Infinera wants to show the advantages of having a selection of advanced coherent modulation formats when upgrading submarine cable systems. Working with Telstra, the optical systems vendor has demonstrated polarisation-multiplexed eight-symbol quadrature amplitude modulation (PM-8QAM) transmission over a distance of 2,200 km.
The field trial, which was conducted over Segment F of the Reach North Asia Loop (RNAL), investigated the performance of several coherent modulation formats with a new higher-gain SD-FEC (forward error correction) code. Segment F runs between Wada in Japan and Pusan in South Korea.
Using PM-8QAM, Infinera was able to deliver a 50 per cent increase in fibre capacity compared to the most common coherent modulation technique, PM-QPSK, in a typical installation under the same conditions.
Infinera had previously demonstrated PM-8QAM transmission in a terrestrial experiment with research and education network DANTE last year, but thinks the technology will be particularly useful for submarine cable operators.
“When regeneration within the submerged cable is simply not an option, the ability to trade margin for additional capacity can be crucial in submarine networks. Having the full suite of modulation formats on a single card can also dramatically lower inventory costs, while allowing the service provider to maximise cable capacity,” said Steve Grubb, Infinera Fellow and the architect behind the RNAL demonstrations.
In subsea more than anywhere else it is up to the transmission technology to adapt to the link, rather than designing the network to fit the achievable reach, writes Emily Burmeister, principal subsea development engineer, Infinera, in the company’s blog. Infinera’s FlexCoherent technology can do this adaptation on the line card itself by allowing the user to software-configure the modulation format and thus the reach and spectral efficiency.
Burmeister said the team had tested 16QAM, but it could not be supported across such distance on the legacy wet plant, but when the modulation format was switched to 8QAM, the performance increased to deployable levels. The more points in the constellation (see diagram, above), the closer together they are, and hence the easier it is for the receiver to mistakenly assign each bit and thus induce an error, she explains.
Telstra’s Andrew Hankins, head of engineering, Global Enterprise and Services, said: “We believe that the current and future capabilities of the DTN-X platform and our use of the newest technologies will continue to give Telstra a clear advantage in the Pacific region, both in terms of capacity and the ability to help deliver reliable services quickly to meet customer demand.”
Telstra previously collaborated with Infinera on one of the world’s first demonstrations of PM-3QAM modulation over Segment 5 of the AAG cable between California and Hawaii, a distance of 4,250 km. The use of PM-3QAM increased the useful capacity of the cable by 50 per cent compared to binary phase-shift keying (BPSK) modulation.
The PM-3QAM format is commercially available on Infinera systems using FlexCoherent technology, while PM-8QAM will be offered in a future release.