Did the Optics Industry Blunder by Switching Intra-Datacenter Links from NRZ to PAM4?
Will More DSP like PAM6 and Coherent Follow, or Will WDM and Parallel Save the Day?
Session Organizer and WDM Team Captain: Chris Cole, II-VI Incorporated, USA
WDM Team Provocateurs: Shigeru Kanazawa, NTT, Japan; Boris Murmann, Stanford University, USA: Bardia Pezeshki, Avicena Tech, USA; Peter Winzer, Nubis Communications, USA
Session Organizer and DSP Team Captain: Ilya Lyubomirsky, InPhi, USA
DSP Team Provocateurs: Yi Cai, ZTE, USA; Dan Sadot, Ben Gurion University, Israel; Henry Sun, Infinera, Canada; Xiang Zhou, Google, USA
Description from WDM Team:
PAM4 was chosen for bandwidth limited electrical channels by the IEEE Ethernet Group in 2012. Modulation format for 50G λs was debated by the IEEE in 2015. Shannon provided clear guidance to stick with NRZ because the optical channel is limited by SNR and not bandwidth. Unfortunately, because the optics industry is the tail on the IC industry dog, PAM4 was chosen to reuse ASIC SerDes technology already in development. This unnecessarily and permanently locked in lower SNR and higher power for optical links. PAM4 50G λs will ship in the millions despite availability of mature 50GBaud technology. Shannon was again ignored by the IEEE for 100G λs and appears likely to be ignored for 200G λs. However, emerging applications not tied to Ethernet are returning to communication theory fundamentals and defining higher channel count lower-order modulation WDM and Parallel links.
Questions for Discussion from WDM Team:
- What are the SNR and power advantages of NRZ over PAM4 optics?
- How does photonic integration change channel count vs. modulation trade-off?
- How does co-packing impact channel characteristics and optimum modulation?
- Are Coherent telecom advantages of any use in the datacenter?
- Will the datacom optics industry continue its self-destructive migration off the higher-order modulation cliff with each future datacom rate increase?
Description from DSP Team:
Analog direct detection technology dominated transport optics for many years. Capacity per fiber grew simply by increasing the modulation rate and number of λs. Around the year 2000, modulation rate reached a 40Gbaud limit because of fiber chromatic and polarization mode dispersion, and DWDM λ spacing reached a practical 50 GHz limit. DSP Coherent technology came to the rescue by adaptively compensating for fiber impairments and by increasing per λ capacity, resulting in lower deployment and operating costs. Analog direct detection technology went extinct, along with multiple start-ups. History is now repeating in the datacenter. Impairments such as component bandwidth, electrical and optical reflections, laser noise, and chromatic dispersion limit performance of higher data rates. DSP again brings relief and cost reduction by equalizing out these impairments and increasing per λ capacity. In optical communications as in other fields, bets against CMOS, the enabler of DSP, always lose.
Questions for Discussion from DSP Team:
- What are the optimum modulation formats for optical and electrical channels?
- Will reduced-complexity DSP bring Coherent into the datacenter?
- Which is more important inside the datacenter: cost or power?
- Is ADC power reduction tracking CMOS node scaling or is it plateauing?
- Will the datacom optics industry snatch defeat from the jaws of victory and switch back to the Analog direct detection path of obsolescence?
- Introductory presentations by WDM and DSP Team Captains, followed by one content slide and one punch line slide presented by alternating WDM and DSP Team Provocateurs, adding up to 50% of session time.
- Vigorous audience participation after each presentation, with Team Captains facilitating wide ranging discussion, adding up to the other 50% of session time.
- Attendees come prepared with tough questions, insightful comments, and different perspectives to challenge the Provocateurs and broaden the discussion.
- Lack of objectivity, unabashed partisanship, and exuberant support of your favorite Team are strongly encouraged, but please no hooliganism.