Infinera trials terabit Advanced Coherent Toolkit with Telstra

Optical transport systems provider Infinera (NASDAQ:INFN) says it has partnered with Australia's Telstra to validate the capabilities of the systems house's upcoming Advanced Coherent Toolkit (ACT). The ACT, which Infinera plans to offer commercially later this year, is designed to support significantly greater reach for such next-generation coherent transmission formats as 16QAM while paving the way for terabit superchannels through the use of Nyquist-based subcarriers and other advanced technologies.

Infinera's transmission trial with Telstra involved using the ACT and other new-to-Infinera capabilities to transmit high-capacity coherent-based wavelengths across the 9,000-km Endeavour submarine cable system that runs from Sydney, Australia, to Oahu, HI, according to Pravin Mahajan, director of product and corporate marketing at Infinera. The test represents the company's first publicized support of polarization-multiplexed (PM)-8QAM and PM-16QAM modulation formats, as well as a new DSP architecture that enables coherent processing on both the transmit (including digital-to-analog conversion capabilities) and receive ends of the optical transmission.

The trial also leveraged several elements of the upcoming ACT:

• Nyquist-based subcarriers within the carrier channels that compose a superchannel
• Flexible channel spacing for those subcarriers, to enable a carrier channel to contain various numbers of Nyquist subcarriers
• A new soft-decision forward error correction (SD-FEC) scheme
• SD-FEC gain sharing, which enables a "strong" channel to share its FEC boost with a "weak" channel so that both channels can be sent via the same modulation format
• Matrix-enhanced phase-shift-keying (ME-PSK), a higher-performing variant of BPSK for submarine network applications.
  

All of the ACT elements are important, but the Nyquist subcarriers are the most eye catching. Mahajan says Infinera has developed a way to share the 37.5-GHz devoted to each superchannel carrier into a flexible number of Nyquist subcarriers; the flexible channel spacing capability will enable the system to split 35 GHz of spectrum into the number of subcarriers the transmission requires, limited by the capabilities and characteristics of the fiber link to be used. An Infinera slide deck shows an example using six subcarriers; Mahajan declined to discuss the maximum number of subcarriers the technology will support, but said fewer than 10 would be employed in a typical application.

Use of the subcarriers lowers the baud rate of the individual transmissions, raises the Q factor, and improves the transmission's resistance to nonlinear impairments, Mahajan explained. It also should pave the way for terabit superchannels.

Mahajan declined to describe the data rates involved in the Telstra trial, saying that Telstra wanted to keep the potential capacity of the Endeavour submarine network under wraps for the time being. However, he said the ACT will enable a 40% to 60% capacity-reach improvement over conventional coherent transmission approaches. The current generation of the company's photonic integrated circuit (PIC) is designed with a maximum transmission capability of 1.2 Tbps.

Mahajan says that Infinera will begin to roll out elements of the ACT in the first half of this year, with application to both terrestrial and submarine networks.

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