How DSP smarts continue to improve optical transport

• Kim Roberts explains the signal processing techniques Ciena is using for its WaveLogic 6 coherent DSP.
• Roberts explains how the techniques squeeze, on average, a 15 per cent improvement in spectral efficiency.
• The WaveLogic 6 Extreme chip can execute 1,600 trillion (1.6 x 10¹⁵) operations per second and uses the equivalent of 4km of on-chip copper interconnect.

Part 2: WaveLogic 6's digital signal processing toolkit

Kim Roberts, Ciena’s vice president of WaveLogic Science and winner of the 2019 John Tyndall Award

Bumping into Kim Roberts on the way to the conference centre at OFC, held in San Diego in March, I told him how, on the Ciena briefing about its latest WaveLogic 6 coherent digital signal processor (DSP), there had been insufficient time to dive deeply into the signal processing techniques used.

“What are you doing now?” said Roberts.

“I’m off to the plenary session to catch the keynotes.”

Chatting some more, I realised I was turning down a golden opportunity to sit down with a leading DSP and coherent modem architect.

“Is that offer still open?” I asked.

He nodded.

We grabbed a table at a nearby cafe and started what would prove to be an hour-long conversation.

High-end coherent DSPs

Many leading coherent modem vendors unveiled their latest designs in the run-up to the OFC show. It is rare for so many announcements to be aligned, providing a valuable glimpse of the state of high-performance optical transport.

Nokia announced its PSE-6s, which has a symbol rate of up to 130 gigabaud (GBd) and supports 1.2 terabit wavelengths. Infinera announced its 1.2-terabit ICE-7, which has a baud rate of up to 148GBd, while Fujitsu detailed it is using its 135GBd 1.2-terabit wavelength coherent DSP for its 1FINITY Ultra optical platform.

Meanwhile, Acacia, a Cisco company, revealed its 140GBd Jannu 1.2-terabit DSP has been shipping since late 2022. Acacia announced the Jannu DSP in March 2022.

All these coherent DSPs are implemented using 5nm CMOS and are shipping or about to.

And Ciena became the first company to detail a coherent DSP fabricated using a 3nm CMOS process. The WaveLogic 6 Extreme supports 1.6-terabit wavelengths and has a symbol rate of up to 200GBd. 

Ciena’s WaveLogic 6 Extreme improves spectral efficiency by, on average, 15 per cent. WaveLogic 6 Extreme-based coherent modems will be available from the first half of 2024.

Customer considerations

Kim Roberts begins by discussing what customers want.

“With terrestrial systems, it is cost-per-bit [that matters], and if you're not going very far, it is cost-per-modem,” says Roberts.

For the shortest reaches (tens of km), 100 gigabit may be enough while 200 gigabit or more is overkill. Here, a coherent pluggable module does the job.

“What matters is the cost per modem to get the flexibility of coherent connectivity so that you can plug it in and it works,” says Roberts.

With medium and long-haul terrestrial routes, cost-per-bit and heat-per-bit are the vital issues. With heat, area and volume of the coherent design are important. “I need volume to get the heat out of the chip on the card and into the air,” says Roberts.

Another use case is where spectral efficiency is key, for networks where fibre is scarce. An operator could be leasing dark fibre, or it could be a submarine network.

Ciena’s WaveLogic 6 Extreme’s 15 per cent improvement in spectral efficiency improves capacity over the same link. “Equivalently, you can go a dB (decibel) further or have a dB more signal margin,” says Roberts.

A common refrain heard is that spectral efficiency is no longer improving due to the Shannon limit being approached. Shannon’s limit is being approached because of the considerable progress already made by the industry in coherent optics.

“There is no 6dB to be had like in the old days,” says Roberts. “WaveLogic 3 was 2.5dB better than WaveLogic 2, but those multiple dBs are no longer there.”

The returns are diminishing, but striving for improvements remains worthwhile. “If you're an operator that cares about spectral efficiency, that's important,” he says.

Nonlinearity mitigation

Roberts returns to the issue of Shannon’s limit, based on the work of famed mathematician and information theorist, Claude Shannon.

“Shannon defines a theoretical limit for the capacity of a channel having linear propagation with additive Gaussian noise,” says Roberts.

This defines a strict mathematical limit, and it is pointless to go beyond that; he says: “In terms of linear performance, modems are getting close to the limit, within a couple of dB.”

Shannon’s limit doesn’t wholly define fibre since the channel is nonlinear.

Roberts says there is a whole research area addressing the bounds given such nonlinearities.

“We're a long way from those theoretical nonlinear limits, but what matters is what's the practical limit, and it's getting hard,” he says

Increasing transmit power improves the optical signal-to-noise ratio (OSNR) and strengthens nonlinearities. Indeed, the nonlinearities grow faster with increased transmit power until, eventually, they dominate.

Because tackling nonlinearities is so complicated, Ciena’s approach is to approximate the problem as a linear Gaussian noise channel and do everything possible to mitigate the effects of nonlinearity rather than embrace it.

This is done by compensating at the transmitter the nonlinearities expected to happen along the fibre. The receiver performs measurements on a second-by-second basis and sends the results back. These are used as estimates of the anticipated nonlinearity about to be encountered and subtracted from the symbols to be sent.

Transmit decompensation: “Instead of being a rectangular grid, it is swirly; that is what we transmit,” says Roberts. “It looks ugly, but what we’re transmitting is the effects of the nonlinearities that will happen in the fibre, subtracted off.” Source: Ciena.

Even though the exact nonlinearity is unknown, this is still a valid approximation. “It gives a quarter to one dB of performance improvement,” says Roberts

Edgeless clock recovery

Robert explains other clever signal processing techniques that buy a 6 per cent spectral efficiency improvement.

With wavelength division multiplexing (WDM), the laser-generated signals are placed next to each other across the fibre’s spectrum.

For WaveLogic 6, when running at its maximum symbol rate of 200 gigabaud, the spectrum occupies a 200GHz-wide channel.

Usually, the signal in the frequency domain is not perfectly square-shaped; the signal rolls off in the frequency domain so that in the time domain there is no inter-symbol interference. “But [as a result] you’re wasting spectrum; you are not fully using that spectrum,” says Roberts.

Source: Ciena

With WaveLogic 6, Ciena has created an idealised flat-topped, vertically edged signal spectra allowing the signals to be crammed side by side thereby making best use of the fibre’s spectrum (see diagram).

The challenge is that the clocking information used for data recovery at the receiver resided in this roll-off region. Now, that is no longer there so Cienahas developed another method to recover clock information.

A second challenge with signal recovery is that the transmit laser and the receive laser are not rigidly fixed in frequency. Being so close together, care is needed to recover only the wavelength - signal - of interest.

Yet another complication is how a rectangle in the frequency domain causes the signal in the time domain to ‘ring’ and go on forever.

“There are several signal processing methods that we had to develop to make this possible,” says Roberts.

Frequency-division multiplexing

Ciena also uses frequency division multiplexing (FDM), a technique it first introduced with the WaveLogic 5 Extreme.

The difference between WDM and FDM, explains Roberts, is that WDM uses different lasers to generate the wavelengths while FDM is generated by applying digital techniques to the same laser. “You are digitally combining different streams,” he says.

This is useful because it turns out that each fibre route has an optimum baud rate because of nonlinearities.

“If I’m using the full symbol rate of 200GBd, I can divide that into parallel streams, which behave as if they were independent circuits as far as nonlinearity is concerned,” says Roberts. “The optimum number of FDM in your spectrum is proportional to the square root of the total amount of dispersion, so high dispersion, more FDMs, low dispersion, just one.”

Ciena first added the option of four FDM with the WaveLogic 5. Now, WaveLogic 6 implements 1,2,4, and 8 FDM channels.

Source: Ciena.

“For short distances, you want to go one signal at 200 gigabaud, or smaller if you're reducing baud rate, but if you're going very long distances, lots of dispersion, you go at eight parallel streams being sent at 25 gigabaud each,” he says.

But introducing FDM causes notches in the near-idealised rectangular spectrum mentioned earlier. Ciena has had to tackle that too.

“If you measure the spectrum, it's completely flat, there are no notches between the FDMs, there is no wasted spectrum,” says Roberts.

Multi-dimensional coding

Multi-dimensional coding is a further technique used by Ciena to improve optical transmission, especially in troublesome cables where there are much nonlinearity and noise. It is challenging to get information through.

To understand multi-dimensional constellations, Roberts uses the example of a 16-QAM constellation, which he describes as a two-dimensional (2D) representation in one polarisation.

But if both polarisations of light are considered one signal, it becomes a 4D, 256-point (16x16) symbol. This can be further extended by including the symbols in adjacent time slots to form an 8D representation.

Ciena introduced this technique with its WaveLogic 3 Extreme coherent DSP, which supported the multi-dimension coding scheme 8D-2QAM to improve the reach or capacity of long-reach spans.

Now Ciena has introduced a family of such multi-dimensional schemes with WaveLogic 6 Extreme, executing in regions of very high nonlinearity and noise. These include 4, 8, and 16-dimensional constellations.

Source: Ciena

An example where the technique is used includes cases where there is twice as much noise as there is signal. “So the signal-to-noise ratio is -3dB,” says Roberts. Yet even here, 100 gigabits can still get through.

WaveLogic 6 Nano

Ciena also announced its 3nm CMOS WaveLogic 6 Nano DSP aimed at pluggable coherent modules. Is the Nano’s role to implement a subset of the signal processing capabilities of the Extreme?

Here, the customer's requirements are different: heat, space and footprint are the dominant concerns. The Nano has to fit the heat envelope of the different sizes of pluggables, says Roberts. The optical performance is chosen based on fitting that heat requirement.

One of the merits of 3nm FinFET transistor technology is that if you don't clock a circuit, only 1 per cent of the heat is generated compared to when it's clocked, notes Roberts: “So, for different features, I can turn off the clock.”

A suitcase still full of tools?

At the time of the WaveLogic 5 launch, Roberts mentioned that there were still many tools left in the suitcase of ideas. Is this still true with the WaveLogic 6?

For Roberts, the question is: will it be economically viable to put in new capabilities based on the heat and performance and in terms of the size, schedule, and the amount of work involved?

Then, with a broad smile, he says: “There is room to occupy us as to how to get the next 10 to 20 per cent of spectral efficiency.”

And with that, we each set off for a day of meetings.

Roberts headed off to his hotel before his 10am meeting. I set off for the OFC exhibition hall and a meeting with the OIF.

As I walked to the convention centre, I kept thinking about the impromptu briefing and how I so nearly passed up on Roberts’ expertise and generosity.