1. Scaling Single-Wavelength Optical Interconnects to 180 Gb/s with PAM-M and Pulse Shaping
Stefanos Dris, Paraskevas Bakopoulos, Nikolaos Argyris, Christos Spatharakis, Hercules Avramopoulos
National Technical University of Athens
Photonics Communications Research Laboratory
photonics.ntua.gr

2. The Forecast: Cloudy with a 100% chance of Traffic
Global datacenter traffic is predicted to increase at a Compound Annual Growth Rate of 25% over the next 5 years
If we look at the forecast for 2019, although traffic between datacenters is gaining ground, intra-datacenter traffic will constitute ~73% of all traffic handled.
This is what we are focusing on today: Optical interconnects for intra-datacenter connectivity.
2019
Source: CISCO
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

3. The Way Ahead
Ethernet speeds growing at ~17% per year (compared to 25% per year for datacenter traffic)
Current trends pointing to >100 Gbit/s per optical lane
>400 Gbit/s to be achieved with multiplexing (WDM, multi-core, etc.)
Bandwidth-efficient modulation formats using intensity modulation and direct detection (IM/DD) are a promising approach
So how do we handle all this traffic?
Ethernet speeds are growing at ~17% per year (compare this to the 25% growth rate of datacenter traffic, and you have a problem in your hands)
The trend in industry and research is pointing toward higher throughputs per optical lane, moving to 100 Gbit/s in the future
400G will then probably be achieved by exploiting multiplexing: Whether that is spatial (multi-core fibers or multi-fiber ribbons/cables or WDM)
BW-efficient modulation formats relying on intensity-modulation and direct detection are a promising way of achieving these goals at reasonable cost and energy consumption
Source: Ethernet Alliance
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

4. Advanced Modulation Formats
PAM-M
Nyquist PAM-M
Optical PAM-4 has been extensively researched, with the fastest achieved so far in the lab being ~56 Gbaud (112 Gbit/s)*.
PAM-8 has been demonstrated up to ~37.4 Gbaud (112 Gbit/s).
Sub-Carrier Modulation (SCM): Half-Cycle 16- QAM Nyquist-SCM has achieved 56 Gbit/s (14 Gbaud) in the lab.
Fujitsu has demonstrated 116 Gbit/s with optical DMT.
single-cycle NRZ-SCM
half-cycle Nyquist-SCM
What are the options for future formats?
Optical PAM-4 has been extensively researched. When we were carrying out this work, the highest rate we had seen published in the literature was ~56 Gbaud (so 112 Gbit/s), though more recently a group at the Hong Kong polytechnic managed 70 Gbaud optical PAM-4.
Optical PAM-8 has also been attempted at up to around 37.4 Gbaud (also 112 Gbit/s).
We’re also seeing other schemes such as 16-QAM SCM, while Fujitsu are pushing ard for a multi-carrier approach based on DMT, and has demonstrated 116 Gbit/s.
An example of an SCM implementation will be presented later today by my colleague, Christos Spatharakis, later today, so check that out if you’re interested.
Discrete Multi-Tone
source: Fujitsu
*Zhong et al., PTL 2015: 70 Gbaud (140 Gbit/s) DD-FTN μm
*See also, 112 Gbit/s – See P. Bakopoulos et al. (SPIE Phot. West 2016)
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

5. This Work
Evaluate benefit of employing pulse-shaping (RRC) for PAM-M signaling in optical interconnects via simulation
Experimental verification of a PAM-M optical interconnect based on commercial off-the-shelf (COTS) components:
1550 CW laser source, 40 Gbit/s (23.4 GHz BW) Electro-Absorption Modulator (EAM), photoreceiver, 1 km+ SMF
Emulate direct drive by a server/switch ASIC: Low driving swing (<1V) from an Arbitrary Waveform Generator (AWG)
High baudrate operation in a bandlimited channel
40 Gbaud PAM-4 and PAM-8 operation with RRC pulse-shaping: 80 & 120 Gbit/s
60 Gbaud PAM-4 and PAM-8 operation: 120 & 180 Gbit/s
Our objectives in this work-
We wanted to see if employing pulse-shaping (root raised cosine) with PAM-M signaling could yield any benefit. We performed simulations that I will show you next.
We also carried out an experimental validation of a PAM-M optical interconnect, based on COTS components: At the heart of the system is an electro-absorption modulator which was directly driven by an Arbitrary Waveform Generator. The idea here was to emulate the case where we have direct drive by a server/switch ASIC with very low driving swing. So NO high-power RF amplifiers were employed.
We wanted to push the limits of our components, and operate at very high baudrates of 40 and 60 Gbaud, for both PAM-4 and PAM-8, for total throughputs of 80, 120 and 180 Gbit/s.
Leave out: For the 40 Gbaud cases, we employed RRC pulse-shaping. This was not possible with the 60 Gbaud signals, since the AWG we used has a sampling rate of 65 Gsa/s, so there wasn’t enough oversampling at 60 Gbaud.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

6. 40 Gbaud PAM-4 Simulations
Simulation of an EAM-based optical interconnect in VPITransmissionMaker
Objective: Evaluate efficacy of pulse-shaping in bandlimited channels
MATLAB-generated
40 Gbaud PAM-4
RRC-shaped with α=0.1
unshaped
Analog electrical filter to emulate Tx bandwidth limitation
MATLAB Rx DSP
Resampling & Symbol clock recovery
RRC Rx filter (only for shaped signals)
Equalization
Symbol detection & BER estimation
Single-Mode Fiber
500, 1000, 2000, 4000 m
Input electrical signals were limited to 1 Vpk-pk, to emulate limited swing of a DAC ASIC
We performed simulations in the VPITransmissionMaker software for 40 Gbaud PAM-4. The goal here was to evaluate the effect of pulse-shaping in a bandlimited optoelectronic channel.
Multi-level signals were generated in MATLAB: We had 2 cases: one with RRC pulse shaping with alpha (roll-off factor) 0.1, and the unshaped case (basically NRZ-like pulses).
It’s important to mention here that we made sure that we constrained the amplitudes of the input signals to 1 Vpk-pk, to emulate the case where the Tx DAC has limited output swing, as would be the case in a real system. Because RRC pulse-shaping results in large overshoots in the transitions, the power of the signal is ‘backed-off’ compared to the simple NRZ case. So for the pulse-shaped case our PAM-4 symbols are closer to each other and we start with a “disadvantage” in this respect.
We added an analog electrical filter to emulate a bandlimited channel. We simulated 4 different lengths of fiber (500, 1000, 2000 and 4000 m) and the received signals were processed with MATLAB DSP.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

7. 40 Gbaud PAM-4 Simulations
Bandlimited channel frequency response models
3 different channel models were used, and their frequency responses are shown here.
We had a Chebyshev type I response with 3dB BW of 22 GHz;
A Chebyshev type II, also with 3 dB BW of 22 GHz – the difference being that the type I has ripples in the passband and a much sharper roll off. The Type II rolls off more slowly.
We also had a non-bandlimited Chebyshev Type II channel, with a large BW of 40 GHz
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

8. 40 Gbaud PAM-4 Simulations
Bandlimited channel frequency response models
We can show the Power Spectral Density of the unshaped signal PAM-4 signal here in green
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

9. 40 Gbaud PAM-4 Simulations
Bandlimited channel frequency response models
As well as of the RRC-pulse-shaped signal.
So you can see how the various channel responses would affect the signals in terms of spectral content.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

10. 40 Gbaud PAM-4 Simulations
Chebyshev I, 22 GHz
Unshaped
On to the simulation results:
First let’s look at the unshaped case, for 500, 1000, 2000 and 4000 m. This is for the Chebyshev Type I channel (so the one with the sharp roll-off).
Basically the performance is the same for the 3 shorter distances, but the 4000 m transmission causes a little bit of penalty due to chromatic dispersion starting to affect the signal.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

11. 40 Gbaud PAM-4 Simulations
Chebyshev I, 22 GHz
Unshaped
Unshaped
RRC-shaped
RRC-shaped
Let’s add the BER curves for the pulse-shaped PAM-4 signal: We immediately see that the performance is better for this case. And this is despite the fact that we had to back-off the input power of the electrical signal, to accommodate the overshoots of the pulses.
If we cut across at the 10-3 BER line, we can look at the required Rx optical power that achieves a BER of 10-3, as a function of transmission length. So this gives a quick visual way of seeing which case performs better. For the first channel model, pulse-shaping is clearly better.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

12. 40 Gbaud PAM-4 Simulations
Equalizer
RRC: 27 taps
Unshaped: 37 taps
Chebyshev I, 22 GHz
bandlimited, steep roll-off
Equalizer
RRC: 17 taps
Unshaped: 11 taps
Chebyshev II, 22 GHz
bandlimited, smooth roll-off
We can now look at the results for all 3 channel models.
The take-away is that pulse-shaping improves performance only when the channel is bandlimited, and has a steep roll-off. For the Chebyshev Type II case which has a smoother roll-off, we saw no penalty for the unshaped case.
The interesting result is the one with the 40 GHz channel (i.e. the one where we were not bandlimited). Here we see that performance is the same for the 500 and 1000 m cases, but as we move to longer distances, RRC-pulse-shaping gives better results. And the reason is chromatic dispersion: The unshaped PAM-4 signal has a much larger spectral content, and is therefore more affected by CD. Therefore employing pulse-shaping is useful also for channels with large bandwidths, and long lengths of fiber 2000 m and beyond (where significant dispersion is accumulated).
Chebyshev II, 40 GHz
not bandlimited
Equalizer
RRC: 5 taps
Unshaped: 9 taps
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

13. Experimental Setup
The Keysight M8195A AWG was used to drive an Electro-Absorption Modulator (EAM) with 3-dB BW of GHz.
No additional electrical amplification in order to emulate the output of an ASIC (swing was ~ mV).
Transmission over 1250 m of single-mode fiber (SMF).
Reception was performed using a limiting photoreceiver (35 GHz BW) - low input optical powers were used
An Agilent DSOX93304Q real time scope (33 GHz analog BW, 80 GSa/s) was used to digitize and capture the received signals
Let’s look at the experimental part.
This is the setup.
Read…
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

14. PAM-M Formats Tested
Root-Raised Cosine (α=0.1) PAM-4 at 40 Gbaud (80 Gbit/s)
Unshaped PAM-4 at 60 Gbaud (120 Gbit/s)
And here are the modulation formats experimentally tested:
We had 40 Gbaud PAM-4 WITH pulse shaping – so 80 Gbit/s throughput
As well as unshaped PAM-4 at 60 Gbaud, for 120 Gbit/s
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

15. PAM-M Formats Tested
Root-Raised Cosine (α=0.1) PAM-4 at 40 Gbaud (80 Gbit/s)
Unshaped PAM-4 at 60 Gbaud (120 Gbit/s)
Root-Raised Cosine (α=0.1) PAM-8 at 40 Gbaud (120 Gbit/s)
Unshaped PAM-8 at 60 Gbaud (180 Gbit/s)
Back-to-back and transmission over 1250 m SMF
Same symbol rates were used with PAM-8.
So the throughputs with PAM-8 were 120 and 180 Gbit/s on a single wavelength.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

16. Experimental Setup
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

17. Offline Rx DSP
The DSP processing consisted of root raised cosine Rx filtering (only for the Nyquist-PAM cases), symbol clock recovery and equalization.
Channel estimation was carried out with a symbol-spaced Feed Forward Equalizer (FFE), whose coefficients were determined using the adaptive normalized Least Mean Squares (LMS) algorithm.
Equalization was carried out with a static FIR filter with the coefficients from the FFE.
A large number of taps were required to reverse the effects of the channel (due to severe bandwidth limitations, as well as the overall uneven frequency response of the component chain).
Frequency-domain equalization (FDE) can be used as a lower-complexity approach, given the large number of taps needed.
Read…
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

18. 40 Gbaud, 80 Gbit/s PAM-4 Rx Amplitude PDFs
electrical RRC PAM-4
transmission
no equalization
after equalization
B2B Optical
The best way of visualizing the signal quality of PAM-M system is by looking at the received amplitude probability density functions- especially when the system relies on Rx-side DSP.
So looking at the 40 Gbaud, RRC-shaped PAM-4 case:
-we start out with the input electrical signal out of the AWG (this is without any equalization)
This is then converted to optical PAM-4 by the EAM, and here’s the back-to-back optical signal. We can see overlap of the PAM-4 levels if no equalization is used. But after equalization, we get 4 very clear peaks.
This is our signal after transmission – with and without equalization.
So this is a nice way of looking at the evolution of the quality of the signal as we traverse the link’s components.
no equalization
after equalization
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

19. 40 & 60 Gbaud PAM-4 Rx Amplitude PDFs
AWG electrical output
Optical Back-to-Back
After 1250 m transmission
RRC-shaped 40 Gbaud PAM-4 (80 Gbit/s)
no equal.
with equal.
no equal.
with equal.
unshaped 60 Gbaud PAM-4 (120 Gbit/s)
no equal.
with equal.
no equal.
with equal.
So these are the PDFs I just showed you for the 40 Gbaud PAM-4 – the electrical input, the B2B case, and after transmission over 1250 m SMF. These are all for the highest Rx optical power.
And we can compare these to the 60 Gbaud PAM-4, and immediately see the difference – the pre-equalization signals are even worse, but after equalization we can somewhat restore the signal quality.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

20. 40 & 60 Gbaud PAM-8 Rx Amplitude PDFs
AWG electrical output
Optical Back-to-Back
After 1250 m transmission
RRC-shaped 40 Gbaud PAM-8 (120 Gbit/s)
no equal.
with equal.
no equal.
with equal.
unshaped 60 Gbaud PAM-8 (180 Gbit/s)
no equal.
with equal.
no equal.
with equal.
The same plots for the PAM-8 case – these are all in the paper, you can have a more detailed look if you’re interested.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

21. BER vs received power RRC 40 Gbaud PAM-4
Let’s have a look at the BER curves we obtained.
Here’s the 40 Gbaud PAM-4, B2B and after transmission (only a small Tx penalty is observed).
The 3 dotted lines show typical FEC thresholds for 3 different codes with increasing overhead (and complexity).
Performance is so good at the highest Rx powers, that we can employ a very low-overhead HD FEC in this case – the lowest line corresponds to a code with only 3% overhead – so very low complexity.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

22. BER vs received power RRC 40 Gbaud 60 Gbaud PAM-4 PAM-4
For the 60 Gbaud PAM-4 case, stronger FEC is needed: Still, hard-decision FEC with FEC threshold of 10-3 is sufficient (it would correspond to an overhead of ~7-11%).
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

23. BER vs received power RRC 40 Gbaud 60 Gbaud PAM-4 PAM-4 RRC 40 Gbaud
On the right we have the PAM-8 results, here for the 40 Gbaud case. So this is also below the 10-3 FEC threshold, and the same HD code with 11% overhead could be used.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

24. BER vs received power RRC 40 Gbaud 60 Gbaud PAM-4 PAM-4 RRC 40 Gbaud
The 60 Gbaud PAM-8 predictably requires the strongest FEC of all. Since we were limited by Rx power, we could not do better than what you see here- A soft-decision FEC like the ones used in telecom applications would be needed – these have overheads of 20-30%.
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

25. Summary & Conclusions
Simulations: Pulse-shaping generally improves performance in severely bandlimited channels, and makes system more resilient to dispersion (for long lengths of fiber)
Demonstrated successful transmission over 1250 m with a single-wavelength optical interconnect:
80 Gb/s RRC PAM-4
120 Gb/s PAM-4 and RRC PAM-8
180 Gb/s PAM-8
Digital equalization at the receiver with a symbol-spaced FIR filter was necessary to ensure that the effects of the severely bandlimited channel could be reversed
The sub-volt electrical driving scheme we have shown is achievable with state-of-the-art CMOS electronics found in switch or server ASICs, and does not require power-hungry linear RF modulator drivers. It is therefore very relevant for datacenter applications, where energy-efficiency is important
Read…
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

26. Acknowledgements
Supported in part by EU-funded ICT projects PhoxTroT, Mirage and Nephele
Special thanks to Keysight Technologies for providing the arbitrary waveform generator
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping

27. BER vs received power
RRC 40 Gbaud
PAM-4
60 Gbaud PAM-4
RRC 40 Gbaud
PAM-8
60 Gbaud PAM-8
The 60 Gbaud PAM-8 predictably requires the strongest FEC of all. Since we were limited by Rx power, we could not do better than what you see here- A soft-decision FEC like the ones used in telecom applications would be needed – these have overheads of 20-30%.
As a final note, we can compare the 2 different implementations that reach 120 Gbit/s (with 60 Gbaud PAM-4 or 40 Gbaud PAM-8). What we observed for our particular link is that the PAM-4 was slightly better by 0.5 dB at a BER of 10-3.
120 Gbit/s with 60 Gbaud PAM-4 is ~0.5 dB better than RRC-shaped 40 Gbaud PAM-8 (at BER=10-3)
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Scaling single-wavelength optical interconnects to 180 Gb/s with PAM-M and pulse shaping