Next-generation coherent adds sub-carriers to capabilities

Briefing: DWDM developments

Part 2: Infinera's coherent toolkit 

Source: Infinera

Infinera has detailed coherent technology enhancements implemented using its latest-generation optical transmission technology. The system vendor is still to launch its newest  photonic integrated circuit (PIC) and FlexCoherent DSP-ASIC but has detailed features the CMOS and indium phosphide ICs support.

The techniques highlight the increasing sophistication of coherent technology and an ever tighter coupling between electronics and photonics.        

The company has demonstrated the technology, dubbed the Advanced Coherent Toolkit, on a Telstra 9,000km submarine link spanning the Pacific. In particular, the demonstration used matrix-enhanced polarisation-multiplexed, binary phased-shift keying (PM-BPSK) that enabled the 9,000km span without optical signal regeneration. 

Using the ACT is expected to extend the capacity-reach product for links by the order of 60 percent. Indeed the latest coherent technology with transmitter-based digital signal processing delivers 25x the capacity-reach of 10-gigabit wavelengths using direct-detection, the company says.  

Infinera’s latest PIC technology includes polarisation-multiplexed, 8-quadrature amplitude modulation (PM-8QAM) and PM-16QAM schemes. Its current 500-gigabit PIC supports PM-BPSK, PM-3QAM and PM-QPSK. The PIC is expected to support a 1.2-terabit super-channel and using PM-16QAM could deliver 2.4 terabit. 

“This [the latest PIC] is beyond 500 gigabit,” confirms Pravin Mahajan, Infinera’s director of product and corporate marketing. “We are talking terabits now.”

Sterling Perrin, senior analyst at Heavy Reading, sees the Infinera announcement as less PIC related and more an indication of the expertise Infinera has been accumulating in areas such as digital signal processing.  

 

Nyquist sub-carriers    

Infinera is the first to announce the use of sub-carriers. Instead of modulating the data onto a single carrier, Infinera is using multiple Nyquist sub-carriers spread across a channel. 

Using a flexible grid, the sub-carriers span a 37.5GHz-wide channel. In the example shown above, six are used although the number is variable depending on the link. The sub-carriers occupy 35GHz of the band while 2.5GHz is used as a guard band. 

“Information you were carrying across one carrier can now be carried over multiple sub-carriers,” says Mahajan. “The benefit is that you can drive this as a lower-baud rate.” 

Lowering the baud rate increases the tolerance to non-linear channel impairments experienced during optical transmission. “The electronic compensation is also much less than what you would be doing at a much higher baud rate,” says Abhijit Chitambar, Infinera’s principal product and technology marketing manager. 

While the industry is looking to increase overall baud rate to increase capacity carried and reduce cost, the introduction of sub-carriers benefits overall link performance. “You end up with a better Q value,” says Mahajan. The ‘Q’ refers to the Quality Factor, a measure of the transmission’s performance. The Q Factor combines the optical signal-to-noise ratio (OSNR) and the optical bandwidth of the photo-detector, providing a more practical performance measure, says Infinera.

Infinera has not detailed how it implements the sub-carriers. But it appears to be a combination of the transmitter PIC and the digital-to-analogue converter of the coherent DSP-ASIC.   

It is not clear what the hardware implications of adopting sub-carriers are and whether the overall DSP processing is reduced, lowering the ASIC’s power consumption. But using sub-carriers promotes parallel processing and that promises chip architectural benefits.

“Without this [sub-carrier] approach you are talking about upping baud rate,” says Mahajan. “We are not going to stop increasing the baud rate, it is more a question of how much you can squeeze with what is available today.“   

SD-FEC enhancements

The FlexCoherent DSP also supports enhanced soft-decision forward-error correction (SD-FEC) including the processing of two channels that need not be contiguous.   

SD-FEC delivers enhanced performance compared to conventional hard-decision FEC. Hard-decision FEC decides whether a received bit is a 1 or a 0; SD-FEC also uses a confidence measure as to the likelihood of the bit being a 1 or 0. This additional information results in a net coding gain of 2dB compared to hard-decision FEC, benefiting reach and extending the life of submarine links.    

By pairing two channels, Infinera shares the FEC codes. By pairing a strong channel with a weak one and sharing the codes, some of the strength of the strong signal can be traded to bolster the weaker one, extending its reach or even allowing for a more advanced modulation scheme to be used.

The SD-FEC can also trade performance with latency. SD-FEC uses as much as a 35 percent overhead and this adds to latency. Trading the two supports those routes where low latency is a priority.      

Matrix-enhanced PSK

Infinera has implemented a technique that enhances the performance of PM-BPSK used for the longest transmission distances such as sub-sea links. The matrix-enhancement uses a form of averaging that adds about a decibel of gain. “Any innovation that adds gain to a link, the margin that you give to operators is always welcome,” says Mahajan.

The toolkit also supports the fine-tuning of channel widths. This fine-tuning allows the channel spacing to be tailored for a given link as well as better accommodating the Nyquist sub-carriers.

Product launch

The company has not said when it will launch its terabit PIC and FlexCoherent DSP.  

“Infinera is saying it is the first announcing Nyquist sub-carriers, which is true, but they don’t give a roadmap when the product is coming out,” says Heavy Reading’s Perrin. “I suspect that Nokia [Alcatel-Lucent], Ciena and Huawei are all innovating on the same lines.” 

There could be a slew of announcements around the time of the OFC show in March, says Perrin: “So Infinera could be first to announce but not necessarily first to market.”