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How 802.11 ac wave 2 MU-MIMO works

  • 24 October 2019
  • 7 replies
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Userlevel 3
  • Participator
  • 85 replies

Hi,

we have a question about how MU-MIMO works. 

In our aps infraestructure (3915 and 3912 models) we only have 20MHz channels to avoid overlaping channels. 

The aps are 2x2:2MU-MIMO. ¿our question is?

With a 20MHz using one antenna the aps can una 2 spatial streams to send trafic to 2 MU-MIMO clients? Or this only works when we have 40MHz Channel with 2 antennas working?

 

Sorry for the english 


7 replies

Userlevel 6

Correct.  MU-MIMO will be limited by the number of spatial streams that the AP supports.  So if your AP model is a 2x2:2, it only supports 2 spatial streams...and therefore can only potentially transmit at the same time to 2 wireless clients.

The channel width (20MHz/40MHz doesn’t have anything to do with this limitation)

Userlevel 3

OK thanks,

Then, using 802.11ac wave 2, 4x4:4 MU-MIMO APs, with a ssid with 20Mhz we can serve 3 spatial streams to 3 different MU-MIMO clients 1x1:1?

Also , we can serve 3 clients: 2clientes 1x1:1 and other 2x2:2?

 

Thanks in advance

Userlevel 6
With 11ac Wave 2 APs, you are limited to transmitting to a maximum of 4 clients at a time no matter what (but the AP can use up to eight spatial streams for those 4 clients, *if* you actually had an AP that supported 8 spatial streams) There’s also another limitation that you can only use a maximum of four spatial streams for each client. Some examples are below...if your AP supported 4 spatial streams.
  • The AP can transmit to 2 different clients with 1 spatial stream each (this would consume 2 spatial streams on the AP)
  • You could transmit to 4 different clients using 1 spatial stream for each client (this would consume 4 spatial streams on the AP)
  • You could transmit to 2 different clients using 2 spatial streams for each client this also would consume 4 spatial streams on the AP)
 You get the idea.  But with your AP only supporting 2 spatial streams, your only possible combination is much more limited….to just the first one below.You can transmit to 2 clients with 1 spatial stream each (uses 2 spatial streams on the AP)You can transmit to 1 client with 2 spatial streams (which is technically no longer even MU-MIMO)  
Userlevel 3

Hi Chris,

in addition to this… I think in a building that have classrooms. We install one AP for each class 2x2:2MU-MIMO… I have speak with other wireless stuff and was talking about the spatial streams. We only can uso different spatial streams if the users not are phisical in the same place, in order to use beanforming to use this different 2 spatial streams.

If this case is true 802.11ac wave 2 and 802.11ax dont have more possibilities than 802.11ac wave1. Also we need that the users have devices using 802.11ac wave 2 ( more battery usage) or 802.11ax (I dont know devices in the market at now)…

 

This thoughts are in order to have a ROI of the wifi infraestructure 802.11ac wave2.. I think is very difficult to justify in our scenario. 

 

Thanks in advance for the info.

Userlevel 6

FES,

 

We only can uso different spatial streams if the users not are phisical in the same place, in order to use beanforming to use this different 2 spatial streams.

Absolutely correct.  There needs to be enough spatial diversity between two wireless clients in order make MU-MIMO work as intended. “Spatial Diversity” most of the time equates to the wireless clients being located in different physical areas.

 

If this case is true 802.11ac wave 2 and 802.11ax dont have more possibilities than 802.11ac wave1. Also we need that the users have devices using 802.11ac wave 2 ( more battery usage) or 802.11ax (I dont know devices in the market at now)…

Well, .11ac wave-1 did not have MU-MIMO *at all* so both wave-2 and 11ax have that difference. :)

And yes, the wireless clients would need to support at least 11ac wave-2 in order for MU-MIMO to function.

This thoughts are in order to have a ROI of the wifi infraestructure 802.11ac wave2.. I think is very difficult to justify in our scenario. 

In terms of ROI, are you looking to compare to an existing 802.11n infrastructure?

(You currently have 802.11n APs?)

Userlevel 7

FYI, the APs that you’ve mentioned doesn’t support MU-MIMO.

 

-Ron

Userlevel 6

Ronald,

Both AP models are 11ac wave-2.  Can’t say w/o locating the spec sheet for the 3912, but the 3915 is the same hardware as the AP7632 which does support MU-MIMO - so don’t see why it wouldn’t.  If it doesn’t, it’s because of a software limitation, not hardware.

 

Okay….so found the spec sheet on the 3915. MU-MIMO is confirmed to be supported.

The AP3915 is built using the latest in technology, including 802.11ac Wave 2, dynamic radio management, spectrum analysis with interference classification, beamforming, multi-user MIMO, self-forming and self-healing meshing, 

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