What is used to respond with an uplink transmission to an MU-RTS trigger frame in the 802.11ax PHY?

In the IEEE 802.11ax standard, uplink multi-user (UL MU) transmissions are coordinated by the Access Point (AP) using a Trigger frame. The MU-RTS is a specific type of Trigger frame used to solicit Clear-to-Send (CTS) frames to reserve the medium. Following this, a basic Trigger frame allocates specific resource units (RUs) to multiple stations for simultaneous uplink transmission. The stations respond to this trigger by sending data encapsulated in a High Efficiency Trigger-Based PPDU (HE TB PPDU). This specific PPDU format is designed exclusively for triggered uplink transmissions in 802.11ax.

A. HE SU PPDU: This format is used for single-user transmissions, not for the multi-user uplink scenario initiated by a trigger frame.

B. HE MU PPDU: This is a downlink multi-user format, used by an AP to transmit simultaneously to multiple stations, not for an uplink response from a station.

D. VHT PPDU: This is a frame format from the older 802.11ac (VHT) standard and is not used in the 802.11ax (HE) PHY for this purpose.

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1. IEEE Std 802.11ax™-2021, IEEE Standard for Information Technology—Telecommunications and information exchange between systems Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Amendment 1: Enhancements for High-Efficiency WLAN.

Section 27.3.1.4 (HE trigger-based PPDU format): "An HE TB PPDU is transmitted by a non-AP HE STA in response to a Trigger frame." This directly states that the HE TB PPDU is the response to a Trigger frame.

Section 26.10 (UL MU operation): This section details the entire uplink multi-user procedure, specifying that the AP sends a Trigger frame to solicit HE TB PPDUs from one or more stations.

2. Khorov, E., Levitsky, A., & Akyildiz, I. F. (2020). A Tutorial on IEEE 802.11ax High Efficiency WLANs. IEEE Communications Surveys & Tutorials, 22(1), 197-216.

Section IV-A (Uplink MU Transmissions), Paragraph 1: "To enable UL MU transmissions, the standard introduces a new mechanism based on a so-called Trigger Frame (TF)... In response to the TF, the STAs that are indicated in the TF simultaneously start transmitting data to the AP. The corresponding PPDU format is called HE Trigger-Based (TB) PPDU." (DOI: https://doi.org/10.1109/COMST.2019.2946907)

3. Cisco. (2020). 802.11ax: The Sixth Generation of Wi-Fi White Paper.

Section: Uplink MU-MIMO, Paragraph 2: "The AP tells stations that it is ready to receive their data by sending a new control frame called a trigger frame... The stations can then send their data simultaneously. The frames they send are a new format called High-Efficiency Trigger-Based (HE-TB) PPDU." This vendor documentation confirms the operational use of the HE TB PPDU in response to a trigger frame.