Does OFDMA Really Work? Part 2

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sfx2000

Part of the Furniture
11ax testing is many levels more complex that what was done with earlier revisions...

HE frames on the UL are triggered by the AP, and this is done when conditions are suitable ( delta time < 400ns) - if not, the AP shouldn't even try if there is more than 400ns arrival time from the client STA's - frequency stability is also significant in that the delta center freq must be less than 350 Hz (yes, and that ain't much on a 5GHz channel)

Client STA power control is also a thing - as the AP needs to have all the OFDMA RU's arrive at a similar power level, and they have to all arrive in the assigned RU groups, otherwise the RU's, and possibly the entire frame, is lost...

MU-MIMO is also about arrival time, and the I/Q offsets - the AP has no idea of north/south, but it does know about the channel timing, and the sounding reports from the client STA's - and with 11ax - MU links are reciprocal - if conditions are not conducive for DL-MU, it's not likely that UL-MU is going to be any better...

Back in the 4G standards development era - LTE looked at this, as did 802.16m (Mobile Wimax) - LTE decided that the complexity of OFDMA would not have much benefit, and went with a SC-FMDA (also known as LP-OFDMA), which is basically OFMDA with a single RU group...

OFDMA in 802.11ax is an evolution of work done in 802.16m - and we all know how successful that was...

I wouldn't worry too much about OFDMA to be honest... there are other comms tech where there is benefit (DOCSIS 3.1 is a great example), but on WLAN/WWAN, it has not shown results that were promised...

OFDMA-RU.png


OFDMA-UL-spectrum.png
 

sfx2000

Part of the Furniture
Is even the Home Scenario (section 5.1) realistic?

Assuming perhaps that every client STA is WiFi-6 in the home - which is unlikely... 2.4GHz clients on a WiFi-6 AP will generally run in 2.4 11n (20MHz), and my mix of 5GHz is N/AC, with a couple of AX (as mentioned).

QC_WP_51.PNG

Because of this - there's little to no gain because of OFDMA as most clients are WiFi 4/5..

The busy office (5.2) assumes the same - all client STA's are WiFi 6, and the classroom scenario is even worse - how many schools have the budget to replace all their chromebooks with shiny new ones, even if there were affordable devices with WiFi-6

Don't get me started on WiFi-6E - I'm trying to be family friendly, and I have some pretty strong opinions there (and salty words ;) )
 
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sfx2000

Part of the Furniture
Well, see if you can restrain your self from the "salty" words. :)I'd like your take on 6E, please.

I'll be nice ;)

1) Dedicated Radio or the barrens of ISM band - one either has to implement a dedicated radio to access the 6E expansion, or kick all the existing WiFi-6/5 clients over to 2.4GHz

2) Automated Frequency Coordination - 6E is not the primary user in that band, so there's the "database" that the regulatory agency (here in the US, it's the FCC and whoever gets the contract to establish and maintain it) - that does smell a bit - more so than DFS did in the UNII-2 bands - it's supposed to be anonymous, but the data is there, waiting to be monetized and even hacked...

I can appreciate the expansion in the upper bands - would have been nice at some point in the past for FCC to open up ISM a little bit more, allowing 12, 13, 14, but that horse has left the barn...
 

sfx2000

Part of the Furniture
1) Dedicated Radio or the barrens of ISM band - one either has to implement a dedicated radio to access the 6E expansion, or kick all the existing WiFi-6/5 clients over to 2.4GHz

And all is not entirely lost in the 2.4GHz band - which is probably worth another thread/post - I have ideas and an open call to the vendors that can help make that band more useful for WiFi 4, 5, and 6...

@thiggins - ping me in PM - this might actually be a useful article starter on the main site.
 

kernol

Very Senior Member
I read that when it first came out. Look at the # of devices in each scenario.
Fully understood - and in my "quiet" home environment - continue to have OFDMA disabled ... along with MIMO on my AX86U :).
 
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