Coherent discourse 3

• The ICE6 Turbo can send two 800-gigabit wavelengths over network spans of 1,100-1,200km using a 100.4 gigabaud (GBd) symbol rate.
• The enhanced reach can reduce the optical transport equipment needed in a network by 25 to 30 per cent.

Infinera has enhanced the optical performance of its ICE6 coherent engine, increasing by up to 30 per cent the reach of its highest-capacity wavelength transmissions.

Ron JohnsonThe ICE6 Turbo coherent optical engine can send 800-gigabit optical wavelengths over 1,100-1,200km compared to the ICE6’s reach of 700-800km.

ICE6 Turbo uses the same coherent digital signal processor (DSP) and optics as the ICE6 but operates at a higher symbol rate of 100.4GBd.

“This is the first time 800 gigabits can hit long-haul distances,” says Ron Johnson, general manager of Infinera’s optical systems & network solutions group.

Baud rates

Infinera’s ICE6 operates at 84-96GBd to transmit two wavelengths ranging from 200-800 gigabits. This gives a total capacity of 1.6 terabits, able to send 4x400 Gigabit Ethernet (GbE) or 16x100GbE channels, for example.

Infinera’s ICE6's coherent DSP uses sub-carriers and their number and baud rates are tuned to the higher symbol rate.

The bit rate sent is defined using long-codeword probabilistic constellation shaping (LC-PAS) while Infinera also uses soft-decision FEC gain sharing between the DSP’s two channels.

The ICE6 Turbo adds several more operating modes to the DSP that exploit this higher baud rate, says Rob Shore, senior vice president of marketing at Infinera.

Reach

Infinera says that the ICE6 Turbo can also send two 600-gigabit wavelengths over 4,000km.

Rob Shore

“This is almost every network in the world except sub-sea,” says Shore, adding that the enhanced reach will reduce the optical transport equipment needed in a network by 25 to 30 per cent.

“One thousand kilometres sending 2x800 gigabits or 4x400GbE is a powerful thing,” adds Johnson. “We’ll see a lot of traction with the content providers with this.”

Increasing symbol rate

Optical transport system designers continue to push the symbol rate. Acacia, part of Cisco, has announced its next 128GBd coherent engine while Infinera’s ICE6 Turbo now exceeds 100GBd.

Increasing the baud rate boosts the capacity of a single coherent transceiver while lowering the cost and power used to transport data. A higher baud rate can also send the same data further, as with the ICE6 Turbo.

“The original ICE6 device was targeted for 84GBd but it had that much overhead in the design to allow for these higher baud rate modes,” says Johnson. “We strived for 84GBd and technically we can go well beyond 100.4GBd.”

This is common, he adds.

The electronics of the coherent design - the silicon germanium modulator drivers, trans-impedance amplifiers, and analogue-to-digital and digital-to-analogue converters - are designed to perform at a certain level and are typically pushed harder and harder over time.

Baud rate versus parallel-channel designs

Shore believes that the industry is fast approaching the point where upping the symbol rate will no longer make sense. Instead, coherent engines will embrace parallel-channel designs.

Already upping the baud rate no longer improves spectral efficiency. “The industry has lost a vector in which we typically expect improvements generation by generation,” says Shore. “We now only have the vector of lowering cost-per-bit.”

At some point, coherent designs will use multiple DSP cores and wavelengths. What matters will be the capacity of the optical engine rather than the capacity of an individual wavelength, says Shore.

“We have had a lot of discussion about parallelism versus baud rate,” adds Johnson.

Already there is fragmentation with embedded and pluggable coherent optics designs. Embedded designs are optimised for high-performance spectral efficiency while for pluggables cost-per-bit is key.

This highlights that there is more than one optimisation approach, says Johnson: “We have got to develop multiple technologies to hit all those different optimisations.”

Infinera will use 5nm and 3nm CMOS for its future coherent DSPs, optimised for different parts of the network.

Infinera will keep pushing the baud rate but Johnson admits that at some point the cost-per-bit will start to rise.

“At present, it is not clear that doubling the baud rate again is the right answer,” says Johnson. “Maybe it is a combination of a little bit more [symbol rate] and parallelism, or it is moving to 200GBd.”

The key is to explore the options and deliver coherent technology consistently.

“If we put too much risk in one area and drive too hard, it has the potential to push our time-to-market out,” says Johnson.

The ICE6 Turbo will be showcased at the OFC show being held in San Diego in March.