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Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35240
Experimental demonstration of advanced
modulation formats for data center networks on
200 Gb/s lane rate IMDD links
JINLONG WEI,*TALH A RAHMAN, STE FAN O CALA BRÒ , NEBOJSA
STOJA NOVIC, LIANG ZHANG, CHANGSONG XIE, ZHICHENG YE,AND
MAXIM KUSCHNEROV
Optical and Quantum Communication Laboratory, Munich Research Center, Huawei Technologies, Riesstr.
25, Munich 80992, Germany
*jinlongwei2@huawei.com
Abstract:
This work contributes experimental demonstrations and comprehensive comparisons
of various modulation and coding techniques for 200 Gb/s intensity modulation and direct
detection links including four-level pulse amplitude modulation (PAM-4), PAM-6, trellis-coded
modulation (TCM) over PAM and discrete multi-tone (DMT) transmission. Both C-band
Mach-Zehnder modulator and O-band electro-absorption modulated laser transmitters were
examined for intra-data center applications based on state-of-the-art commercial components.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Following the standardization of 400 gigabit ethernet (GE) [1,2] and driven by the upgrade of
switch bandwidth for data center network (DCN) applications [3], the next generation (NG)
Ethernet pluggable modules targeting 800 Gb/s or 1.6 Tb/s will soon be needed to support the
NG switches with a bandwidth of 25.6 Tb/s and 51.2 Tb/s, respectively. Figure 1illustrates the
typical scenarios of DCN based on a mesh architecture and their corresponding optical module
evolution roadmap from a historic point of view. 800 Gb/s optical interconnects mainly address
the connections from leaf to top of rack (TOR) switches with up to 100 m reach and between
spline to leaf switches of 500 m to 2 km. Coherent solutions are promising to convey such
high-capacity data over one or two wavelengths, but are not well suited for intra-DC applications
due to the relatively high transceiver cost and complexity. Intensity modulation and direct
detection (IMDD) solutions are preferred. Recently, an 800G multi-source agreement had been
announced [3] and a part of the announcement considered the use of 200 Gb/s/lambda for DCN
applications. Indeed, various IMDD systems transmitting 200
+
Gb/s per wavelength have been
demonstrated [4–22] to verify the technical feasibility of pluggable modules supporting 800 Gb/s
or beyond for links of up to 2 km of single-mode fiber (SMF).
Efforts have been made to tackle the technical challenges of realizing
>
200 Gb/s IMDD links
by introducing novel components such as special high-speed DACs [4,5] or self-developed special
ultra-broadband optical modulators [6–8,17]. Even with such high-performance components,
powerful digital signal processing (DSP) is required to mitigate various linear and nonlinear
distortions. Recently, lab demonstrations were performed to show the feasibility of 200 Gb/s/
λ
short-reach DCN enabled by advanced modulation formats and DSP and based on commercial off-
the-shelf components with strong bandwidth limitation [9–15,18–22]. Demonstrated modulation
formats mainly include four level pulse amplitude modulation (PAM-4) [10,15,18–21], PAM-6
[12,14,18,20,21], PAM-8 [21,22], and discrete multi-tone (DMT) [4,13,20]. Trellis coded
modulation (TCM) can further improve the power sensitivity of PAM and DMT schemes
[11,13,16,20]. The commercial transmitters mainly rely upon Mach–Zehnder intensity modulators
(MZM) [10–14,18–21] and electro-absorption modulated lasers (EMLs) [9,18]. The former can
#409905 https://doi.org/10.1364/OE.409905
Journal © 2020 Received 10 Sep 2020; revised 25 Oct 2020; accepted 28 Oct 2020; published 5 Nov 2020
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35241
Fig. 1. Typical DCN optical Interconnect Roadmap.
be viewed as a benchmark transmitter, while the latter are more likely to be incorporated in real
800GE products in order to leverage the 400 GE ecosystem and Ethernet standard compatibility.
However, there has been lack of a comparative study of various modulation formats, DSPs, and
hardware, which would be practically important for choosing the right system and component
configuration. This work extends our previous work [20] and contributes a comprehensive
experimental demonstration and direct comparison of 224 Gb/s data links based on various
advanced modulation formats as PAM-4, PAM-6, TCM on two dimensional eight level PAM
(2D-PAM8) and DMT, by using both an MZM and an EML. With respect to [20] we offer a
significantly more detailed analysis and we include measurement results of an O-band EML-based
optical link.
2. Experimental setup
Figure 2depicts the experimental setup for the investigated 200 G IMDD links. The transmitters
rely on offline DSP to generate the waveforms of interest, and use a 120-GS/s 35-GHz arbitrary
waveform generator (AWG) to convert the digital waveform into an analog signal. A linear driver
with bandwidth of 50 GHz is used to amplify the analog signal before modulating a MZM or an
EML. The MZM (EML) has a 33-GHz (40-GHz) 3-dB bandwidth. The laser externally fed to
the MZM and the laser integrated with the EML operate at 1550 nm and 1310 nm, respectively.
After transmission over SMF, the optical signal reaches a combined optical receiver comprising a
variable optical attenuator (VOA), a semiconductor optical amplifier (SOA), and a photodiode
(PD) with a bandwidth of approximately 60 GHz. The SOA gain is adjustable so that the input
power injected into the PD can be optimized. The detected signal is then converted into a digital
signal by a 160 GS/s oscilloscope with a bandwidth of 60 GHz. The digital signal is finally
subject to offline DSP. It is interesting to note that the received signal spectra for the MZM and
EML links are quite similar in the absence of digital pre-distortion (DPD), as indicated in the
inset of Fig. 2. This is because although the MZM has a smaller 3-dB bandwidth than then EML
it has a very slow frequency roll-off.
The offline DSP stack for the transmitter and the receiver, which is dependent on the modulation
format, is also presented in Fig. 2. At the transmitter, DPD is optionally used for PAM schemes.
The DPD aims to compensate the limited bandwidth of the transceiver [12]. Hard clipping is
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35242
Fig. 2.
Experimental setup for 200G IMDD systems using various advanced modulation
formats and commercial EML and MZM.
applied after DPD to limit the signal peak-to-average power ratio (PAPR) at 8 dB before loading
the samples to the AWG.
It is worth mentioning an alternative scheme for bandwidth pre-compensation, namely the
Tomlinson-Harashima precoding (THP) [21,22]. As a replacement of receiver side decision
feedback equalizer (DFE), THP can avoid the error propagation issue. THP is beneficial only
when extremely strong bandwidth limitation and serious fiber chromatic dispersion issues exist
[21], which does not apply to this demonstration. In addition, THP leads to an increased number
of amplitude levels, which makes it incompatible with a standard Viterbi equalizer for PAM.
Therefore, THP is not considered here.
2.1. PAM
Both PAM-4 and PAM-6 are demonstrated in this work, which correspond to baud rates (bit
rates) of 112 Gbaud and 90 Gbaud (224 Gb/s and 225 Gb/s), respectively. The transmitter DSP
includes a pseudorandom binary sequence generator, a PAM encoder mapping the bit stream into
symbols, and an optional DPD. For PAM-6, the encoder firstly maps the bits to a cross 32-QAM
constellation, whose I and Q projections are then sequentially transmitted [11,12].
In the receiver DSP, the signal is first resampled to a rate of 2 samples per symbol. An
automatic gain controller followed by timing recovery (TR) and an anti-aliasing low-pass filter is
then applied. The signal is then equalized by a full Volterra feed-forward equalizer (FFE) which
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35243
contains both linear and nonlinear kernels, as indicated in Eq. (1).
y(k)=wdc +
M1−1
Í
k1=0
w1(k1)x(k−k1)
+
M2−1
Í
k1=0
M2−1
Í
k2=k1
w2(k1,k2)x(k−k1)x(k−k2)
+
M3−1
Í
k1=0
M3−1
Í
k2=k1
M3−1
Í
k3=k2
w3(k1,k2,k3)x(k−k1)x(k−k2)x(k−k3)
(1)
Where xis the sequence of received samples at one sample per symbol and M
1
,M
2
and M
3
are
the memory lengths for linear, 2
nd
- and 3
rd
-order Volterra kernels, respectively. w
dc
,w
1
,w
2
, and
w
3
are the tap coefficients corresponding to the direct current (DC), linear, 2
nd
- and 3
rd
-order
Volterra kernels. The nonlinear Volterra FFE can be trained to deliver a duobinary (DB) or
full-response signal (virtually without inter-symbol interference), depending on which target
spectrum shows the better match to the channel frequency response [11,12]. The training symbols
are the transmitted symbols and the DB version of the transmitted symbols for the full-response
and DB FFEs, respectively. A following noise cancellation (NC) unit is used to suppress and
whiten the noise [11,12,23]. The NC unit relies upon noise correlation after the equalizer. The
correlation can be calculated via approaches such as the Burg algorithm [23] and the noise term
from neighbor symbols can be subtracted from the current symbol. If the nonlinear Volterra
FFE is trained to deliver a full-response signal, the following processing directly calculates the
system bit error rate (BER), otherwise maximum likelihood sequence estimator (MLSE) for the
DB channel with a memory length of 1 is used.
2.2. 2D PAM-8
2D PAM-8 TCM encodes five bits to two PAM-8 symbols (
±
1,
±
3,
±
5,
±
7), as indicated in Fig. 3.
This is achieved by generating an additional redundant bit by means of an 8-state convolutional
encoder similar to that shown in [24], as indicated in Fig. 3(a). The levels are divided into two
groups A
=
(
±
7,
±
3) and B
=
(
±
5,
±
1) according to the set partitioning rule shown in Fig. 3(b) and
four symbols make eight subsets from S
0
to S
7
, as represented by Fig. 3(c). Subsequently, the
mapper selects a 2-dimensional PAM-8 symbol pair out of 64 possibilities. The 8-state trellis is
shown in Fig. 3(d). The decoder selects the best candidate from each subset S and one of four
branches entering each state is selected. The TCM decoder uses 8-state MLSE and decodes 2
PAM-8 symbols (i.e. 5 bits) per step. The combination of DB equalization and 2D PAM-8 is not
considered because of its complexity.
2.3. DMT
The DSP at the transmitter is similar to classic DMT systems [25], except that 16-state TCM
is employed to increase the effective Euclidean distance and improve the system performance.
Before the TCM encoder, the input bit stream is parallelized and bit/power loading based on
Chow’s algorithm [25] is applied. The IFFT size is 1024 and a cyclic prefix (CP) of 16 samples
is added. The TCM encoder is implemented in a way similar to that shown in Section 2.2. The
DMT features a high peak-to-average power ratio (PAPR) compared with PAM systems. Hard
clipping is used to limit the PAPR at 10 dB.
In receiver DSP, the signal is firstly re-sampled to 1040 samples per DMT symbol. The
synchronization process is realized by a sliding window and correlation technique to identify
the beginning of the DMT symbols. A FFE using conventional Volterra base functions or
absolute value (ABS) functions by introducing abstract-terms x(k
1
)
|
x(k
2
)
|
and x(k
1
)
|
x(k
2
)
|2
to
replace the standard Volterra terms shown in Eq.(1) is used to mitigate system nonlinearities [13],
including signal-to-signal beating interference (SSBI) caused by the direct-detection process,
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35244
Fig. 3.
(a) 2D PAM-8 encoder. (b) 2D PAM-8 set partitioning. (c) 2D PAM-8 symbol
group subsets. (d) 2D PAM-8 8-state trellis.
and modulator nonlinearity. After serial to parallel (S/P) conversion, CP removal and FFT, the
signal is transformed to frequency domain and the 1-tap equalization is used to compensate the
linear distortions of the system. A following TCM decoder and a QAM de-mapper yield the
estimated bit sequence before the system BER is calculated.
For all modulation schemes described above including PAM-4, PAM-6, 2D PAM-8, and DMT,
we consider a Reed Solomon forward error correction (FEC) code with an overhead of 12% and
a threshold bit error rate (BER) of 2
×
10
−3
throughout the paper. Note that TCM could reduce
error floor, 2D PAM-8 and DMT may consider the KP4 FEC code with 5.9% overhead and a
threshold BER of 2×10−4when low overhead FEC is required.
3. Results
3.1. DPD effects
As described above, a significant challenge of a 200G IMDD link stems from the limited
bandwidth of available commercial transceivers. We considered two efficient approaches to
address this challenge. The first one is to adopt DB equalization as indicated in Fig. 4(a), which
dramatically reduces the bandwidth requirement at the cost of increased SNR requirement. The
second one is to apply DPD at the transmitter. Figure 4(b) shows the effect of the DPD on the
signal spectrum. Without DPD, there exists a significant gap at high frequencies between the
received signal (before the nonlinear Volterra FFE) and both the DB and full response spectra
targets. Full response equalization results in a large noise enhancement, whereas DB equalization
can partly filter out the high-frequency noise components at the cost of an increased number
of amplitude levels and a higher signal-to-noise ratio (SNR) requirement. Such a trade-off
determines the choice of the receiver DSP. In the presence of DPD, the received signal before
the nonlinear Volterra FFE shows a very flat spectrum and a good match with the full response
target up to 40 GHz, beyond which a gap still exists. Whereas a stronger DPD could close the
gap, it would dramatically increase the signal PAPR and effectively degrade the overall system
performance.
Figure 4(c) presents the 225-Gb/s PAM-6 system BER versus the received optical power (ROP)
for the setup with MZM with and without DPD in the case of optical back-to-back (BtB) and
1-km SMF transmission. In the absence of DPD, DB equalization together with MLSE with
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35245
Fig. 4.
(a) Simulated spectrum of 224 Gb/s PAM-4 and PAM-6 and their corresponding
DB counterparts. (b) Measured 90-Gbaud PAM-6 signal spectra before and after DB and
full response equalization. (c) 90-Gbaud PAM-6 system BER versus received optical power
with and without DPD.
a memory length of 1 were found to be the best choice for the receiver DSP. In the presence
of DPD, the receiver DSP only needs a full response equalizer. This is because MLSE does
not bring about significant gain in this scenario with negligible inter-symbol interference. For
reasonably moderate bandwidth limited channel cases such as to PAM-6 here, the benefits of
using DPD at the transmitter with full response equalizer at the receiver compared to using
FFE
+
MLSE at the receiver are two-fold: first, it enables better optical power sensitivity. As
indicated in Fig. 4(c) 1.6-dB and 1.2-dB improvements in optical power sensitivity are obtained
for optical BtB and 1 km SMF transmission, respectively, while, additionally, the error floor is
slightly improved. Second, DPD enables a receiver DSP architecture excluding a MLSE without
performance degradation.
Although only PAM-6 is presented here as an example to indicate the importance of DPD,
the situation is similar for 2D-PAM8 which also has a baud rate of 90 Gbaud and a spectral
efficiency of 2.5 bit/s/Hz. For PAM-4, which has higher baud rate (112 Gbaud), a nonlinear
Volterra DB FFE and a MLSE are preferred in the receiver DSP even if DPD is applied. This
is because the channel shows a very sharp roll-off beyond 45 GHz and, to avoid an excessive
increase of the PAPR, it is not desirable to pre-compensate the signal near the Nyquist frequency.
If higher bandwidth transceivers are available in future, PAM-4 will require a similar transceiver
DSP configuration as described above for PAM-6. The performance shown in the following
mainly refers to DSP configurations with DPD. These considerations also apply to the O-band
EML-based link since this exhibits a similar end-to-end bandwidth compared to MZM case, as
indicated in the inset of Fig. 2.
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35246
3.2. Nonlinearity
Figure 5illustrates the system nonlinearity and the effectiveness of nonlinear Volterra FFE for
PAM-4 and PAM-6, respectively. The receiver nonlinear Volterra FFE configuration is denoted
as “[M
1
,M
2
,M
3
]”, where M
1
,M
2
, and M
3
are the memory lengths for linear, 2
nd
- and 3
rd
-order
Volterra kernels, respectively, as indicated in Eq. (1). A long memory for the linear terms is
used because of the reflections in our discrete setup [11]. Note that a compact integrated setup
would exhibit less reflection and therefore require a lower number of linear taps. Three cases
including a linear FFE, a nonlinear FFE with linear and 2
nd
-order kernels, and a nonlinear
FFE with linear, 2
nd
and 3
rd
-order kernels are considered. For both PAM-4 and PAM-6, Fig. 5
clearly shows that the use of nonlinear equalization improves both optical power sensitivity
and error floor, regardless of the transmitter choice. For the setup with MZM as indicated in
Fig. 5(a), the 3
rd
order nonlinear FFE brings about much more significant improvement than the
2
nd
order nonlinear FFE, indicating that the system is dominated by the 3
rd
order nonlinearity.
On the other side, for the EML case, as indicated in Fig. 5(b), the 3
rd
order nonlinearity is much
weaker compared to the MZM case. In general, the EML-based link exhibits a smaller overall
nonlinearity. However, it suffers from a higher error floor especially for PAM-6 due to the fact
that the EML has a lower extinction ratio (ER) compared to the MZM. A similar conclusion
applies to 2D-PAM8 for MZM case.
Fig. 5.
BER as a function of ROP for 224 Gb/s DB PAM-4 and 225 Gb/s PAM-6 subject to
different Volterra FFE configurations for (a) MZM and 1 km SMF, and (b) EML and 5 km
SMF.
The complexity of the Volterra filter can be reduced by introducing restrictions on the
tap spacing among samples participating in the same kernel without significant performance
degradation [19]. However, we adopt a full Volterra equalizer here to check the best achievable
performance.
3.3. System BER performance
The overall BER performance for each modulation scheme is presented in Fig. 6. We have chosen
to consider DPD and DB FFE with DB-MLSE for PAM-4 and DPD and full response nonlinear
Volterra FFE for PAM-6, respectively. We consider 2D-PAM8 with DPD, full response nonlinear
Volterra FFE, and a TCM-MLSE decoder after the FFE.
As indicated in Fig. 6(a)-(b), assuming FEC with a threshold BER of 2
×
10
−3
, PAM-6 with
DPD achieves the best receiver optical power sensitivity (-9.4 dBm) for the optical BtB case and
exhibits about 1 dB penalty for 1 km SMF compared to optical BtB. DB PAM-4 shows 0.7 dB
power penalty at the threshold BER compared to PAM-6 for optical BtB. DMT shows similar
optical power sensitivity to DB PAM-4 for optical BtB case, while 2D-PAM8 exhibiting slightly
better sensitivity than DB PAM-4 and DMT. For the 1 km SMF transmission case, 2D PAM-8
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35247
Fig. 6.
BER versus ROP of various modulation formats for MZM at (a) optical BtB, (b) 1
km SMF.
has the best optical power sensitivity of -8.6 dBm, with about 0.2 dB gain compared with all the
other three schemes which exhibit similar sensitivity.
Especially, 2D-PAM8 brings about a much improved error floor compared to all other schemes
for both optical BtB and 1 km SMF cases, indicating its potential to support simpler FEC with
higher BER threshold. Although DMT uses also TCM, its inherit disadvantages of high PAPR
and strong intra-sub-carrier beating noise upon square-law direct detection limit its error floor
performance.
The benchmark setup already shows the great potential of non-TCM PAM schemes in view
of their comparable or better optical power sensitivity but simpler DSP architecture compared
with the TCM schemes. Therefore, the EML setup focuses on examining the two PAM schemes.
For the EML case shown in Fig. 7, DB PAM-4 and PAM-6 achieve in the optical BtB case a
sensitivity of -8.5 dBm and -8 dBm, respectively, at the threshold BER of 2
×
10
−3
, which show
about 0.6 dB penalty compared to the MZM counterpart as well as increased error floor due to the
reduced ER of the EML. PAM-6 shows a high error floor than DB PAM-4 due to its sensitivity to
ER. After 5 km SMF, both DB PAM-4 and PAM-6 show negligible penalties and similar error
floors compared to the optical BtB case, since the accumulated fiber chromatic dispersion at
1310 nm is very small.
Fig. 7.
BER versus ROP of various modulation formats for EML at (a) optical BtB, and (b)
5 km SMF.
The overall performance of each modulation scheme for MZM and EML cases are summarized
in Table 1. It is interesting to note that the EML achieves a better optical power sensitivity
compared to the MZM for DB PAM-4 at a BER of 2
×
10
−3
after SMF transmission. This is
attributed to the lower nonlinearity of the EML based link and the negligible dispersion compared
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35248
to C-band transmission with the MZM, as indicated in section 3.2. Concerning the error floor, in
the EML case DB PAM-4 is preferable to PAM-6. For the MZM case, 2D-PAM8 and DMT show
no power penalty compared to DB PAM-4 and PAM-6.
Table 1. Summary of performance and FEC/DSP requirement
MZM case EML case
Modulation
Format
224G DB
PAM-4
225G PAM-6 225G 2D
PAM-8
224G DMT 224G DB
PAM-4
225G PAM-6
MZM 1 km
and EML 5km
ROP
sensitivity
-8.5 dBm -8.4 dBm -8.6 dBm -8.4 dBm -9 dBm -8 dBm
DSP DPD, NLa
DB-FFE,
DB-MLSE
DPD, NL FFE DPD, NL FFE,
TCM
IFFT/FFT, NL
FFE, TCM
DPD, NL
DB-FFE,
DB-MLSE
DPD, NL FFE
aNL: nonlinear Volterra
3.4. Complexity
In addition to the performance, Table 1also lists the DSP requirements for the considered
modulation schemes. For both MZM and EML cases, PAM-6 requires the least complex
DSP under the current component bandwidth constraints. Although 2D PAM-8 with superior
error floor performance in the MZM case could potentially allow less complex FEC, the DSP
complexity advises against its practical implementation, unless a simple FEC is required. DMT
in the MZM case shows no advantages on optical power sensitivity, error floor and complexity
over PAM-4 and PAM-6, indicating it is not a strong candidate.
4. Discussions
It is clear that the choice of the modulation format is a trade-off between optical link power
budget and DSP complexity. The optical link power budget is determined by both optical power
sensitivity and transmitter power. Table 1clearly shows that DB PAM-4 using the EML achieves
the best optical power sensitivity due to the negligible dispersion in O band. Depending on the
implementation platform, an EML and a MZM could have similar output power, but an EML
requires a smaller driving voltage and is more power efficient [26]. In the EML case PAM-6 shows
a relatively high error floor as a consequence of the ER limitation. Regarding the complexity,
PAM-6 needs the simplest DSP. However, when the next generation of components with 45 GHz
bandwidth or beyond is available, PAM4 could reduce the DSP requirements significantly (like
PAM-6 does) and become the overall best choice with respect to performance and complexity
because of its intrinsic noise tolerance.
5. Summary
Experimental demonstrations of 200 Gb/s per lane IMDD short-reach links have been undertaken
based on modulation candidates including PAM-4, PAM-6, TCM-encoded 2D-PAM8, and
DMT using both a C-band MZM and an O-band EML. PAM-6 shows better trade-off between
performance and DSP complexity using current commercial components with strong bandwidth
limitation but it exhibits the worst error floor when using EML due to ER limitation, making it
very undesirable. When the next generation of components with improved bandwidth is available,
PAM4 could reduce the DSP requirements significantly and become the best choice because of
its intrinsic noise tolerance. TCM-encoded 2D-PAM8 gives rise to the best error floor but its
complexity may prohibit its practical use unless the system requires a FEC with lower overhead
Research Article Vol. 28, No. 23 / 9 November 2020 / Optics Express 35249
but higher threshold BER. DMT does not show any advantage in either performance or DSP
complexity.
Disclosures
The authors declare no conflicts of interest.
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