The 8QAM Sweet Spot: The Right Way to Do Advanced Modulation
Infinera Technology Evangelist
Coherent transmission opens up a fascinating toolbox of modulation options, as we’ve seen with Infinera’s pioneering FlexCoherent™ modulation technology.
For those of you who are not aware of the issue, here’s a quick summary.
Phase modulation (part of modern coherent transmission) allows us to encode varying numbers of bits per symbol in order to increase spectral efficiency (and thereby optical fiber capacity).
The current “workhorse” modulation technology is Pol-muxed (PM) Quadrature Phase Shift Keying (QPSK), which carries four bits per modulation symbol.
In various trials, Infinera has demonstrated alternative modulations that have longer reach but lower capacity than QPSK (i.e. BPSK, Enhanced BPSK and 3QAM), as well as modulations that have greater spectral efficiency, but shorter reach than QPSK (i.e. 8QAM and 16QAM).
The key to FlexCoherent modulation is to present all modulations in a software selectable format on a single line card so that the service provider can choose the optimum balance between optical reach and fiber capacity, and historically QPSK has been the most commonly used modulation precisely because it offers a great reach/capacity balance.
Looking at one of these newer modulations in particular, in two recent trials we’ve seen how PM-8QAM not only offers a 50% increase in fiber capacity versus QPSK, but also represents a “sweet spot” in terms of optical reach on both existing optical fiber types, and new types of large area/low loss fiber, such as OFS Terawave™.
In the first trial with Telstra, we achieved a reach of 2,200 kilometers over an existing submarine fiber.
In trial announced today, we achieved an astonishing increase in reach over OFS Terawave fiber – 7,400 kilometers, in fact – which would be enough to close Atlantic submarine routes! Just to be clear the specific Terawave fiber we used in this test is a type that’s optimized for terrestrial transmission, and we may have been able to do even better if we’d used the submarine-optimized Terawave SLA+ or ULA Ocean Fibers.
These numbers are important because optical reach has a direct impact on the total cost of ownership for a DWDM system. Basically, the longer the reach, the lower the cost. But since fiber capacity also affects cost of ownership, we can imagine that there are reach values that are “just enough” to close a given set of routes without resorting to regeneration, and yet by using one of the higher order modulation formats we could achieve much greater fiber capacity.
In other words, we can imagine a set of sweet spots emerging because a given proportion of city pairs are a given distance apart.
These sweet spots are reasonably well understood. We know that analysts generally break down markets on the basis of reach – so that the ultra-long haul market would be distances of greater than 3,000 kilometers; long haul is distances greater than 1,800 kilometers; and at distances of less than 1,800 kilometers we have regional and metro networks.
So it’s clear that, with reach numbers of between 400 and 1,200 kilometers on conventional fiber depending on the specific type of fiber and amplification used, 16QAM is not a viable long haul modulation format. But 8QAM, with reach of over 2,000 kilometers on conventional fiber, certainly could be.
When we include optical protection techniques, the picture becomes even more clear. Figure 1 shows two common topologies: a ring and a mesh. Generally speaking, metro networks still tend to be laid out in rings. Regional and long haul networks are more likely to be logical meshes, but historically the fiber may have been deployed in a physical ring.
In the ring example, we seen Nodes A and B are connected by a relatively short working path, shown by the solid green line. Let’s say that in this example, X = 500 kilometers – well within the reach of 16QAM modulation. But if there is a fiber cut between A and B, the path length increases dramatically to three times that, or 1,500 kilometers. This is currently beyond the vendor claims we’ve seen for 16QAM modulation, even using Raman amplification.
In the mesh example, the differences are generally less extreme, but the absolute distances are more likely to be longer because we’re now looking at a regional or long haul deployment.
So you could imagine that Y = 1,000 kilometers; which would be possible to close with a very high performance 16QAM solution, probably including Raman amplifiers. But if the A-B link breaks and there is purely optical protection, then the link length increases to 2,000 kilometers, and once again 16QAM would not be able to cope.
But in both cases 8QAM could close the link, and so we can immediately appreciate there could be a reach sweet spot for this modulation technology.
By the way, if you’re wondering why I’m emphasizing optical protection, it’s because the companies who are heavily promoting 16QAM technology are also companies who promote optical protection.
Let’s move on to implementation, because 8QAM is not an “easy” modulation technology for vendors who are using discrete optical components.
Table 1 shows the data rates for the “new breed” of line cards that some vendors are now announcing. Each line card can run in several different modes – which are usually a simplified form of Infinera’s FlexCoherent capability.
In BPSK mode the card runs at 50 Gb/s, in QPSK mode it runs at 100 Gb/s, in 8QAM mode it runs at 150 Gb/s, and in 16QAM mode (which is the mode that is normally heavily marketed) it runs at 200 Gb/s.
I’ve already mentioned that 16QAM probably has too short a reach to be generally useful, and it’s not clear how many of these “200 Gb/s” line cards are being used in that mode today.
Oddly none of the vendors who are promoting 200 Gb/s operation seem to be talking about 8QAM (apart from hero experiments), and this may be because it delivers an odd sort of data rate of 150 Gb/s for this type of discrete component card design.
We know already that some DWDM systems struggle to provide a non-blocking switching capability across the backplane between line cards at even 100 Gb/s, and that means it could be difficult to amalgamate these separate 8QAM signals into a useable data rate – typically a multiple of 100 Gb/s.
Last August I took part in a field trial on the GÉANT network with a team of colleagues, and we showcased a prototype terabit PIC technology. We showed BPSK, 3QAM, QPSK, 8QAM and 16QAM technology over this link, using the same line card with FlexCoherent modulation.
But an important aspect of Infinera’s implementation, shown in Figure 3, is that the production version of this line card will deliver the full 1.2 Tb/s capacity for all of the “terrestrial” modulation types – which means QPSK, 8QAM and 16QAM. That means that instead of having to implement anywhere between six and 12 line cards to achieve 1.2 Tb/s, an Infinera customer can simply plug in one card.
Moreover the DTN-X platform that uses this line card will have a full 12 Tb/s non-blocking OTN switching capability to turn all of the link bandwidth into a virtual pool of digital capacity. Any service – including 1GbE, 10GbE, 40GbE, 100GbE and future 400GbE can be easily and efficiently supported using this OTN switching capability – including 8QAM modulation. Thus it is not only important to have the line card with the right modulation options; the system must be designed with forward scale to accommodate those line cards when they become available. Below is a video showing how any DTN-X ever deployed can be upgraded from 5 Tb/s to 12 Tb/s in service to accommodate 1.2 Tb/s line cards when they become generally available.
So we firmly believe that 8QAM will offer a “reach sweet spot” for high order modulation, but we also believe it’s essential to choose the right implementation to get the best out of this technology.
Terawave is a trademark of OFS Fitel, LLC.