The multi-user transmission (both OFDMA and MU MIMO) on uplink in 802.11ax (HE) is initiated by a trigger frame from an Access Point Station (AP STA), as shown in the following figure.

Figure 14.

As specified in standard IEEE P802.11ax/D8.0; the trigger frame contains all the user related information, and common information across users. The following figure depicts the format of the trigger frame.

Figure 15.
Figure 16.

To generate a trigger frame using the NI WLAN Generation Toolkit APIs, you must use two generation toolkit sessions. One session to configure the trigger frame content and another one for transmit PHY. The transmit PHY could be one of the OFDM standards 802.11a/g, 802.11n, 802.11ac, or 802.11ax.

You must complete the following steps to create the trigger frame MSDU bits.

  1. Configure the first session for 802.11ax Trigger-based PPDU.
  2. Enter the common settings such as bandwidth, GI, etc., directly to the session.
  3. Configure the payload parameters on the session to derive the other common parameters such as L-SIG Length, Pre-FEC Padding factor, etc. Or configure L-SIG Length, Pre-FEC Padding Factor, PE Disambiguity, and LDPC Extra Symbol Segment directly.
  4. Configure the per user information such as MCS index, RU allocation etc. with the “userx” active channel.
  5. Call the niWLANG Create Trigger Frame MSDU Bits VI or niWLANG_CreateTriggerFrameMSDUBits function to create the bits.
    Note For the list of trigger frame parameters, refer to the following table.
Table 8.
Trigger Frame Subfield Value
Common Information field

User Info field

Trigger Type

Set to 0. Only Basic Trigger is supported.

UL Length

Either derived from specified per user payload settings, or specified using L-SIG Length property/NIWLANG_L_SIG_LENGTH attribute

More TF

Set to 0

CS Required

Specified using CS Required property/NIWLANG_CS_REQUIRED attribute

UL BW

Specified using the Channel Bandwidth property/NIWLANG_CHANNEL_BANDWIDTH attribute

GI And LTF Type

Specified using the following properties:

  • Guard Interval Type/NIWLANG_GUARD_INTERVAL_TYPE
  • HE-LTF Size/NIWLANG_HE_LTF_SIZE

MU-MIMO LTF Mode

Specified using the MU-MIMO LTF Mode Enabled property/NIWLANG_MU_MIMO_LTF_MODE_ENABLED attribute

Number of HE-LTF Symbols and Midamble Periodicity

Specified using the Number of HE-LTF Symbols property/NIWLANG_NUMBER_OF_HE_LTF_SYMBOLS attribute and Midamble Periodicity property/NIWLANG_MIDAMBLE_PERIODICITY attribute.

UL STBC

Specified using the STBC All Streams Enabled property/NIWLANG_STBC_ALL_STREAMS_ENABLED attribute

LDPC Extra Symbol Segment

Either derived from specified per user payload settings or specified using LDPC Extra Symbol Segment property/NIWLANG_LDPC_EXTRA_SYMBOL_SEGMENT attribute

AP Tx Power

Specified using the AP Tx Power property/NIWLANG_TRIGGER_FRAME_AP_TXP attribute

UL Packet Extension

Either derived from specified per user payload settings or specified using following properties:

  • Pre-FEC Padding Factor property/NIWLANG_PRE_FEC_PADDING_FACTOR attribute
  • PE Disambiguity property/NIWLANG_PE_DISAMBIGUITY attribute

UL Spatial Reuse

Set all to 0

Doppler

Set to 0 if Midamble Periodicity property/NIWLANG_MIDAMBLE_PERIODICITY attribute is set to None/NIWLANG_VAL_NONE, set to 1 otherwise.

UL HE-SIG-A2 Reserved

Set all to 1

Reserved

Set to 1

AID12

Specified using the AID12 property/NIWLANG_AID12 attribute

RU Allocation

Specified using the RU Size and RU Offset properties/NIWLANG_RU_SIZE and NIWLANG_RU_OFFSET attributes

UL FEC Coding Type

Specified using the FEC Coding Type property/NIWLANG_FEC_CODING_TYPE attribute

UL MCS

Specified using the MCS Index property/NIWLANG_MCS_INDEX attribute

UL DCM

Specified using the DCM Enabled property/NIWLANG_DCM_ENABLED attribute

SS Allocation

Specified using following properties:

  • Number of Space Time Streams property/NIWLANG_NUMBER_OF_SPACE_TIME_STREAMS attribute
  • Space Time Stream Offset property/NIWLANG_SPACE_TIME_STREAM_OFFSET attribute

UL Target RSSI

Specified using the Target RSSI property/NIWLANG_TRIGGER_FRAME_TARGET_RSSI attribute

Reserved

Set to 1

Trigger Dependent User Info

Set to 32

After completing the aforementioned steps, generate the trigger frame by completing the following steps.

  1. Configure the second session to any one of the OFDM standards: 802.11a/g, 802.11n, 802.11ac, or 802.11ax.
  2. Configure the session to generate user-defined bit pattern.
  3. Input the MSDU bits created by the first session to the current session as a user-defined bit pattern.
  4. Configure MAC Frame Type as Trigger Frame.
  5. Configure the MAC Header settings and MAC Padding Duration (s).
  6. Call the niWLANG RFSG Create and Download Waveforms VI, or niWLANG_RFSGCreateAndDownloadWaveform/niWLANG_RFSGCreateAndDownloadMIMOWaveforms functions to generate the Trigger Frame waveform.

An example demonstrating the creation and the generation of trigger frame is located at: LabVIEW\examples\RF Toolkits\WLAN\generation\Basic or Documents\National Instruments\WLAN Toolkit\Examples\C\Generation