contact button

Truly Open Subsea Cables with Spectrum Sharing in the Asia-Pacific Region

David Kuan

May 7, 2024
By David Kuan
Sr. Director, APAC Subsea Sales

As I travel through the APAC region to meet with Infinera’s subsea customers and prospects, one consistent area of interest is spectrum sharing as a service on open submarine cables. This type of service is attractive to operators that don’t need the capacity of an entire fiber pair, but that prefer to keep more control over their own destiny and have superior cost of ownership than is possible when leasing multiple fixed-data-rate wavelength services like 100 GbE or 400 GbE.

I see three recurring concerns that these potential spectrum tenants have in terms of spectrum security, optical power management, and fairness of spectrum allocation.

The good news I can bring to them is that Infinera’s Intelligent Power Management (IPM) solution can help them with these issues and has been deployed on multiple cables in the APAC region, such as Asia Direct Cable (ADC), an important new system that interconnects China, Japan, the Philippines, Singapore, Thailand, and Vietnam.

Spectrum Security

Tenants on the same fiber pair are allocated spectrum by IPM, which then programs the dual wavelength-selective switch (WSS) within Infinera’s FlexILS terminal based on a route-and-select approach. This is different from spectrum sharing solutions that use a single-WSS, broadcast-and-select architecture. A route-and-select approach ensures spectrum privacy, as I show in Figures 1a and 1b.

A single-WSS, broadcast-and-select architecture does not provide secure spectrum
Figure 1a: A single-WSS, broadcast-and-select architecture does not provide secure spectrum

 

A dual WSS with a route-and-select architecture ensure spectrum security and privacy
Figure 1b: A dual WSS with a route-and-select architecture ensure spectrum security and privacy

Optical Power Management

Submarine cable repeaters operate in a constant power mode, which means that it’s the responsibility of the fiber pair operator to ensure that the correct total optical power, as well as the wavelength distribution of this power, is injected into the fiber pair at all times. Service transponders provide some of this load, but where cables are not fully loaded, amplified spontaneous emission (ASE) and idler devices are used to generate broadband optical power and precise optical power respectively.

IPM’s Smart Shield includes the ability to automatically insert ASE in the case where a spectrum tenant is operating outside of its agreed optical power levels, regardless of the root cause. This ensures that operational stability is maintained for all remaining fiber pair users.

Fairness of Capacity

If we assume that a modern subsea repeater has a bandwidth of about 4.6 THz, how should we divide this spectrum fairly between potential tenants? Equally sized frequency bands do not equate to equal capacity because in an uncompensated fiber pair, the low-frequency (red) end of the spectrum has significantly better optical signal-to-noise ratio (OSNR) than the high-frequency (blue) end.

A shared spectrum provider may decide to offer equally sized frequency bands, and savvy tenants will insist on lower frequencies in a first-come, first-served manner.

But an alternative is to offer more equal-capacity bands using Equalized G-OSNR, which takes into account nonlinear variations and is transmission-technology independent. This latter point is critical because spectrum sharing tenants often choose this type of service partly because they want the freedom to choose their own transponder vendor.

Infinera IPM’s Smart Optimize function is designed to optimize the Q value (quality factor) on a per-channel basis across the whole fiber spectrum, and it can be used to create equal-capacity blocks for spectrum sharing.

Sharing spectrum by equal-frequency blocks vs. equal-capacity blocks
Figure 2: Sharing spectrum by equal-frequency blocks vs. equal-capacity blocks

Terrestrial Benefits

Spectrum sharing services can be between cable landing stations or between data centers (DC-DC). Both approaches are used today but DC-DC services must include power management capabilities that extend into the terrestrial backhaul network. ADC, for example, has backhaul extensions in Singapore, Hong Kong, and Japan. Double and even triple protection options available in Infinera’s submarine solutions help to ensure that these optically expressed routes stay operational.

In some cases, terrestrial backhaul fibers may be close to full capacity, and operators who are leasing fiber may need the option to extend the fiber spectrum to Super C or even C+L transmission. My colleague Geoff Bennett wrote a blog last year on how hitless C+L upgrades to terrestrial backhaul cables can be achieved.

Summary

Spectrum sharing services are becoming increasingly popular as a new service type on open submarine cables. They offer advantages that include best-of-breed transponder flexibility and lower total cost of ownership. But they have to be implemented with bulletproof spectrum security, reliable optical power management, and the ability to offer equal-capacity spectrum blocks. Infinera delivers all these capabilities and more.