Hi Julian
Have you sorted out this question? I study for my Wireless exam and come across a youtube video from network chuck on 802.11ax.
I did some searching and this is what i found:
MU-MIMO is only on 802.11ac wave2, and only work downstream.
meaning: the AP can transmit at the same time to multiple devices but transmitting to the AP, the client devices still have to take their turns.
so: upstream only 1 client can transmit downstream multiple clients can get transmission from the AP radios at the same time.
802.11ax will have real MU-MIMO ergo transmitting and receiving to and from multiple devices at the same time.
in the meantime I asked a question that i didn't get:
if an AP got multiple SSID-s each on different channels, how come multiple clients can't transmit at the same time to the AP? they are not interfering with each other since they are on different channels. how do they do "Channel Assessment" (CCA) ? they actually SCAN every channel prior to transmission to see if the medium is idle?
the answer was: the AP has a time allocation for each channel.
so the way i see it : radio is a shared medium. either transmit or receive. 1 antenna can either transmit or receive. multiple antennas can form multiple spatial streams and transmit to multiple devices at a time but when the clients transmit uplink they still have to take their turns.
Please, feel free to correct me if i am wrong.