The trigger frame is a control frame of the MAC header, which contains the association IDs (AIDs) of the intended stations (STAs) and their respective spatial stream or RU allocation. Upon receiving a trigger frame, the STA’s whose AID matches the ones specified in the trigger frame are required to transmit their data and spatial stream or RU using HE TB PPDU.
Trigger-based (TB) PPDU in 802.11ax is transmitted in response to a trigger frame. The trigger frame allocates resources and carries the information required to construct a TB PPDU.
Trigger FRame Format
Frame control > Duration > RA > TA > Common Info > User Info List > Padding > FCS
Client # 2 > AID 02 > RU allocation , MCS , spatial streams > target RSSI
Client # 2 > AID 03> RU allocation , MCS , spatial streams > target RSSI
The AP can transmit a trigger frame in legacy HT, VTH , HE data rates.
The common info field of the trigger frame specifies the trigger frame type, TX power of the trigger frame in dBm, and several key parameters of the following TB PPDU, such as the L-SIG length field, BW, GI, and HE LTF. The trigger frame type used by the AP for receiving QoS data with the UL MU operations is called a basic trigger.
The user info list field contains a user info field per participating station. The user info filed specifies the AID, RU allocation, spatial stream allocation, and MCS to be used by each participating STAs in the TB PPDU.
Upon receiving the trigger frame, each participating STA first computes the path loss between AP and the station by subtracting the RX RSSI in dBm from the TX power mentioned in the common info field.
Example trigger frame types:
Trigger Type: Basic
Trigger Type: Buffer Status Report Poll (BSRP)
Trigger Type: MU BAR