Wifi 6: Will OFDMA & MU-MIMO operate simultaneously?

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Once OFDMA is enabled on the new WiFi 6 AP's, will OFDMA and MU-MIMO operate simultaneously? 

For example, let's say you have a factory with 100 clients connected to an R730 AP.  (For simplicity, I'll just look at the 5 GHz running 40MHz wide channels.)  The majority of the clients are lightweight and using under 500Kbps of bandwidth and, say, 10 AP's are using ~4Mbps. 
  1. Is it an either/or situation in that the AP will decide to use either use OFDMA or MU-MIMO?
  2. Or will it transmit a few frames with OFDMA and then a few frames with MU-MIMO?
  3. Or will it transmit 8 MIMO spacial streams while simultaneously splitting each stream into 18 RU's and allocating them to the clients as needed?
I've scoured the internet and can't find any answers on this.  So any insight would be appreciated.

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Paul Ainslie

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Posted 4 weeks ago

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Paul Ainslie

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I found this here explaining that, "...the combination of MU-MIMO and OFDMA in a single packet, while quite possible in theory, becomes complicated in practice and support is deferred to wave 2."  That should be a big improvement for high density environments!!
(Edited)
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Michael Brado, Official Rep

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Regarding OFDMA, one key point to remember about 802.11ax (or Wi-Fi 6) is it's not labeled as Extremely High Throughput (802.11ac was labeled VHT for Very High Throughput), 802.11ax is labeled as HE for High Efficiency.  The majority of frames in 802.11 are 300 Bytes or less so the theory is all of these frames can be consolidated in smaller RU's and sent at the same time, allowing more time slices for that one client that wants to download a movie.  The purpose of the client device matters just as much as the OFDMA.   If all the clients are downloading movies, then they won't request smaller spectrum chunks, they are going to request the full channel. If all of the devices are simply checking in and no demand is placed, then you won't see any increase in speed, and with only 4 clients, you won't see much improvement in anything since you can do that in 802.11ac.

For more information, there is a podcast in the US called Clear To Send that spent most of 2019 discussing Wi-Fi 6 and broke down a lot of stuff around it and I think they did a really good job.  You can find that here https://www.cleartosend.net/802-11ax-series/
(Edited)
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Paul Ainslie

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Interesting, I didn’t know that the majority of frames in 802.11 are 300 Bytes or less. In that case, the smaller RU’s will certainly improve efficiency!!


What if 95 clients are doing lightweight browsing and 4 clients are doing heavy streaming? Would/could several spatial streams be devoted to the streamers while the other spatial streams handle lightweight clients via OFDMA? Or would/could it switch between MIMO and OFDMA on a frame by frame basis?


I looked thru the cleartosend.net blogs (very well written) but couldn’t find anything about how the two can be used together.
(Edited)