Triggering the usage of MU-MIMO with 802.11ac devices

I don't know if this is is allowed here of more suitable for /r/homenetworking, in any case it may still be relevant for enterprise & business settings.

Objective: Trigger MU-MIMO in a setup with 1 AP and 2 WiFi clients, all 802.11ac MU-MIMO capable. I'm particularly interested in capturing the exchange of sounding frames (sent by the AP) and management frames containing feedback matrices (sent by the clients).

Problem: Recently I have gained access to a few TP-Link Talon AD7200 routers, which are equipped with 802.11ac radios. To test their MIMO capabilities, I have followed the method described below to trigger the use of MU-MIMO.

However, after running iperf3 tests and analyzing packet captures, I can't find any sounding frames or frames containing feedback matrices, which suggests MU-MIMO isn't being triggered.

My questions:

• In general, how does an AP 'decide' to use MU-MIMO? What conditions must be satisfied?
• Can you suggest a good method to trigger MU-MIMO with the hardware I have?
• Can you spot something fundamentally wrong with my method and setup?

My Setup (w/ diagram):

• AP :
  • TP-Link Talon AD7200
  • OS: OpenWRT LEDE (v17.01)
  • WiFi: 802.11ac, freq. 5170–5250, 80 MHz channel bandwidth
• Clients : TP-Link Talon AD7200, running OpenWRT LEDE (v17.01)

• For convenience and SSH access, all nodes are accessible via a wired LAN, in a different subnet from that used in the WiFi network.

   +-----------------------------------+ | |eth0 | |192.168.1.w | wlan0 +---------+ | 192.168.10.y o client 2| | /+---------+ | | / eth0 | eth0 +---------+192.168.1.1 | 192.168.1.v +---------+/ wlan0 | gateway o-------------|--------------o ap o 192.168.10.1 +---------+ | +---------+\ | | \ | | \+---------+ | wlan0 o client 1| | 192.168.10.x +---------+ | |eth0 | |192.168.1.z +-----------------------------------+ 
  

Methodology: As a first step, I tried to understand the exact 802.11ac capabilities of the routers, and confirmed they are MU-beamformer/ee capable:

• Evidence from AP
• Evidence from client

Then, I've set one of the TP-Links as AP, and two TP-Links as clients, located 1 to 2 meters from the AP, in diametrically opposed directions. All of the nodes are at the same height.

I start an iperf3 server at the AP - iperf3 -s - and run iperf3 in reverse mode (i.e., with the -R option) at both clients to generate downlink TCP traffic (i.e., from the AP to the clients). iperf3 reports throughputs between 300 and 500 Mbps, in both clients.