Just like to chime in to clear out the link to the latest proposal from the IMDD proponents as Chris pointed out below:
Towards an 800G-LR4 IMDD Specification Consensus - Nov 2022 Update (ieee802.org)
I’d like to comment that there was misconception that coherent can do several thousand of kilometers for long haul in telecom, so it can easily do 10km with “margin”.
Telecom space leverage the aid of EDFA and DWDM to compensate fiber loss for long distance
In Datacom space under unamplified link configuration (no EDFA is available), Chris and Bill made it clear that Coherent has no link budget advantage over IMDD.
Thanks,
Frank
From: Chris Cole <chris@xxxxxxxxxxxxxxx>
Sent: Sunday, November 13, 2022 11:37 AM
To: STDS-802-3-B400G@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_B400G] Oct 2022 Session Webpage Update
This email is from outside of our organization. Exercise caution responding or opening attachments/links.
Nowhere have I compared 1 Coherent laser to 1 IMDD laser. That would be silly. Excerpting for your convenience:
"... the Coherent link budget requires more total laser source optical power, i.e. less link margin than IMDD."
Please note the "total" in the sentence.
Their number for the average launch power (min) is -0.9dBm. To get the total source power (min) we add 5dB for EA loss and 6 dB for four channels, which gives us 10.1dBm.
Now we can make an Engineering comparison of Coherent (16) vs. IMDD (10.1) total laser source power (min):
This does raise the question how the Coherent proponents see the above inequality as justifying characterizing their margin as significantly better. The casual observer would characterize a 5.9dB deficit as worse or significantly worse.
I think actual performance numbers for IMDD and coherent at 10km is a good thing (with apples to apples comparison).
As for the link budgets, I am thoroughly confused as to where all the dB’s are going.
What takes 2 16QAM transmitters (1 laser) takes 4 PAM IMDD Tx lasers.
So counting 1 coherent laser versus 1 PAM laser doesn’t seem fair.
The 16 dBm coherent laser is split 50/50 for Tx and Rx, leaving 13 dBm to go to 2 Tx modulators (X/Y polarizations).
4 – 4 dBm PAM 4 lasers = 10 dBm of laser power for IMDD to achieve the same 800 Gbps.
Hence only a 3 dB delta in Tx laser power at 800 Gbps for coherent versus IMDD.
The slides indicate -10 dBm Tx power (per pol or both X/Y?). I presumes the slides are indicating that they
can close the link budget (fiber and interconnect), but without a large margin.
Hence, your statement Coherent has no link budget advantage over IMDD.
(I am just a Matlab jockey trying to learn in months what you folks have spent years on.
i.e. please excuse my ignorance. Just trying to make sense of things.)
Warning - External Email
Dear 802.3df TF participants,
Throughout this year we have tried to shift the critically important debate between Coherent and IMDD approaches from Marketing to Engineering, i.e. from adjectives to numbers.
We have shown in several ways that Coherent has no link budget advantage over IMDD in short-reach unamplified applications, like inside the datacenter. This is easy to understand
because Coherent throws away 23dB in the transmitter, which dwarfs the 6.3dB of the actual link, for example see email below.
Yet despite this, we are again seeing
a presentation, incorrectly claiming that Coherent has a significant margin over IMDD, without numbers to back-up the adjective. The Coherent and IMDD link budgets presented so far to the TF have no unallocated loss. Further, the Coherent link budget requires
more total laser source optical power, i.e. less link margin than IMDD.
If the authors were to replace assertions with numbers in their
presentation, I would speak in favor of the Task Force allowing the post-deadline updated presentation to be given.
Dear 802.3df participants,
I received several private emails asking about my Coherent receiver sensitivity comment during yesterday's call. In a previous reflector thread, I used a simple link budget analysis
but it looks like further explanation is in order.
In Telecom, this is a tremendous advantage because to first order it can double span reach. It is therefore understandable that it's brought up in discussion of datacom giving the
impression of similar advantage. The opposite is true. For typical SiPh implementations
the huge Coherent transmitter loss results in overall link budget loss over IMDD. As commented previously, Eric deserves credit for being transparent about this, as the more typical approach by proponents has been to hide it.
The better absolute Coherent receiver sensitivity is more than cancelled out by lower coherent transmitter output power, for example -10dBm in TF proposals. IMDD EML transmitter
output power can be well above 0dBm. Unlike in Telecom, there are no EDFAs to increase the optical power.
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