The Uplink transmission of frames from 802.11ax capable Stations to an 802.11ax Access Point requires a Trigger frame to be sent by the Access Point to the stations who would participate in the Multi-User UpLink transmission.

The Trigger Frame will assign RUs, determine TX power for transmission and spatial streams to the HE capable stations. The Access Point might also use the Buffer Status report sent implicitly by the HE Stations to determine the RUs to be assigned to different stations. The Access Point of its own accord (explicitly) send a BSRP Trigger frame to obtain the buffer status report from different stations.

The MU-RTS, BSRP Trigger frame and the basic Trigger frames have been discussed previously in previous articles. The below diagram depicts an UpLink Multi-User OFDMA transmission.

Co-channel Interference and BSS Coloring