ADCs key for high baud-rate coherent systems
Increasing the baud rate of coherent modems benefits optical transport. The higher the baud rate the more data can be sent on a wavelength, reducing the cost-per-bit of traffic.
But engineers have become so good at designing coherent systems that they are now approaching the Shannon limit.
At the OFC show earlier this year, Ciena showcased a coherent module operating at 107 gigabaud (GBd). And last year, Acacia, now part of Cisco, announced its next-generation 1.2 terabits-per-second (Tbps) wavelength coherent module operating at up to 140GBd.
The industry believes that increasing the baud rate to 240+GBd is possible, but each new symbol-rate hike is challenging.
All the components in a modem – the coherent DSP and its digital-to-analogue (DAC) and analogue-to-digital (ADC) converters, the optics, and the analogue drive circuitry – must scale in lockstep.
Gigabaud and giga-samples
Coherent DSPs continue to improve in optical performance with each new CMOS process. The latest DSPs will use 5nm CMOS, while the semiconductor industry is developing 3nm CMOS and beyond.
Optical device performance is also scaling. For example, a 220GBd thin-film lithium niobate modulator has been demonstrated in the lab, while photodetectors will also achieve similar rates.
However, the biggest challenge facing coherent modem engineers is the analogue drive circuitry and the coherent DSP’s ADCs and DACs.
A key performance metric is its sampling rate measured in giga-samples-per-second (Gsps).
According to Nyquist, a signal needs to be sampled at twice its baud rate to be perfectly reconstructed. But that doesn’t mean sampling is always done at twice the baud rate. Instead, depending on the DSP implementation, the sampling rate is typically 1.2-1.6x the symbol rate.
“So, for a 200 gigabaud coherent modem, the DSP’s converters must operate at 240+ giga-samples per second,” says Tomislav Drenski, marketing manager, wireline, at Socionext Europe.
Socionext
Socionext is a system-on-chip specialist founded in 2015 with the combination of the system LSI divisions of Fujitsu and Panasonic, with its headquarters in Japan. Its European arm focuses on mixed-signal design, especially ADC, DACs and serialisers/ deserialisers (Serdes).
The company has developed 8-bit converters for several generations of long-haul optical designs, at 200Gbps, 400Gbps and greater than 1Tbps (see bottom photo). These optical systems used ADCs and DACs operating at 65, 92 and 128Gsps, respectively.
Socionext works with leading coherent optical module and network system providers but is also providing 5G and wireless ASIC solutions.
“We design the ADCs and DACs, which are ultra-high-speed, state-of-the-art circuit blocks, while our partners have their ideas on how the DSP should look,” says Drenski. “They provide us the DSP block, and we integrate everything into one chip.”
It is not just the quality of the circuit block that matters but how the design is packaged, says Drenski: “If the crosstalk or the losses in the package are too high, then whatever you have got with the IP is lost in the packaging.”
Any package-induced loss or added capacitance decreases bandwidth. And bandwidth, like sampling rate, is key to achieving high baud-rate coherent systems.
Design considerations
An important ADC metric is its resolution: the number of bits it uses to sample a signal. For high-performance coherent designs, 8-bit ADCs are used. However, because of the high sampling rate required and the associated jitter performance, the effective number of bits (ENOB) – an ADC metric – reduces to some 6 bits.
“People are asking for 10-bit converters for newer generations of design; these are shorter reach, not ultra-long-haul,” says Drenski.
Extra bits add fidelity and enable the recovery of higher-order modulated signals. Still, for ultra-long-haul, where the optical loss is more significant, using a 10-bit ADC makes little sense.
For 5G and wireless applications, higher resolutions, even going up to 14bit, is the recent trend. But such solutions use a lower sampling rate – 30Gsps – to enable the latest, direct-RF applications.
ADC architecture
An interleaved architecture enables an 8-bit ADC to sample a signal 128 billion times a second.
At the input to the ADC sits a sample-and-hold circuit. This circuit feeds a hierarchy of interleaved ‘sub-ADCs’. The interleaving goes from 1 to 4, then 4 to 16, 16 to 64, with the sub-ADCs all multiplexed.
“You take the signal and sample-and-hold it, then push everything down to many sub-ADCs to have the necessary speed at the end, at the output,” says Drenski.
These sub-ADCs must be aligned, and that requires calibration.
An ADC has three key metrics: sampling rate, bandwidth and ENOB. All three are interdependent.
For example, if you have a higher bandwidth, you will have a higher frequency, and clock jitter becomes a limiting factor for ENOB. Therefore, the number of sub-ADCs used must be well balanced and optimised to realise the high sampling frequencies needed without affecting ENOB. The challenge for the designer is keeping the gain, bias and timing variations to a minimum.
Drenski says designing the ADC is more challenging than the DAC, but both share common challenges such as clock jitter and also matching the path lengths of the sub-DACs.
240 gigabaud coherent systems
Can the bandwidth of the ADC reach 240+GBd?
“It all comes down to how much power you can spend,” says Drenski. “The more power you can spend to linearise, equalise, or optimise, the better.”
Noise is another factor. The amount of noise allowed determines how far the bandwidth can be increased. And with higher bandwidth, there is a need for higher clock speeds. “If we have higher clock speeds, we have higher complexity, so everything gets more complicated,” says Drenski.
The challenges don’t stop there.
Higher sampling rates mean the number of sub-ADCs must be increased, affecting circuit size and power consumption. And limiting the power consumption of the coherent DSP is a constant challenge.
At some point, the physical limitations of the process – the parasitics – limit bandwidth, independent of how the ADC circuitry is designed.
Coherent optics specialists like Acacia, Nokia, ADVA and Lumentum say that 220-240 gigabaud coherent systems are possible and will be achieved before the decade’s end.
Drenski agrees but stresses just how challenging this will be.
For him, such high baud rate coherent systems will only be possible if the electronics and optics are tightly co-integrated. Upping the bandwidth of each essential element of the coherent system, like the coherent DSP’s ADCs and DACs, is necessary but will not work alone.
What is needed is bringing both worlds together, the electronics and the optics.
Infinera’s ICE6 crosses the 100-gigabaud threshold
Coherent discourse 3
- The ICE6 Turbo can send two 800-gigabit wavelengths over network spans of 1,100-1,200km using a 100.4 gigabaud (GBd) symbol rate.
- The enhanced reach can reduce the optical transport equipment needed in a network by 25 to 30 per cent.
Infinera has enhanced the optical performance of its ICE6 coherent engine, increasing by up to 30 per cent the reach of its highest-capacity wavelength transmissions.
The ICE6 Turbo coherent optical engine can send 800-gigabit optical wavelengths over 1,100-1,200km compared to the ICE6’s reach of 700-800km.
ICE6 Turbo uses the same coherent digital signal processor (DSP) and optics as the ICE6 but operates at a higher symbol rate of 100.4GBd.
“This is the first time 800 gigabits can hit long-haul distances,” says Ron Johnson, general manager of Infinera’s optical systems & network solutions group.
Baud rates
Infinera’s ICE6 operates at 84-96GBd to transmit two wavelengths ranging from 200-800 gigabits. This gives a total capacity of 1.6 terabits, able to send 4×400 Gigabit Ethernet (GbE) or 16x100GbE channels, for example.
Infinera’s ICE6’s coherent DSP uses sub-carriers and their number and baud rates are tuned to the higher symbol rate.
The bit rate sent is defined using long-codeword probabilistic constellation shaping (LC-PAS) while Infinera also uses soft-decision FEC gain sharing between the DSP’s two channels.
The ICE6 Turbo adds several more operating modes to the DSP that exploit this higher baud rate, says Rob Shore, senior vice president of marketing at Infinera.
Reach
Infinera says that the ICE6 Turbo can also send two 600-gigabit wavelengths over 4,000km.
“This is almost every network in the world except sub-sea,” says Shore, adding that the enhanced reach will reduce the optical transport equipment needed in a network by 25 to 30 per cent.
“One thousand kilometres sending 2×800 gigabits or 4x400GbE is a powerful thing,” adds Johnson. “We’ll see a lot of traction with the content providers with this.”
Increasing symbol rate
Optical transport system designers continue to push the symbol rate. Acacia, part of Cisco, has announced its next 128GBd coherent engine while Infinera’s ICE6 Turbo now exceeds 100GBd.
Increasing the baud rate boosts the capacity of a single coherent transceiver while lowering the cost and power used to transport data. A higher baud rate can also send the same data further, as with the ICE6 Turbo.
“The original ICE6 device was targeted for 84GBd but it had that much overhead in the design to allow for these higher baud rate modes,” says Johnson. “We strived for 84GBd and technically we can go well beyond 100.4GBd.”
This is common, he adds.
The electronics of the coherent design – the silicon germanium modulator drivers, trans-impedance amplifiers, and analogue-to-digital and digital-to-analogue converters – are designed to perform at a certain level and are typically pushed harder and harder over time.
Baud rate versus parallel-channel designs
Shore believes that the industry is fast approaching the point where upping the symbol rate will no longer make sense. Instead, coherent engines will embrace parallel-channel designs.
Already upping the baud rate no longer improves spectral efficiency. “The industry has lost a vector in which we typically expect improvements generation by generation,” says Shore. “We now only have the vector of lowering cost-per-bit.”
At some point, coherent designs will use multiple DSP cores and wavelengths. What matters will be the capacity of the optical engine rather than the capacity of an individual wavelength, says Shore.
“We have had a lot of discussion about parallelism versus baud rate,” adds Johnson.
Already there is fragmentation with embedded and pluggable coherent optics designs. Embedded designs are optimised for high-performance spectral efficiency while for pluggables cost-per-bit is key.
This highlights that there is more than one optimisation approach, says Johnson: “We have got to develop multiple technologies to hit all those different optimisations.”
Infinera will use 5nm and 3nm CMOS for its future coherent DSPs, optimised for different parts of the network.
Infinera will keep pushing the baud rate but Johnson admits that at some point the cost-per-bit will start to rise.
“At present, it is not clear that doubling the baud rate again is the right answer,” says Johnson. “Maybe it is a combination of a little bit more [symbol rate] and parallelism, or it is moving to 200GBd.”
The key is to explore the options and deliver coherent technology consistently.
“If we put too much risk in one area and drive too hard, it has the potential to push our time-to-market out,” says Johnson.
The ICE6 Turbo will be showcased at the OFC show being held in San Diego in March.
Building the data rate out of smaller baud rates
In the second article addressing the challenges of increasing the symbol rate of coherent optical transport systems, Professor Andrew Lord, BT’s head of optical network research, argues that the time is fast approaching to consider alternative approaches.
Coherent discourse 2
Coherent optical transport systems have advanced considerably in the last decade to cope with the relentless growth of internet traffic.
One-hundred-gigabit wavelengths, long the networking standard, have been replaced by 400-gigabit ones while state-of-the-art networks now use 800 gigabits.
Increasing the data carried by a single wavelength requires advancing the coherent digital signal processor (DSP), electronics and optics.
It also requires faster symbol rates.
Moving from 32 to 64 to 96 gigabaud (GBd) has increased the capacity of coherent transceivers from 100 to 800 gigabits.
Last year, Acacia, now part of Cisco, announced the first 1-terabit-plus wavelength coherent modem that uses a 128GBd symbol rate.
Other vendors will also be detailing their terabit coherent designs, perhaps as soon as the OFC show, to be held in San Diego in March.
The industry consensus is that 240GBd systems will be possible towards the end of this decade although all admit that achieving this target is a huge challenge.
Baud rate
Upping the baud rate delivers several benefits.
A higher baud rate increases the capacity of a single coherent transceiver while lowering the cost and power used to transport data. Simply put, operators get more bits for the buck by upgrading their coherent modems.
But some voices in the industry question the relentless pursuit of higher baud rates. One is Professor Andrew Lord, head of optical network research at BT.
“Higher baud rate isn’t necessarily a panacea,” says Lord. “There is probably a stopping point where there are other ways to crack this problem.”
Parallelism
Lord, who took part in a workshop at ECOC 2021 addressing whether 200+ GBd transmission systems are feasible, says he used his talk to get people to think about this continual thirst for higher and higher baud rates.
“I was asking the community, ‘Are you pushing this high baud rate because it is a competition to see who builds the biggest rate?’ because there are other ways of doing this,” says Lord.
One such approach is to adopt a parallel design, integrating two channels into a transceiver instead of pushing a single channel’s symbol rate.
“What is wrong with putting two lasers next to each other in my pluggable?” says Lord. “Why do I have to have one? Is that much cheaper?”
For an operator, what matters is the capacity rather than how that capacity is achieved.
Lord also argues that having a pluggable with two lasers gives an operator flexibility.
A single-laser transceiver can only go in one direction but with two, networking is possible. “The baud rate stops that, it’s just one laser so I can’t do any of that anymore,” says Lord.
The point is being reached, he says, where having two lasers, each at 100GBd, probably runs better than a single laser at 200GBd.
Excess capacity
Lord cites other issues arising from the use of ever-faster symbol rates.
What about links that don’t require the kind of capacity offered by very high baud rate transceivers?
If the link spans a short distance, it may be possibe to use a higher modulation scheme such as 32-ary quadrature amplitude modulation (32-QAM) or even 64-QAM. With a 200GBd symbol rate transceiver, that equates to a 3.2-terabit transceiver. “Yet what if I only need 100 gigabits,” says Lord.
One option is to turn down the data rate using, say, probabilistic constellation shaping. But then the high-symbol rate would still require a 200GHz channel. Baud rate equals spectrum, says Lord, and that would be wasting the fibre’s valuable spectrum.
Another solution would be to insert a different transceiver but that causes sparing issues for the operators.
Alternatively, the baud rate could be turned down. “But would operators do that?” says Lord. “If I buy a device capable of 200GBd, wouldn’t I always operate it at its maximum or would I turn it down because I want to save spectrum in some places?”
Turning the baud rate down also requires the freed spectrum to be used and that is an optical network management challenge.
“If I need to have to think about defragmenting the network, I don’t think operators will be very keen to do that,” says Lord.
Pushing electronics
Lord raises another challenge: the coherent DSP’s analogue-to-digital and digital-to-analogue converters.
Operating at a 200+ GBd symbol rate means the analogue-to-digital converters at the coherent receiver must operate at least at 200 giga-samples per second.
“You have to start sampling incredibly fast and that sampling doesn’t work very well,” says Lord. “It’s just hard to make the electronics work together and there will be penalties.”
Lord cites research work at UCL that suggests that the limitations of the electronics – and the converters in particular – are not negligible. Just connecting two transponders over a short piece of fibre shows a penalty.
“There shouldn’t be any penalty but there will be, and the higher the baud rate, you will get a penalty back-to-back because the electronics are not perfect,” he says.
He suspects the penalty is of the order of 1 or 2dB. That is a penalty lost to the system margin of the link before the optical transmission even starts.
Such loss is clearly unacceptable especially when considering how hard engineers are working to enhance algorithms for even a few tenths of a dB gain.
Lord expects that such compromised back-to-back performance will ultimately lead to the use of multiple adjacent carriers.
“Advertising the highest baudrate is obviously good for publicity and shows industry leadership,” he concludes. “But it does feel that we are approaching a limit for this, and then the way forward will be to build aggregate data rates out of smaller baud rates.”
OIF moves to raise coherent transmission baud rate
"We want the two projects to look at those trade-offs and look at how we could build the particular components that could support higher individual channel rates,” says Karl Gass of Qorvo and the OIF physical and link layer working group vice chair, optical.
Karl Gass
The OIF members, which include operators, internet content providers, equipment makers, and optical component and chip players, want components that work over a wide bandwidth, says Gass. This will allow the modulator and receiver to be optimised for the new higher baud rate.
“Perhaps I tune it [the modulator] for 40 Gbaud and it works very linearly there, but because of the trade-off I make, it doesn’t work very well anywhere else,” says Gass. “But I’m willing to make the trade-off to get to that speed.” Gass uses 40 Gbaud as an example only, stressing that much work is required before the OIF members choose the next baud rate.
"We want the two projects to look at those trade-offs and look at how we could build the particular components that could support higher individual channel rates”
The modulator and receiver optimisations will also be chosen independent of technology since lithium niobate, indium phosphide and silicon photonics are all used for coherent modulation.
The OIF has not detailed timescales but Gass says projects usually take 18 months to two years.
Meanwhile, the OIF has completed two projects, the specification outputs of which are referred to as implementation agreements (IAs).
One is for integrated dual polarisation micro-intradyne coherent receivers (micro-ICR) for the CFP2. At OFC 2015, several companies detailed first designs for coherent line side optics using the CFP2 module.
The second completed IA is the 4x5-inch second-generation 100 Gig long-haul DWDM transmission module.
