DL MU OFDMA bit-by-bit

There are a lot of blogs, podcast and videos at the internet explaining 802.11ax at a high level. And some have done testing with 802.11ax compatible devices. But I have not found anyone that explains 802.11ax at a deep level. So why not me

The last year, since I bought the Perahia and Staceys book “Next Generation Wireless LANs”, I have been interested in the PHY-level of 802.11. And to go deep at 802.11ax I had to buy the 802.11ax, Draft 4.0.
There are so many new topics in the 802.11ax technologies so I had to make usecases for some of the topics and I have choosen the MU OFDMA process. This first blogarticle, in a series of articles, are about the frame where the AP sends data down to stations that needs data, the DL MU OFDMA frame. This frame is sent in i HE MU PPDU format, one of the four different frame formats in the 802.11ax standard.

Later on I will cover other aspect of the MU OFMDA process, like the MU-RTS/CTS process, the uplink OFDMA (UL MU OFDMA) process and the Acknowledgement process

Nothing of this is testet in real world, it’s picked out of the 802.11ax draft

DL MU OFDMA is the process where the AP sends data down to several stations that need/want data in a parallell process. In this slides I have used a example where four stations receives data in parallell. The AP have, before it starts to send data, decided how it should allocate its RUs.

A overview of this frame (PPDU) is like this

DL OFDMA transmission overview

The presentation (slides) could be downloaded at this link (pdf)

DL OFDMA, bit-by-bit

If someone have constructive feedback I would be grateful

Useful links

  • Cleartosend 802.11ax podcast-series,  link
  • David Colemans presentation at WLPC_US 2019, link
  • Wifininjas, link
  • IEEE 802.11ax draft 4.0 ($400), link


4 thoughts on “DL MU OFDMA bit-by-bit

  1. Hello Gjermund, thank you very much for all your posts regarding Wi-Fi 6 testing. I am writing my master thesis about this new amendment right now and I have some questions.
    Is there any bit in the Beacons and Probe Requests indicating support for DL OFDMA or UL OFDMA? In the HE MAC Capabilities Information tab, I see only a bit responsible for OFDMA RA Support, so OFDMA UORA. What about basic DL and UL OFDMA?
    The similar problem I see with MU-MIMO, I see a bit responsible for MU Beamformer (HE Phy Capabilities Information tab, bits 24-39), so information if AP can do MU-MIMO, but there is no MU Beamformee bit.
    I expected to see such bits as for example, it is easy to validate “theoretical” support for TWT, BSS Coloring, or 1024-QAM in Beacons and Probe Requests. But OFDMA and MU-MIMO is a mystery for me.
    Thank you very much!


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