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One More Reason to Deploy 600G Generation Coherent Now: Fiber Capacity

headshot of Paul Momtahan

July 21, 2020
By Paul Momtahan
Director, Solutions Marketing

Back in May I wrote a blog that listed the top five reasons to deploy 600G generation coherent technology: it’s available now, and it offers low cost per bit, cost-effective transport for 400 Gigabit Ethernet, low power consumption, and minimized footprint. However, there is one additional very good reason to deploy this technology now: fiber capacity, or more specifically how much bandwidth can you get out of the 4,800 GHz of the extended C-band, which is also the spectral efficiency.

Maximizing fiber capacity and spectral efficiency becomes an issue as the C-band spectrum of a fiber is maxed out. How big an issue it is depends on whether the fiber is owned or leased, whether additional fibers are available, the cost of these additional fibers, and the cost to light additional fibers with optical line systems equipment such as mux/demux filters, ROADMs, and amplifiers. Another factor is the length of the fiber path, with a longer path incurring more cost and longer delays.  This is illustrated by the following four scenarios:

Scenario 1: Operator owns cable/duct with spare fibers

If the operator owns the cable/ducts with spare unlit fibers, then there is an opportunity cost to the fiber –  it could have been leased to a third party as a dark fiber – in addition to the cost to light the fiber. There will also be a cost in terms of time, as fiber characterization may be needed and equipment ordered, installed, and commissioned. In this scenario, the value of increased fiber capacity/spectral efficiency will be lower relative to the other scenarios, at least for shorter distances. However, with longer fiber routes, the higher cost of lighting the fiber makes increased fiber capacity/spectral efficiency more valuable.

Scenario 2: Operator leases fiber and the cost of additional fibers is low

Depending on the regulatory regime, location, and number of competitive dark fiber providers, the availability of additional fiber may be high and the cost low. This is often the case between data centers in major metropolitan areas in Europe and North America. For example, dark fiber is typically readily available and very cost-effective between the 20 data centers of London’s Docklands. In this scenario, fiber capacity/spectral efficiency is less critical relative to scenarios 3 and 4.

Scenario 3: Operator leases fiber and the cost of additional fibers is high

As the cost of additional fibers increases with a less competitive environment or longer distances, the ability of increased fiber capacity/spectral efficiency to avoid the cost and delay of new fibers becomes very valuable.

Scenario 4: Additional fiber is not available

A worst-case scenario is that additional fiber is just not available for lease or that an operator that owns cable/duct has used up all the fibers. In this case, one option may be to put in additional fibers, with costs and delays related to planning, materials, rights of way, and installation. A 2013 report put the costs per meter of duct in the U.K. at £100 on roads, £60 on footpaths, and £40 on grass verges, with an additional £2.75 per meter for the 48-fiber cable itself. OTELCO puts the cost of aerial fiber in the U.S. at between $18,000 and $22,000 per mile.

Given the high costs and long delays of installing additional fibers, maximizing the capacity of existing fibers is very valuable. This is especially true if there is no option to put in additional fibers due to factors such as the regulatory regime, an inability to secure rights of way, or financial constraints.

600G Generation Coherent: Higher-order Modulation

Spectral efficiency is influenced by how tightly you can pack the wavelengths together, which is a function of the wavelength spectral shape/roll-off and the gap between wavelengths that is required to account for things like laser drift. However, the primary determinant of spectral efficiency is the number of bits per symbol, which is a function of the modulation.

Figure 1: Modulation and bits per symbol

The 200G generation of coherent typically supported only PM-QPSK with 4 bits per symbol, PM-8QAM with 6 bits per symbol, and PM-16QAM delivering 8 bits per symbol. 600G generation technology adds PM-32QAM with 10 bits per symbol and PM-64QAM with 12 bits per symbol, as shown in Figure 1. PM-32QAM can therefore deliver 25% better fiber capacity/spectral efficiency than PM-16QAM, while PM-64QAM can deliver 50% better fiber capacity/spectral efficiency. The CHM2T sled for the Groove (GX) Series G30 and GX G25 can deliver 38.4 Tb/s (i.e., 64 x 600G wavelengths) to distances of 150+ km with PM-64QAM, and 32 Tb/s (i.e., 64 x 500G wavelengths) to 350+ km with PM-32QAM.

Figure 2: Time domain hybrid modulation

Another tool that 600G generation coherent can provide to maximize spectral efficiency is time domain hybrid modulation. This provides the ability to mix different QAM symbols in the time domain. For example, hybrid 16QAM/32QAM that alternates 16QAM symbols and 32QAM symbols, as shown in Figure 2, delivers a modulation with capacity and spectral efficiency that is the average of the two individual modulations. As the input signal quality to the FEC block is the average of the lower- and higher-order modulation, the reach will get near to the average of the two individual modulations.

Figure 3: Increased fiber capacity/spectral efficiency

As shown in Figure 3, these two tools enable the 600G generation technology in the GX G30 CHM2T sled and the GX G25 to maximize fiber capacity/spectral efficiency over a wide range of distances. Network operators that have already deployed the GX G30 can leverage its modular architecture to upgrade to 600G generation coherent with the CHM2T sled.

This increased fiber capacity/spectral efficiency has particular value for scenarios where the cost of leasing additional fibers is high, where additional fiber is not readily available, or where distances are long. It can also free up spectrum on the fiber that can be sold as a service to other operators or enterprises, generating additional revenue.

For more information on 600G generation coherent technology, download the new Infinera Groove (GX) Series 600G Generation Coherent Application Note.