Problem: I cannot trigger the usage of MU-MIMO in a setup with 1 AP and 2 WiFi clients, all of which are (apparently) 802.11ac MU-MIMO capable.

My questions are:

• Has anybody been able to make use of MU-MIMO with the TALON AD7200? If yes, what was the configuration you've followed (i.e., how many clients, their position and distance relative to the router, nr. of antennas in clients, what type of traffic did you use, special configurations in the router, etc.)?
• Does the number of antennas on each node (i.e., clients and AP) matter to trigger MU-MIMO? If yes, what are the allowed combinations?
• Does the throughput demand by the clients matter to trigger MU-MIMO?
• In general (or in particular for the AD7200), how does an AP 'decide' to use MU-MIMO? What conditions must be satisfied?

Details: I have acquired a TP-Link Talon AD7200 router, which is equipped with a 802.11ac radio, and - according to the brochure - compatible with MU-MIMO.
To test its MIMO capabilities, I have used it together with 2 clients, positioned 1 to 2 meters away from the AP, in diametrically opposed directions. All of the nodes are at the same height.

To generate traffic, I use the iperf3 tool, making sure to generate 'downstream' TCP streams (i.e., from the AP to the clients), which reach throughputs up to 500 mbps.

I've confirmed that all nodes - AP and 2 clients - are MU-MIMO compatible because all have their 'MU-beamformer/ee capable' bit set in their Beacon/Association WLAN frames:

• Evidence from AP
• Evidence from client

I know that MU-MIMO is not being used because I don't see the exchange of MU-MIMO sounding frames or frames containing feedback matrices.

For the record, I'm using a MacBook Pro 2015 (OS X v10.11.5) running the 'Sniffer' tool included in the 'Wireless Diagnostics' application to capture WLAN frames over the air, and I have confirmed I can capture MU-MIMO sounding over the air (e.g., frames of the type 0x0015, VHT NDP announcements)

Thanks in advance!