DL OFDMA, the RF Perspective

My previous blog post was on how the RF spectrum looks like during a UL ODFMA transmission.

This article is about the DL OFDMA transmission. The DL ODFMA transmission can be preceded by an MU-RTS (multi-user request to send) and CTS (clear-to-send). I keep it to a later article.

The Acknowledgment process
The most efficient mechanism to get the 802.11 acknowledgments back from the STAs (non-Ap STA) during DL OFDMA is to include a Trigger frame as the first MPDU in the A-MPDU.
The STAs will respond with a BlockAck in a HE TB PPDU immediately and in parallel. Just like the UL OFDMA process.
This picture (found on the internet) shows this process

DL ODFMA with Trigger for BA
This is a new method of sending the 802.11 acknowledgment. For 20 years this 802.11 acknowledgment frame has been sent in non-HT format as a Control frame on a mandatory data rate. In 802.11ax and DL OFDMA is one of the options is to send it in a subdivided trigger-based frame.
I will explain it in a later article.


A short summary of DL OFDMA
– In this example, the AP will data downlink to two STAs (clients)
– The AP decides to subdivide the 20MHz channel into two 106-tones RUs and send the data (payload) til client1 (AID1) in the lower 106-tones RU and the data(payload) to client2 (AID2) in the higher 106-tones RU.
– This receiving STAs sends their acknowledgment as BlockAck inside their assigned RU, based on the content in the Trigger frame, uplink to the AP in a TB PPDU format


The DL ODFMA frame
The frame been used when sending data down to multiple non-AP STAs in parallel is called MU-PPDU (multi-user). I have a deep-dive article on this.
Basically, the transmitting procedure is like this:
– First the legacy preamble
– The HE-preamble. It contains the HE-SIG-B field with information about the resource allocation for the rest of the frame.  It tells each receiving STA which RU the STA will receive its data on.
– The HE short and long training symbols
– The A-MPDU to each non-AP STA in their assigned RU, where the first MDPU is the Trigger information for Block Acknowledgment

From the perspective of the AP it looks like this:

MU PPDU , AP perspectice


The non-AP STA, let’s say AID1, will see in the HE-SIG-B field that the data to AID1 is sent in the lower 106-tones RU. The STA with AID1 will therefore only look at the subcarriers that are assigned for him, both short/long training fields and the data.
So from the perspective of AID1, the transmission looks like this:

MU PPDU , STA perspectice



The receiving STA will use the Trigger information from the first MPDU in the A-MPDU in the downlink data frame to build the frame that sends back the BlockAck. The frame type used for this is called HE TB PPDU and is the same method as used with UL OFDMA. UL OFDMA is explained in this article.

In this example, the receiving STA sends their BlockAck like this:

HE TB PPDU BlockAck, separated


Just like any other 802.11 frames, it starts with preambles using the full channel. And the BlockAck information is sent in their assigned RU.

From the AP perspective, those transmissions will be received in parallel.

HE TB PPDU BlockAck, parallel

This was explained in my previous article


This is DL ODFMA briefly explained


A friend of mine has sent me some packet captures with the Multi-User frame down to several clients. He uses SOHO-routers in beta code for this.
He has explained how he does it and today I was able to capture MU-PPDU from a Cisco Catalyst 9115 access point down to a Samsung S10.

I will put together an article on “how to capture DL OFDMA”


















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