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.
Professor Andrew Lord
One-hundred-gigabit wavelengths, long the networking standard, have been replaced by 400-gigabit ones while state-of-the-art networks now use 800 gigabits.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.”