (i.e., client data
rate) or the symbol rate (with a fixed bit rate).
0163-6804/13/$25.00 © 2013 IEEE
41
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 42
X-pol, inphase (I)
With the recent
DAC
introduction of DSPs
MZM1
X-pol
in both the optical
DSP/ASIC
π/2
transmitter and
receiver (along with
coherent detection),
X-Q
Binary
signal
FEC
encoder
Digital
spectral
shaping
QAM
mapping
various polarization-
DAC
PBC
Y-I
multiplexed QAM
DAC
MZM1
Y-pol
signals can be
π/2
(a)
MZM2
generated by using a
DAC
Y-pol, quadrature (Q)
single I/Q optical
modulator for each
X-pol.
Rx front end
polarization and
X0°
detected using a
universal optical
MZM2
Signal
front end.
O/E
Pol. X90°
and
O/E
phase
diverse Y90°
O/E
hybrid
Y0°
O/E
A/D
j
DSP/ASIC
CD
comp.
A/D
A/D
j
A/D
CD
comp.
LO
(b)
2x2
adaptive
equalizer
and
phase
recovery
QAM
decision
and
de-mapping
FEC
code
decoder
QAM
decision
and
de-mapping
FEC
code
decoder
Y-pol.
Figure 1. DSP-enabled universal polarization-multiplexed QAM a) transmitter and b) receiver. DAC: digital-to-analog converte; PBC: polarization beam combiner; MZM: Mach-Zehnder modulator; CD: chromatic dispersion; ASIC: application-specific integrated circuit.
Through an intuitive analysis, we argue that the
total transponder expense using symbol-rateadaptable technology will be greater than when
using bit-rate-adaptable technology, as well as
requiring the more costly flex-grid ROADMs for
channel routing. This article also reveals that
although regular 2M QAMs allow for coarse rate
adaptation, the much finer rate adaptation realized by using either the recently proposed timedomain hybrid QAM [9] or code-rate-selectable
forward error correction (FEC) [10] is particularly useful for fully exploiting the potential benefit provided by bit-rate-adaptable transponders.
The article then proposes a methodology for
evaluating these choices. The method calculates
the (normalized) cost to build out an entire
long-haul transport network with each of the
transponder options. The cost model includes a
charge for the infrastructure shared by all transport links (including fiber), which results in a
cost proportional to both distance and optical
bandwidth used (e.g., $/[km GHz]). The network is represented by a realistic broad distribution of transport link lengths. With these models
it is shown how bit-rate-adaptable transponders
can be used to improve the network economics
within a traditional fixed-bandwidth wavelengthdivision multiplexed (WDM) grid.
This article is organized as follows. We introduce the basic concept of DSP-enabled optical
modulation and detection techniques. We devote
a section to bit-rate-adaptable optical transmission technology, where we place a special
emphasis on a newly proposed time-domain
42
hybrid QAM, because it enables fine-grained
rate adaptation, realized with relatively little
additional hardware compared to using coderate-selectable FEC. We discuss symbol-rateadaptable optical transmission technology. We
present the detailed economic analysis for using
bit-rate-adaptable transponders in an exemplarily reach-diverse network. The article is then
summarized.
DSP-ENABLED OPTICAL
MODULATION AND DETECTION
With the recent introduction of DSPs in both
the optical transmitter and receiver (along with
coherent detection), various polarization-multiplexed QAM signals can be generated by using a
single I/Q optical modulator for each polarization and detected using a universal optical frontend, as shown in Fig. 1. For a typical
DSP-enabled transmitter (Fig. 1a), the binary
client signal first goes through a FEC encoder,
and then the FEC-coded binary signals are
mapped into the desired QAM symbols. After
that, various digital spectral shaping techniques
may be applied to the QAM-mapped signals to
improve the transmission performance or quell
transmission impairments.
After digital spectral shaping, the in-phase
and quadrature components of the digital QAM
signals to be modulated on each polarization are
converted into analog signals, which are used to
drive I/Q modulators to upconvert the baseband
IEEE Communications Magazine • March 2013
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 43
DSP/ASIC (only x-pol. is shown)
Rate selectable
clock
DSP/ASIC
Rate
selectable
binary
signal
Matched
clock
Selectable decision tables
QPSK
FEC
encoder
1x3
switch
8QAM
3x1
switch
16QAM
Bits to QAM
symbol mapping
Digital
spectral
shaping
X-I
X-Q
Y-I
Y-Q
X
CD
comp.
(a)
Y
CD
comp.
2x2
adaptive
equalizer
and
phase
recovery
QPSK
1x3
switch
8QAM
3x1
switch
16QAM
QAM decision and
symbol to bit mapping
FEC
code
decoder
(b)
Figure 2. Required line-side DSP for coarse granular bit-rate-adaptable a) transmitter and b) receiver. The additional DSP functions
compared to the universal QAM transmitter and receiver in Fig. 1a are shown in bold.
electrical signal into an optical signal for transmission. For such a polarization-multiplexed
QAM transmitter, a single IQ modulator for
each polarization can be used to generate various QAM modulation formats.
Figure 1b shows a typical digital coherent
receiver. The incoming optical field is coherently
mixed with a local oscillator (LO) through a
polarization- and phase-diverse 90˚ hybrid. This
hybrid separates the in-phase and quadrature
components of the received optical field in both
X- and Y-polarizations, which are then detected
by four balanced photodetectors. The detected
analog electrical signals are digitized by four
analog-to-digital converters (ADCs), and the
digitized signals are then sent to a DSP unit. For
such a digital coherent receiver, the front-end
can be used to receive any QAM-modulated signal, because modulation-specific demodulation
and decoding are carried out in the DSP unit.
The post-transmission DSP consists of four
major functional blocks, that is, the fiber chromatic dispersion (CD) compensation, adaptive
equalization and carrier phase recovery, QAM
decision and symbol-to-bit demapping, and FEC
decoding. Fiber CD is typically compensated
using a frequency-domain-based phase-only digital spectral shaping technique, and this function
can be moved to the transmitter side or split
between the transmitter and receiver for ultralong-haul transmission, where the required computational load may be too heavy for either a
single transmitter or receiver DSP chip. The 2 ¥
2 adaptive equalization performs automatic
polarization tracking, polarization-mode dispersion, and residual CD compensation. This adaptive equalization also helps mitigate the
impairment from narrowband filtering effects
from the ROADM(s).
Both the transmitter and receiver DSPs are
usually implemented in an application-specific
integrated circuit (ASIC) for best overall performance (footprint, power consumption, latency,
etc.).
BIT-RATE-ADAPTABLE OPTICAL
TRANSMISSION TECHNOLOGY
Different flavors of rate-adaptable transponders
can easily be implemented using the DSP techniques described above. In this section we focus
on a bit-rate-adaptable transponder, where we
IEEE Communications Magazine • March 2013
assume that the transponder operates at a constant symbol rate, but the net bit rate can be
selected from a short list. Such a bit-rate-adaptable transponder requires the client side rates to
also be adaptable. Such adaptability could be
implemented with various configurations of the
client optics, for example, with pluggable or
interchangeable interface modules. Otherwise, if
the transport function is integrated with an optical transport network (OTN) switch, such adaptability could be part of the OTN multiplexing
operation. That is, an OTN switch would pack
incoming traffic into an optical data unit (ODU),
which would then be encapsulated in an OTN
frame [11] to be transported. While the most
common (approximate) ODU sizes are 2.5 G
(ODU1), 10 G (ODU2), 40 G (ODU3), and 100
G (ODU4), a variable-size data unit has also
been defined. It is called ODUflex and may hold
any integral multiple of 1.24 Gb/s (known as
ODU0). Thus, ODUflex can support dozens of
sizes from 2.5 G to 100 G, and each of them
could be transported using a pair of bit-rateadaptable transponders operating at a corresponding net bit rate. It is reasonable to assume
the International Telecommunication Union
(ITU) will eventually extend G.709 and ODUflex
up to 400 G.
A coarse-grained bit-rate-adaptable transponder can be implemented by slightly modifying
the DSP (assuming ASIC implementation) as
shown in Fig. 2, where, as an example, a QAM
transponder with three selectable rates is realized by using three different regular QAM formats: quadrature phase shift keying (QPSK),
8-QAM, and 16-QAM. Comparing Fig. 2 to Fig.
1, one can see that the additional DSP effort
required for a bit-rate-selectable transponder is
small compared to a standard single-rate
transponder (assuming identical optical bandwidth), because the computational load for
QAM decision and mapping/demapping is much
smaller than the CD compensation and adaptive
equalization. But for a bit-rate-adaptable
transponder, both the client-side components
and line-side FEC encoder/decoder should be
designed for the highest bit rate, therefore
adding cost compared to the standard single-rate
transponder operating at a moderate bit rate.
However, the cost for client-side components
and line-side DSP should be relatively small
compared to the line-side opto-electronic components (i.e., D/A converters, driver amplifiers,
43
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 44
5 binary bits
5 binary bits
6 binary bits
...
64QAM
32QAM
32QAM
32QAM
32QAM
...
Time
TDM frame
TDM frame
(a)
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1
0.5
(b)
64QAM
Binary bits to QAM
symbol mapping
0
-0.5
-1
-1
-0.5
0
0.5
1
32-64 hybrid QAM
SE = 5.33 b/symbol
(c)
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
1
64-QAM
SE = 6 b/symbol
Figure 3. An exemplary time-domain hybrid 32-64QAM that achieves a SE of
5.33 b/symbol a) in the time domain; b) its constellation; c) conventional 64QAM for comparison.
and I/Q optical modulators at the transmitter
and the front-end at the receiver). Therefore,
the fractional cost increase for a bit-rate-adaptable transponder should be small.
Finer rate granularity can be realized by
using the recently proposed time-domain hybrid
QAM technique [9]. For this technique, two regular 2 m –ary QAMs with different SE are
assigned to different time slots within each TDM
frame. Using this method, any SE that falls
between the SE of the two regular QAMs can be
realized easily by appropriately designing the
TDM frame length and the time slot occupancy
ratio of the two QAMs. An example, shown in
Fig. 3a, has an SE of 5.33 b/symbol implemented
with a TDM frame length of three time slots
with 32-QAM assigned to the first two slots and
64-QAM assigned to the final slot. If the
Euclidean distance between symbols is designed
to be identical for the 32-QAM and 64-QAM
constellations, such a time-domain hybrid QAM
will exhibit a constellation diagram similar to 64QAM but with unequal constellation occupation
probability, as can be seen in Fig. 3b. For comparison, a conventional 64-QAM constellation is
also displayed in Fig. 3c.
The additional DSP needed for a fine-grained
transponder using time-domain hybrid QAM is
shown in Fig. 4, where, as an example, a
transponder with five selectable rates is proposed with three different regular QAMs and
two hybrid QAMs. From Fig. 4 one can see that
the use of hybrid QAMs introduces only a minor
complexity increase in the DSP implementation.
Fine-rate granularity can also be realized by
varying FEC code rates, as is shown in Fig. 5. By
changing the FEC redundancy, while maintaining the same net bit rate, one can also change
the information rate [10]. But this method
requires multiple selectable FEC encoders/
encoders (with different code rates) to be implemented within the same ASIC for optimal cod-
44
ing performance (for every code rate). Because
the implementation complexity of a soft-decision
FEC encoder or decoder is much greater than
the hybrid QAM symbol mapping/demapping,
this FEC-based rate adaptable method is expected to be more costly than the above-described
time-domain QAM method. Furthermore, at the
same net bit rate and using the same optical
bandwidth, time-domain hybrid QAM allows a
system to be operated at a lower SE than the
FEC-based method, where a higher-order QAM
must be used to accommodate the extra FEC
overhead. Because modulation with lower SE is
more tolerant of both fiber nonlinear effects and
laser phase noise, using time-domain hybrid
QAM may also achieve better transmission performance than using an FEC-based bit-rateadaptable method.
The use of bit-rate-adaptable transponders
enables channel-by-channel capacity optimization based on the actual link distance for each
channel: for a shorter-distance demand, the
transponder will switch to a higher-SE modulation format or switch to a less powerful FEC
code (with less overhead) to maximize the transport capacity; for a long-distance demand, the
transponder will switch to a lower-SE modulation format or switch to a more powerful FEC
(with more overhead) to minimize the use of
costly O-E-O regeneration. Because the optical
bandwidth occupied by the signal remains constant, a bit-rate-adaptable transponder is compatible with the conventional ROADM optical
network using a fixed channel grid.
SYMBOL-RATE-ADAPTABLE OPTICAL
TRANSMISSION TECHNOLOGY
Alternatively, a rate-adaptable transponder can
also be designed with variable symbol rates while
holding the net bit rate (i.e. the client-side interface rate) constant. Similar to the bit-rate-adaptable transponder, a coarse-grained symbolrate-adaptable transponder can be implemented
with multiple switchable regular QAMs within
the same ASIC, whereas a finer rate-granularity
can be realized by using either the time-domain
hybrid QAM or code-rate variable FEC.
Using a symbol-rate-adaptable transponder
allows a channel to operate at higher SE for
short distance optical links by switching to a
higher-SE modulation format or switching to an
FEC code using less overhead. Because the use
of a more spectrally efficient modulation format
or the use of a lower-overhead FEC reduces the
signal optical bandwidth (for the same net bit
rate), more capacity can be transmitted over the
network. For a long-distance optical link, this
transponder will switch to a modulation format
with lower SE or to a more powerful FEC code
(with more redundancy) to minimize the use of
costly O-E-O regeneration.
Because the optical bandwidth occupied by
the signal can be different from channel to channel (a shorter-reach channel consumes less optical bandwidth, whereas a longer-reach channel
consumes more bandwidth), a bandwidth-flexible
channel routing technique, such as the recently
introduced flex-grid ROADMs [12], should be
IEEE Communications Magazine • March 2013
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 45
DSP/ASIC (only x-pol. is shown)
DSP/ASIC
Rate
selectable
binary
signal
Matched
clock
QPSK
FEC
encoder
1x5
switch
QPSK/8QAM
8QAM
Rate selectable
clock
Selectable decision tables
5x1
switch
Digital
spectral
shaping
8QAM/16QAM
16QAM
Bits to QAM
symbol mapping
X-I
X-Q X
Y-I
Y-Q
(a)
Y
CD
comp.
CD
comp.
QPSK
2x2
adaptive
equalizer
and
phase
recovery
1x5
switch
QPSK/8QAM
8QAM
5x1
switch
FEC
code
decoder
8QAM/16QAM
16QAM
QAM decision and symbol
to bit mapping
(b)
Figure 4. DSP required for an exemplary fine-grain rate-adaptable a) transmitter and b) receiver enabled by time-domain hybrid QAM.
The additional DSP functions compared to the universal QAM transmitter and receiver in Fig. 1 are shown in bold.
DSP/ASIC (only x-pol. is shown)
Selectable decision tables
DSP/ASIC
Fixed-rate
FEC
binary
code 1
signal
encoder
1x2
switch
FEC
code 2
encoder
QPSK
2x3
switch
8QAM
3x1
switch
16QAM
Bits to QAM
symbol mapping
Digital
spectral
shaping
(a)
X-I
X-Q
Y-I
Y-Q
QPSK
CD
2x2
1x3
3x2
comp. adaptive
8QAM
switch
switch
equalizer
16QAM
and
phase
Y CD
QAM decision and
comp. recovery symbol to bit mapping
X
FEC
code 1
decoder
FEC
code 2
decoder
2x1
switch
(b)
Figure 5. Required line-side DSPs for fine granular rate-adaptable a) transmitter and b) receiver using variable FEC code rate (in order
to achieve optimal coding performance for every code rate). The additional DSP functions compared to the universal QAM transmitter
and receiver in Fig. 1 are shown in bold.
used with the symbol-rate-adaptable transponders to improve utilization of the network’s common resources. Unfortunately, the use of flex-grid
ROADMs not only adds extra cost, but also significantly complicates the network management.
In addition to the need for flex-grid
ROADMs, in the following we argue that using
symbol-rate-adaptable transponders in a reachdiverse network will also increase the total
transponder cost compared to using bit-rateadaptable transponders. For a bit-rate-adaptable
transponder, the line-side opto-electronics
always operate at their maximum bandwidth,
whereas the client opto-electronics operate at
variable rates and likely not at the maximum
client speed for most transport links. On the
contrary, for the symbol-rate-adaptable transponder, the client interface rate is fixed; therefore,
the client-side opto-electronics always operate at
their maximum bandwidth, whereas the line-side
opto-electronics operate at variable symbol rates
and likely not at their maximum bandwidth for
most transport links. (In other words, for a
reach-diverse network using the symbol-rateadaptable technology, transponders in many
higher-SE (shorter distance) optical links will
operate at a symbol rate less than the maximum
opto-electronic bandwidth, whereas all the
transponders will operate at their maximum symbol rate for a network using bit-rate-adaptable
technology.) Assuming that the maximum optical
bandwidth for each type of transponder is
designed to be identical, then when transmitting
the same capacity over the same network, more
transponders will be required when using symbol-rate adaptation, because the average capacity transmitted by a symbol-rate-adaptable
transponder is smaller than a bit-rate-adaptable
IEEE Communications Magazine • March 2013
transponder (this will be more evident in the
example in the next paragraph). Note that the
cost for the two types of transponder should be
comparable, because the cost of a high-speed
optical transponder is mostly determined by its
line-side opto-electronic bandwidth. As a result,
the total transponder expense using symbol-rateadaptable technology will be greater than when
using bit-rate-adaptable technology.
As an example, we assume a simple network
having only two reach demands, one for a shortdistance 160 km link and the second for a longdistance 1600 km link. Assume that we need to
transmit 400 Gb/s information over both the
short- and long-distance links, and the transponders utilize single-carrier modulation at maximum 40 Gbaud symbol rate. If we use the
bit-rate-adaptable technology discussed above,
we only need one pair of transponders for the
short-reach link with the transponders operating
at 400 Gb/s net bit rate using polarization modulated (PM)-64-QAM and 50 GHz optical bandwidth, and two pairs of transponders for the
long-distance link with the transponders operating at 200 Gb/s information rate using PM-8QAM and a total of 100 GHz optical bandwidth.
However, if we use the symbol-rate-adaptable
technology discussed above (i.e., fixed 200 Gb/s
client interface rate and variable line-side symbol rate), for the short link we need two pairs of
PM-64-QAM transponders, which operate at
half the symbol rate and use half the optical
bandwidth (i.e., 25 GHz) compared to the bitrat-adaptable technology. For the long-distance
link, we need two pairs of PM-8-QAM transponders, which operate at the same maximum symbol rate and optical bandwidth (i.e., 50 GHz) as
the bit-rate-adaptable technology. So, for this
45
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 46
simple network, the network using symbol-rateadaptable technology requires an additional pair
of transponders, as well as bandwidth-flexible
routing, compared to the network using bit-rateadaptable technology.
ECONOMIC ANALYSIS FOR
BIT-RATE-ADAPTABLE TECHNOLOGY
This section assumes an exemplary reach-diverse
network to calculate how bit-rate-adaptable
transponders could be used to reduce the transport network cost (as compared to traditional
fixed-rate transponders). This study considers
only single-carrier-based modulation and
assumes that:
• All transponders operate at a fixed 40
Gbaud symbol rate and use 20 percent softdecision FEC.
• Five PM regular QAM formats (QPSK, 8QAM, 16-QAM, 32-QAM, and 64-QAM)
are used to generate five coarse-grained
signals (corresponding to net bit rates of
133 Gb/s, 200 Gb/s, 266 Gb/s, 333 Gb/s, and
400 Gb/s, respectively), whereas the timedomain hybrid QAM is used to generate
finer-grained signals.
100%
0.8
Normalized probability
80%
Cumulative percent
0.6
60%
0.4
40%
0.2
20%
0
0
1000
2000
3000
Distance (km)
4000
Cumulative density function
Normalized probability
1
0%
5000
Figure 6. An exemplary probability distribution of link distances demanded of
the transport network.
Reach (km)
4000
Hybrid QAM
Regular QAM
PM-QPSK
3000
PM-8QAM
PM-16QAM
2000
PM-32QAM
PM-64QAM
1000
40Gbaud
0
100
150
200
250
300
Net signal bit rate (Gb/s)
350
400
Figure 7. Estimated transmission reach for different modulation formats all
operating at 40 GBaud symbol rate and 50 GHz channel spacing.
46
• Each channel fits into the standard 50 GHz
WDM grid.
• A hybrid Raman/erbium doped fiber amplifier (EDFA) is used for inline optical amplification.
The transmission reach of each modulation format is estimated based on realistic optical links
using standard single-mode fiber (SSMF) (span
length = 80 km and span loss = 21 dB). We
have assumed that a well designed 133 Gb/s PMQPSK transponder has a reach of 4000 km. We
then use the required optical SNR (OSNR) for
this reach as a baseline to estimate the reaches
of all the other modulation formats, by assuming
that the optimal launch power density is independent of the chosen transmission format (such
that the transmission reach of different modulation formats can be directly deduced from their
OSNR requirements.) Note that the assumption
that the optimal launch power density does not
depend on the chosen transmission format has
been proven to be true (to first order) in a dispersion uncompensated high-SE transmission
system by a recently published theoretical paper
[13]. The analysis further assumes an exemplary
distribution of link distances demanded of the
transport network, as shown in Fig. 6. The estimated transmission reaches for the different
modulation formats are displayed in Fig. 7.
In this analysis, the network cost consists of
three elements:
• The terminal transponders
• The 3-R regenerators
• All the shared (common) infrastructure
such as the fiber, inline optical amplifiers,
and ROADMs
For convenience, we use the PM-8-QAM
transponder as the baseline transponder and
define the unit common cost as the average cost
of all shared infrastructure for 1 km of transmission distance and for consuming 1 GHz of optical bandwidth (i.e., $/[km GHz]). In Fig. 8 we
show the calculated network cost vs. the unit
common cost for five single-rate transponders.
The unit common cost is shown relative to the
cost of a baseline transponder (PM-8QAM) and
the range of that ratio covered in Fig. 8 is representative of the common costs for many real networks. For a continental-scale network with
owned (not leased) fiber and facilities, a ratio
near the center is approximately correct today. It
is projected that the ratio will be closer to values
on the left in about five years. The common
costs for network operators using leased fiber
and facilities will be on the right side of the plot
or even larger than the maximum normalized
unit common cost shown. The assumed cost differences for the five single-rate transponders are
given in Table 1. The cost of a 3-R regenerator
is assumed to be 1.6 times the cost of a terminal
transponder for any modulation format. The
main reason for the difference in calculated network cost for each modulation format is the difference in reach for the single-rate transponders,
resulting in differences in the amount of expensive 3-R regeneration that is required. In general, when common costs are high relative to
transponder costs, the total network cost is minimized by maximizing spectral efficiency (to avoid
lighting another fiber), limited by regeneration
IEEE Communications Magazine • March 2013
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 47
Transponder
PM-QPSK
PM-8-QAM
PM-16-QAM
PM-32-QAM
PM-64-QAM
Cost
0.88
1
1.05
1.12
1.2
Table 1. Assumed normalized cost for the five single-rate transponders.
IEEE Communications Magazine • March 2013
5
4
Normalized network cost
costs. And when common costs are low relative
to transponder costs, minimizing the total network cost is achieved by choosing longer-reach
(and thus lower spectral efficiency) modulation
formats to avoid regeneration.
For the distribution of length demands in
Fig. 6, the reach capabilities in Fig. 7, and the
cost assumptions above, Fig. 8 shows that among
the five single-rate transponders, PM-8-QAM
performs best (in terms of minimizing the network cost) in the moderate to high common-cost
region. In the low common-cost region, PM-8QAM is only slightly worse than PM-QPSK,
whereas PM-16-QAM is noticeably worse than
PM-8-QAM in the low common cost region.
Only on the far right of the figure does PM-16QAM out-perform PM-8-QAM. The network
cost for using PM-32-QAM and PM-64-QAM is
very high across the whole displayed common
cost region. For these assumptions, one can see
that PM-8-QAM could be an optimal option
when using single-rate-based transmission technology in this exemplary network.
The network costs for using bit-rate-adaptable transponders are given in Fig. 9, where
three rate-adaptable transponders with different
degrees of rate granularity are considered. The
coarse-grained transponder has five selectable
rates, 133 Gb/s, 200 Gb/s, 266 Gb/s, 333 Gb/s,
and 400 Gb/s, which are realized by using the
five regular QAM modulation formats, PMQPSK, PM-8-QAM, PM-16-QAM,PM-32-QAM,
and PM-64-QAM, respectively. The fine-grained
transponder employs time-domain hybrid QAM
to realize four additional rates: 166 Gb/s, 233
Gb/s, 300 Gb/s, and 366 Gb/s. For the ultra-finegrained transponder, we have a total of 26
selectable rates uniformly distributed from 133
to 400 Gb/s. They are also realized by using the
time-domain hybrid QAM technique. For all
plots in this figure, the displayed network costs
have been normalized to the corresponding
baseline network cost using single-rate PM-8QAM technology along the entire curve to
emphasize the network savings.
In Fig. 9a, b, and c, we display the results
based on assumptions that the cost of a rateadaptable transponder is 10, 20, and 30 percent
more, respectively, than a single-rate baseline
PM-8-QAM transponder. Because the cost
increase for a rate-adaptable transponder using
time-domain hybrid QAM mainly comes from
the increased maximum client side interface rate
and the FEC decoder/encoder (additional DSP
required for QAM mapping/demapping is minor
compared to other DSP functions), we have
assumed that the cost difference between coarse
granularity and fine granularity (including the
ultra-fine-grain transponder) is negligibly small.
From Fig. 9 we can see that as long as the cost
increase of a rate-adaptable transponder (as
compared to the optimal single-rate baseline
3
PM-QPSK
PM-8-QAM
PM-16-QAM
PM-32-QAM
PM-64-QAM
2
1
0
10-6
10-5
Unit common cost/baseline TRx cost
10-4
Figure 8. Normalized network cost as a function of unit common cost (normalized to the cost of a baseline transponder) for using five different singlerate transponders. TRx: transponder.
transponder) is smaller than 20 percent, using
coarse-grained transponders can reduce the network cost by more than 9 percent in the moderate network common cost region, and the cost
savings can increase to more than 16 percent
using fine-grained transponders and to more
than 23 percent using ultra-fine-grained
transponders (shown in Fig. 9b at 10 –5 ratio of
unit common cost to baseline transponder cost).
In the low common cost region, the network savings can be up to 15 percent for using coarsegrained transponders, 22 percent for using
fine-grained transponders, and 28 percent for
using ultra-fine-grained transponders. For the
extreme case of very high network common cost,
only the use of fine-grained transponders can
reduce the network cost (due to the transponders’ ability to fine-tune the spectral efficiency to
avoid both lighting new fibers and regeneration).
DISCUSSION AND CONCLUSION
It should be noted that instead of using a single
type of rate-adaptable transponder, one can also
use multiple different single-rate transponders
for a reach-diverse network. However, that strategy requires all types of transponders to be
inventoried and spared, which greatly complicates inventory management and increases the
inventory cost compared to using the proposed
rate-adaptable transponder. Another significant
advantage to using a single type of rate-adaptable transponder lies in the fact that its operating rate can be remotely reconfigured, which
significantly reduces the required service provising time in a dynamic network where the num-
47
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 48
ber of dense WDM links demanded may dynamically change.
In conclusion, we have shown that the use of
DSP in both the transmitter and receiver (with
coherent detection) enables a powerful new
transponder design concept, the rate-adaptable
transponder, which opens a new possibility to
reduce the transport network costs for realistic
1.2
Normalized network cost
Coarse granular
Fine granular
Ultra-fine granular
(a)
1
0.8
Assume rate-adaptable TRx cost 10% more
0.6
10-6
10-5
Unit common cost/baseline TRx cost
(a)
10-4
REFERENCES
1.2
Normalized network cost
Coarse granular
Fine granular
Ultra-fine granular
(b)
1
0.8
Assume rate-adaptable TRx cost 20% more
0.6
10-6
10-5
Unit common cost/baseline TRx cost
(b)
10-4
1.2
Normalized network cost
Coarse granular
Fine granular
Ultra-fine granular
(c)
1
0.8
Assume rate-adaptable TRx cost 30% more
0.6
10-6
10-5
Unit common cost/baseline TRx cost
(c)
10-4
Figure 9. Network cost (normalized to corresponding cost of using single-rate
baseline PM-8-QAM TRxs) for using three different levels of granularity of the
rate adaptation under three different transponder (TRx) cost assumptions: the
rate-adaptable TRx increases the transponder cost by a) 10 percent, b) 20 percent, and c) 30 percent compared to the single-rate baseline PM-8QAM TRx.
48
reach-diverse networks. Two types of rate-adaptable transponders have been discussed, and our
analysis has revealed that using bit-rate-adaptable transponders can be more cost effective
than using symbol-rate-adaptable transponders.
While regular 2 M QAM modulation formats
allow the realization of a coarse-grained
transponder, finer rate granularity can be realized by using either the newly proposed timedomain hybrid QAM or a code-rate-variable
FEC (or a combination of both). By examining
the detailed DSP functions, we found that an
ASIC implementation of time-domain hybrid
QAM requires less additional hardware than the
code-rate-variable FEC. By using link-length
demands from an exemplary distance-diverse
network, we demonstrate that using bit-rateadaptable transmission technology can reduce
the network cost by more than 20 percent by
using time-domain hybrid QAM enabled finegrained rate-adaptable transmission technology.
This methodology may prove instructive to other
carriers and their equipment suppliers when
evaluating the numerous pending options for
transport beyond 100G.
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BIOGRAPHIES
X IANG Z HOU [SM] (zhoux@research.att.com) received his
Ph.D. degree in electrical engineering from Beijing University of Posts and Telecommunications in 1999. From 1999 to
2001, he was with Nangang Technological University, Singapore, as a research fellow, doing research on optical
CDMA and wideband Raman amplification. He has been
with AT&T Labs-Research, Middletown, New Jersey, since
October 2001, working on various aspects of long-haul
IEEE Communications Magazine • March 2013
ZHOU LAYOUT_Layout 1 3/1/13 12:38 PM Page 49
optical transmission and photonic networking technologies, including Raman amplification, polarization- and
reflection-related impairments, optical power transients
control (in dynamic optical networks), advanced modulation formats, and digital signal processing for high-speed
transmission (100 Gb/s, 400 Gb/s, and beyond). He has
authored/co-authored more than 100 peer-reviewed journal and conferences publications, and holds 34 U.S.
patents. He currently serves as an Associate Editor of
Optics Express and on the Technical Program Committees
for several IEEE and OSA conferences, including OFC. He is
a member of the OSA.
LYNN NELSON [M] received her Sc.B. degree in engineering
from Brown University, Providence, Rhode Island, and
her M.S. and Ph.D. degrees in electrical engineering from
the Massachusetts Institute of Technology, Cambridge.
From 1997 to 2000 she was with Bell Laboratories,
Lucent Technologies, Holmdel, New Jersey, where she
worked on fiber nonlinearities, wavelength-division multiplexing, and polarization mode dispersion. In 2000 she
became technical manager of the Fiber Systems Testing
Group for the Optical Fiber Solutions (OFS) business unit
of Lucent and remained with OFS after its acquisition by
Furukawa in 2001. Since 2007 she has been with AT&T
Labs-Research, Middletown, New Jersey, where she is
focusing on high-capacity transmission at 100 Gb/s and
beyond, modulation formats, and polarization issues for
IEEE Communications Magazine • March 2013
AT&T’s long-haul network. She has co-authored over 150
peer-reviewed journal and conference publications and
four book chapters, holds 14 U.S. patents, and is a Fellow of the OSA. She served on the Technical Program
Committee for OFC, 2001–2003, and served as Technical
Co-Chair of OFC/NFOEC 2005 and General Co-Chair of
OFC/NFOEC 2007.
P ETER M AGILL received his Ph.D. in physics from the Massachusetts Institute of Technology in 1987, after which he
joined AT&T Bell Labs at Crawford Hill, Holmdel, New Jersey. He worked on the characterization of advanced lasers,
optical access networks, and data-over-cable access protocols. He transferred with Lucent Technologies as it was
spun out of AT&T in 1996 to head their access research
department. He managed the R&D of passive optical network systems and cable modem head-end equipment. In
2000 he returned to AT&T and took over Optical Systems
Research as executive director in Middletown, New Jersey.
He oversaw research on advancing fiber communications
technologies for the entire network (inter-city, metro, and
access), including high-speed transmission systems (400
Gb/s and beyond) and dynamic networks at wavelength
and sub-wavelength rates. In 2012 he became assistant
vice president of Communications Technology in AT&T
Labs, supporting research on wireless technology and networks, network design and optimization, as well as optical
systems.
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