How DSP smarts continue to improve optical transport

• Kim Roberts explains the signal processing techniques Ciena is using for its WaveLogic 6 coherent DSP.
• Roberts explains how the techniques squeeze, on average, a 15 per cent improvement in spectral efficiency.
• The WaveLogic 6 Extreme chip can execute 1,600 trillion (1.6 x 10¹⁵) operations per second and uses the equivalent of 4km of on-chip copper interconnect.

Part 2: WaveLogic 6's digital signal processing toolkit

Kim Roberts, Ciena’s vice president of WaveLogic Science and winner of the 2019 John Tyndall Award

Bumping into Kim Roberts on the way to the conference centre at OFC, held in San Diego in March, I told him how, on the Ciena briefing about its latest WaveLogic 6 coherent digital signal processor (DSP), there had been insufficient time to dive deeply into the signal processing techniques used.

“What are you doing now?” said Roberts.

“I’m off to the plenary session to catch the keynotes.”

Chatting some more, I realised I was turning down a golden opportunity to sit down with a leading DSP and coherent modem architect.

“Is that offer still open?” I asked.

He nodded.

We grabbed a table at a nearby cafe and started what would prove to be an hour-long conversation.

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WaveLogic 5: Packing a suitcase of ideas into 7nm CMOS

• Ciena’s WaveLogic 5 coherent digital signal processor family comprises the Extreme and Nano chips
• The WaveLogic 5 Extreme maximises optical capacity and transmission reach while the Nano is targeted at compact, power-conservative applications

Advancing coherent optical transmission performance to benefit its platforms; targeting the emerging coherent pluggable market opportunity; selling modules directly, and being more vertically integrated. All these aspects were outlined by Cisco to explain its intention to buy the coherent optical transmission specialist, Acacia Communications; a deal set to be completed in the spring of 2020.      

But this also fits the strategy being pursued by Ciena with its next-generation WaveLogic 5 family of coherent DSPs.

Kim Roberts

The WaveLogic 5 continues Ciena’s tradition of issuing a coherent digital signal processor (DSP) family approximately every three years: Ciena announced the WaveLogic 3 in 2012 and the WaveLogic Ai in 2016. 

Ciena has managed to maintain its three-yearly cadence despite the increasing sophistication of each generation of coherent DSP. For example, the WaveLogic 5 Extreme will support 800 gigabits-per-wavelength, double Ciena’s WaveLogic Ai that has been shipping for nearly two years. 

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Kim Roberts: The 2019 John Tyndall Award winner

A Profile

A conceptualiser, mathematician, furniture maker, prolific inventor, sushi lover, creative spirit, team leader and mentor. These are just some of the descriptors applied to Kim Roberts of Ciena by the people that know him. 

Kim Roberts of Ciena. On the wall are some of his 160 patents while on the screen is an image of a 32-point constellation produced by the WaveLogic Ai coherent modem. Source: Ciena.

Roberts has been awarded the 2019 John Tyndall Award by The Optical Society (OSA) and the IEEE Photonics Society in recognition of his “pioneering contributions to the development of practical coherent communication systems”.

“It is well deserved,” says Seb Savory, who first knew Roberts when they both worked at Nortel and who is now an academic at the University of Cambridge working on joint projects with Ciena. Ciena acquired Nortel in 2010.

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Heading off the capacity crunch

Feature - Part 1: Capacity limits and remedies

Improving optical transmission capacity to keep pace with the growth in IP traffic is getting trickier. 

Engineers are being taxed in the design decisions they must make to support a growing list of speeds and data modulation schemes. There is also a fissure emerging in the equipment and components needed to address the diverging needs of long-haul and metro networks. As a result, far greater flexibility is needed, with designers looking to elastic or flexible optical networking where data rates and reach can be adapted as required.

Figure 1: The green line is the non-linear Shannon limit, above which transmission is not possible. The chart shows how more bits can be sent in a 50 GHz channel as the optical signal to noise ratio (OSNR) is increased. The blue dots closest to the green line represent the performance of the WaveLogic 3, Ciena's latest DSP-ASIC family. Source: Ciena.

But perhaps the biggest challenge is only just looming. Because optical networking engineers have been so successful in squeezing information down a fibre, their scope to send additional data in future is diminishing. Simply put, it is becoming harder to put more information on the fibre as the Shannon limit, as defined by information theory, is approached.

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2020 vision

In a panel discussion at the recent Level123 Terabit Optical and Data Networking conference, Kim Roberts, senior director coherent systems at Ciena, shared his thoughts about the future of optical transmission. 

Final part : Optical transmission in 2020

"Four hundred Gigabit and one Terabit are not going to start in long-haul"

Kim Roberts, Ciena 

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Optical transmission's era of rapid capacity growth

Kim Roberts, senior director coherent systems at Ciena, moves from theory to practice with a discussion of practical optical transmission systems supporting 100Gbps, and in future, 400 Gigabit and 1 Terabit line rates. This discussion is based on a talk Roberts gave at the Layer123's Terabit Optical and Data Networking conference held in Cannes recently. 

Part 2: Commercial systems

The industry is experiencing a period of rapid growth in optical transmission capacity. The years 1995 till 2006 were marked by a gradual increase in system capacity with the move to 10 Gigabit-per-second (Gbps) wavelengths.  But the pace picked up with the advent of first 40Gbps direct detection and then coherent transmission, as shown by the red curve in the chart. 

Source: Ciena

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The capacity limits facing optical networking

Ever wondered just how close systems vendors are in approaching the limits of fibre capacity in optical networks? Kim Roberts, senior director coherent systems at Ciena, adds some mathematical rigour with his explanation of Shannon's bound, from a workshop he gave at the recent Layer123's Terabit Optical and Data Networking conference held in Cannes. 

Part 1 Shannon's bound

 Source: Ciena

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