Next-generation super coherent technologies propel 200G for the long haul
Next-generation super coherent technologies are creating a tipping point for optical transport networks—now almost all long haul networks can be built economically with 200G, offering a doubling of capacity at lower cost compared to today’s 100G long haul.
Better, more useful 200G
Next-generation coherent technologies—which include electro-optic innovations plus new features and products—introduce a new type of long haul wavelength, 200G 8QAM.
First generation coherent digital signal processors (DSPs) introduced 100G, which quickly became the dominant long-haul technology due to its high performance and lower cost per bit per kilometer. Second generation coherent DSPs allowed a single line card to operate in the well-established mode of QPSK for 100G wavelengths, as well as 16QAM for 200G. The use of 16QAM increased the number of bits encoded within one analog symbol from two to four, doubling capacity and spectral efficiency while operating within the capabilities of then-state-of-the-art DSP silicon, around 35 gigabaud. Yet the increase in spectral efficiency came at the cost of distance, limiting the reach of 16QAM to approximately 1000 km, and its applicability to metro/regional networks. With 100G QPSK supporting links of greater than 3000 km, there exists an ‘optimization gap’ between 1000 and 3000 km, where 16QAM delivers insufficient performance, and QPSK fails to fully maximize the bandwidth achievable on such links.
As discussed in the Nokia insight article “The super coherent drivetrain - balancing capacity and reach to capture the opportunity in every fiber”, next generation super coherent DSPs can select from multiple modulation formats and operate at 35 gigabaud or a new 45 gigabaud rate. One of the new and unique modes offered by modern super coherent DSPs is an 8QAM modulation format running at 45 gigabaud that operates at 200G data rates. Encoding three bits per symbol, 8QAM improves on the spectral efficiency of 100G QPSK while avoiding the constrained performance of 200G transmission based on 16QAM. It brings true long-haul performance to 200G wavelengths over distances of up to 2000 km.
The impact—200G everywhere
How important will 200G long haul be to CSPs? The short answer is very. A recent Nokia study compared the percentage of wavelength paths of actual European and North American networks in which unregenerated 200G wavelengths with standard erbium doped fiber amplification are viable, first using a 16QAM modulation format, and then using 200G 8QAM modulation format.
200G 16QAM modulation could support only 9 percent of wavelength paths in North America and 31 percent in Europe—whereas 200G 8QAM could support 63 percent of European and more than 16 per cent of North American optical routes – a significant improvement.
200G 8QAM – improving 200G applicability
The study then investigated the impact of regeneration. In long haul DWDM networks, optical-to-electrical-to-optical regeneration is generally avoided due to the increased costs imposed by the regeneration equipment. So why should CSP’s consider regeneration with 200G 8QAM wavelengths? Let’s look at an example.
A 200G demand met with 100G wavelengths requires four interfaces. A 200G demand served by one 200G wavelength with a single regenerator also requires four interfaces (and since these interfaces are programmable, both cases are implemented with the same hardware). Yet the 100G wavelength scenario requires two wavelengths and consumes 100GHz of spectrum, whereas the 200G implementation requires only one wavelength and consumes just 62.5 GHz of spectrum. This preserves spectrum for future growth and results in a 60% improvement in total fiber capacity.
With regeneration considered as an option for wavelength paths exceeding the unregenerated reach of 200G, the study now shows that 200G 16QAM is preferable for 40% of the North American wavelengths and 84% of the European wavelengths. When 200G 8QAM is considered, the viability of 200G more than doubles to 92% of North American wavelengths and now covers 100% of European wavelengths, showing that 200G can economically be deployed almost anywhere in the network, including long haul.
200G 8QAM - the most economical choice for scaling long haul transport.
Changing the way networks are built
With the reach enabled by 200G 8QAM, operators can build long-haul networks economically, using almost exclusively 200G wavelengths. But there’s more to 200G 8QAM than doubling long haul capacity. An optical network built with 200G 8QAM wavelengths uses fewer line cards, consumes less space and power, uses less optical spectrum and delivers lower cost/bit/km than networks built with 100G QPSK and 200G 16QAM. And compared to alternative 8QAM solutions running at only 150G, it offers better alignment with the demand for 100G transport that customers are starting to demand for core router and data center interconnect.
By supporting more modulation formats and higher baud rates, next generation super coherent technologies offer CSP’s new capabilities to optimize every wavelength in their network. Perhaps the most exciting of these new wavelengths is 200G 8QAM. With an ideal combination of modulation and baud rate, it makes 200G long haul a reality.