4000km Transmission of 50GHz spaced, 10494.85-Gb/s
Hybrid 32-64QAM using Cascaded Equalization and
Training-Assisted Phase Recovery
†
X. Zhou1, L. E. Nelson1, R. Isaac1, P. D. Magill1, B. Zhu2, D. W. Peckham3, P. Borel4, and K. Carlson4
(1) AT&T Labs-Research, 200 Laurel Ave South, Middletown, NJ 07748, USA, †zhoux@research.att.com
(2) OFS Labs, Somerset, NJ 08873, USA; (3) OFS, Norcross, GA 30071; (4) OFS, Broendby, Denmark
Abstract: Employing time-domain hybrid QAM, training-assisted phase recovery and cascaded equalization,
we successfully transmitted ten 494.85Gbit/s PDM-32/64QAM DWDM signals at 8.25b/s/Hz net spectral
efficiency (SE) over 40100km, achieving a record terrestrial SEdistance product of 33000 bitkm/s/Hz.
OCIS codes: (060.1660) Coherent communications; (060.2330) Fiber optics communications.
1. Introduction
Following the commercial availability of systems carrying 100-Gb/s WDM channels and based on the likelihood of
400Gb/s as the next-generation transport standard, research on spectrally-efficient systems at bit rates of 400Gb/s has
intensified [1-7]. The premise that optical transport costs are decreased most efficiently by increasing both the perchannel data rate and spectral efficiency (SE) has motivated the utilization of high-order quadrature amplitude
modulation (QAM) formats for 400Gb/s with multiplexing schemes such as OFDM [1,3], OTDM [2], TDM [4], and
single-carrier-FDM with digital pilot tone[5]. Several of these first demonstrations [1,2,4] reported long-haul
transmission reaches of 800 to 2000km, yet the experiments used >50GHz WDM channel spacing, which is sub-optimal
in terms of maximizing the SE.
From an operational point of view, the ability to place 400-Gb/s signals on the 50GHz channel grid at a net
SE>8b/s/Hz is highly attractive, and may be most cost-effective, because such a transport system will be compatible
with current ROADM optical networks, while providing a four-fold increase in the transport capacity. Recently some
progress has been made on this front. In [6] it was demonstrated that Nyquist-shaped PDM-32QAM enables a 400G
WDM channels in a 50GHz grid optical network could be transmitted over 8x100km [6]. The transmission reach was
improved to 12x100km in [7] by using a newly proposed time-domain hybrid QAM technique. However, these reaches,
and the reach reported in [5], are still not adequate for many practical long-haul transmission systems.
In this paper we report the successful transmission of ten 494.85Gbits/s PDM-32/64QAM DWDM signals over
40100km of ultra-large-area (ULA) fiber at a net SE of 8.25b/s/Hz (after excluding the 20% soft-decision FEC
overhead). Time-domain hybrid 32/64 QAM along with novel, improved carrier-phase recovery and equalization
algorithms are utilized. To our best knowledge, this is the longest transmission distance demonstrated for a WDM SE
above 5bit/s/Hz and bit rate>100Gb/s. This result also represents a new record for the SEdistance product for territorial
transport systems.
2. Experimental setup
A schematic of the experimental setup is shown in Fig. 1. The ten 494.85Gb/s, 50GHz-spaced, C-band channels are
derived from odd (192.20 to 192.60THz) and even (192.15 to 192.55THz) sets of multiplexed, 100GHz-spaced external
cavity lasers (ECL) with ~100kHz linewidth. For each channel, five frequency-locked sub-carriers, separated by
9.9GHz are generated from a single laser source using a method very similar to that described in [6-7] and shown in Fig.
1b. All the unwanted harmonics were suppressed to be 40dB lower than the desired five subcarriers. The odd and the
even subcarriers are modulated by two independent IQ modulators (i.e. MOD1 and MOD2), each driven with a
Nyquist-shaped 9.7Gbaud time-domain 32-64 hybrid QAM (SE=5.1015bit/symbol) signal having 215 pseudorandom
pattern length (bandlimiting effect from the D/A converters has been pre-compensated). Then the ten sets of three
49.485Gb/s odd subcarriers and ten sets of two 49.485Gb/s even sub-carriers are passively combined and polarization
multiplexed with 20ns relative delay, resulting in ten 50GHz-spaced DWDM, 494.85Gb/s channels. The ten DWDM
channels are effectively de-correlated because their optical phases are different. Due to close to ideal Nyquist pulse
shaping (roll-off factor=0.01), the 494.85Gb/s signal are well confined within 49.3GHz optical bandwidth (see Fig.1f).
The principle of the time-domain-based 32-64 hybrid QAM is shown in Fig. 1d. Within each time-divisionmultiplexing (TDM) frame, two regular 2m–ary QAMs with different SE (in terms of bit/symbol) are assigned to
different time slots. Using this method, any SE that falls between the SE of the two QAMs can be realized by
appropriately designing the TDM frame length and the time slot occupancy ratio of the two QAMs. Such a time-domain
hybrid QAM technique exhibits several potential advantages as compared to the conventional frequency-domain-based
1
hybrid QAM, such as lower peak-to-average power ratio and better tolerance toward laser phase noise (due to
fundamentally shorter symbol period). For this experiment, each TDM frame consists of 128 symbols, where 28 of them
are 64QAM and 97 are 32QAM. The extra 3 symbols are used as training symbols for carrier phase recovery. For ease
of processing, the Euclidean distances for the 64QAM and 32QAM are designed to be identical, resulting in a 64QAMlike constellation with un-equal constellation occupation probability, as can be seen in Fig. 1e.
(b) 10495Gb/s PDM-32-64 hybrid QAM Tx
(a)
switch
50GHz
WB
WDM
WDM
Raman
Pumps
21 taps
MZM1
VOA
TDM
Syn.
LMS
EQ2
21 taps
Carrier
Syn.
LMS
Carrier
Syn.
801
taps
EQ2
TDM frame
(e)
Pre- equalized
9.7 Gbaud digital
Nyquist 32-64
hybrid QAM
EDFA
POL
MUX
OC
25/50G
IQ
MOD 2
Pre- equalized
9.7 Gbaud digital
Nyquist 32-64
hybrid QAM
MZM2
TDM frame Time
-20
D/A
converters
Sub1
(f)
Sub5
-30
Power (dBm)
EQ1
9.7G
clock
(d) A: 64QAM B: 32QAM
A …
B A … B
Decoding
CD Comp.
y
CMA
.
.
.
IQ
MOD 1
Odd subcarriers
(c)
x
EDFA
OC
2
10
9.7G
clock
ILF
Isolator
100 km ULA fiber
D/A
converters
12.5/25G
ILF
Raman
Pumps
.
.
.
Even subcarriers
WDM
Digital
Coherent
receiver
9
x4
3dB
Coupler
ECLs
1
ILF
switch
EDFA
WDM
10495-Gb/s
PDM-32-64hybrid QAM
Transmitter
Loop Synch Pol
Controller
-40
-50
49.3GHz
BW=1.25GHz
-60
-70
0
50
-50
-50-40-30-20-10 0 10 20 30 40 50
Frequency offset (GHz)
Fig.1. Experimental setup. ILF: interleaver filter. OC: optical coupler; WB: wavelength blocker. CD: chromatic dispersion, EQ: equalizer
Following the polarization multiplexer and a booster erbium-doped fiber amplifier (EDFA), the ten 494.85-Gb/s
channels are sent into a re-circulating transmission loop, which consists of four 100-km spans of ULA fiber having, at
1550nm, 135 um2 average Aeff and 0.179 dB/km average attenuation. As shown in Fig. 1a, the spans are configured for
all-Raman amplification, resulting in total span losses (fiber + components) of between 19.7 and 20.1dB. For each
span, counter-pumps at 1435nm and 1455nm with 310mW and 650mW, respectively, provide an average of 17dB onoff Raman gain, while co-pumps at 1455nm and 180mW provide an average of 3dB on-off gain. After the last span, a
loop-synchronous polarization controller is followed by a wavelength-blocker-based channel equalizer to flatten the
spectrum after each loop.
After transmission, each subcarrier is independently received by passing the signal through a tunable optical filter
with ~37.5GHz -3dB bandwidth and detecting it with the polarization- and phase-diverse coherent receiver. A fourchannel real-time sampling scope (50GSa/s and 16GHz bandwidth) performs the sampling and digitization function,
followed by post-transmission offline DSP of the captured data. The major DSP functional blocks are shown in Fig. 1c.
A new cascaded three-stage equalization strategy is used for polarization demultiplexing and linear distortion
mitigation. First, a decision-independent CMA equalizer (21-tap and T/2-spaced, i.e. EQ1 in Fig. 1c) is used to perform
an initial equalization. The output of this CMA equalizer is then used for TDM frame synchronization, which is
followed by a second equalizer (i.e. EQ2 in Fig. 1c) also with a tap length of 21. The coefficients of EQ2 are first
optimized by using 16000 T/2-spaced samples in a training-based manner in the acquisition stage. After acquisition is
achieved, EQ2 is switched into a decision-directed (DD) blind equalization mode. The carrier frequency and phase
recovery are performed within the DD-LMS loop. The signal-LO frequency offset is initially estimated using the
acquisition training signals, and then is tracked using the recovered carrier phases through a feedback configuration. We
achieved robust carrier phase recovery by using a new training-assisted method: the initial phase is estimated by using
the 3 training symbols and then refined by using a blind phase search algorithm [8] over a small phase-varying range
(/32). This method can effectively mitigate the cyclic phase-slipping problem. The phase-recovered signal at the output
of EQ2 is then sent to the third equalizer, EQ3, for final optimization. EQ3 operates in a decision-directed mode (after
acquisition), but with a longer tap length (801 T/2-spaced taps used in this experiment). Such a long filter is used for
two reasons: 1) the optimal receive filter for a Nyquist signal is significantly longer than a regular NRZ signal; 2) a long
filter allows mitigation of some low-frequency/narrow-band distortion that may occur at the transmitter and the
receiver. Note that the allowable tap length used for EQ2 is limited by the phase noise and therefore cannot be too long.
The equalizer implementation complexity can be greatly reduced by using a frequency–domain based method when the
tap length is long [9], although the time-domain based method is used for this experiment. For bit error ratio (BER)
calculations, errors are counted over 2×106 bits of information for each subcarrier (10×106 bits for each channel).
3. Measurement results
2
In Fig. 2 we show the measured back-to-back performance (average of the five subcarriers) of a single 494.85 Gb/s,
PDM 32/64-hybrid QAM channel with the three-stage equalizer. For comparison, the theoretical prediction and twostage equalizer (EQ1 and EQ2) results also are displayed. The required OSNR at 2x10-2 BER is reduced from 25dB for
the two-stage equalization method to 22.8dB for the three stage method, which is only about 0.6dB higher than the
theoretical required OSNR. The DWDM transmission results are displayed in Fig. 3 to Fig. 5, where the total launch
power entering the transmission fiber for the ten WDM channels is 6.5dBm, corresponding to -3.5dBm per channel
and -10.5dBm per subcarrier, which we have found to be the optimal launch power for 4000km transmission. Fig. 3
shows the measured optical spectra (in 0.1nm noise bandwidth) of the launched DWDM signals and the signals after
4000km transmission. The launch OSNR is above 40.5 dB for all the WDM channels. After 10 circulations (4000km),
the spectrum of the ten 494.85Gb/s channels was flat to within 1dB. Due to the use of all Raman amplification, the
OSNR after 4000km transmission is still greater than 24dB for all ten WDM channels, despite the relatively low perchannel launch power.
In Fig. 4 we show the measured BER and OSNR of the central channel (Ch. 5) with transmission distance ranging
from 400km to 4800km. The measured BERs after 3600, 4000 and 4400km transmission are 1.51x10 -2, 1.94x10-2, and
2.4x10-2, respectively, with corresponding OSNRs of 25, 24.5 and 23.9dB. The measured BERs of all ten 494.85-Gb/s
WDM channels after 40x100km transmission are presented in Fig. 5. The BERs for the five individual subcarriers of
Ch. 5 are also plotted. The BERs of all ten channels are better than 2.010-2, while the BERs of all 50 individual
subcarriers are better than 2.210-2; all are below the 2.410-2 BER threshold for 20% soft-decision FEC using quasicyclic LDPC code [10]. The recovered constellation diagrams for the X- and Y-polarizations of one of Ch. 5’s
subcarriers are shown in Fig. 5 as insets. Because no inline optical dispersion is used, we found that the noise statistics
can be well approximated by an additive Gaussian distribution in both linear and nonlinear regimes, agreeing with [11].
10-1
-20
Power (dBm)
Theory
3-stage EQ
10-2
-30
Launch
BW=0.1nm
-40
-50
-60
1555.5
1556.5
1557.5
1558.5
1559.5
1560.5
-10
10-3
-20
Power (dBm)
Bit error ratio
2-stage EQ
10-4
19
21
23
25
27
29
31
33
35
37
39
30
28
26
24
400
1557.5
22
800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800
Transmission distance (km)
Fig. 4. BER and OSNR of Ch. 5. vs. transmission distance
10-1
Average
sub2
sub4
Bit error ratio
32
OSNR of Ch. 5 (in 0.1nm, dB)
Bit error ratio of Ch. 5
34
0.01
10-2
0.001
10-3
1556.5
1558.5
1559.5
1560.5
Fig. 3. Launched and received optical spectra
36
BER
-50
Wavelength (nm)
Fig. 2. Measured back-to-back results for a 494.85Gb/s channel
OSNR
40x100km
BW=0.1nm
-40
1555.5
OSNR in 0.1 nm bandwidth (dB)
0.1-1
10
-30
X
sub1
sub3
sub5
Y
FEC threshold
10-2
1556
1556
1557
1557
1558
1558
1559
1559
1560
1560
Wavelength (nm)
Fig. 5. BER of all ten 50GHz-spaced WDM channels after 40x100km
4. Conclusions
In summary, we have demonstrated 4000km transmission of ten 494.85Gbits/s, 50-GHz-spaced DWDM signals at
8.25b/s/Hz net spectral efficiency (excluding the 20% soft-decision FEC overhead). The result was enabled by timedomain hybrid 32/64 QAM, as well as novel carrier-phase recovery and equalization algorithms. To our best
knowledge, 40x100km is the longest transmission distance demonstrated for a WDM SE above 5bit/s/Hz (and bit
rate>100Gb/s), and the SEdistance product of 33000 bitkm/s/Hz is a new record for territorial transport systems. Our
results indicate the potential for practical long-haul DWDM systems carrying 400Gb/s channels on the 50GHz grid.
Acknowledgment: We thank Robert Lingle, Jr. of OFS and M. Feuer and K. Reichmann of AT&T Labs for their generous support of these results.
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4