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Re: [802.3_B400G] Oct 2022 Session Webpage Update

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. 

Thank you

Chris

On Thu, Oct 13, 2022 at 11:37 AM Chris Cole <chris@xxxxxxxxxxxxxxx> wrote:
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.

The statement that Coherent detection provides 12-13 dB sensitivity gain over IMDD is correct, however in datacenter context is only half the story. 

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. 

Thank you

Chris

On Thu, Oct 13, 2022 at 8:50 AM John D'Ambrosia <jdambrosia@xxxxxxxxx> wrote:

All,

Updates to presentations at the 12 Oct meeting have been uploaded to the October 2022 Session webpage – https://www.ieee802.org/3/df/public/22_10/index.html.

I am working with Kent Lusted on the minutes for the meeting.  Further updates may be made during this time.  If any are made, this will be noted when the Task Force is notified of the uploading of the unapproved minutes.

Regards,

John D’Ambrosia

Chair, IEEE P802.df Task Force

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