＠NTUST, Taiwan

2021/07/07

# OFDMA in 802.11ax
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[TOC]
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**Goal:**
- [x] Show the background on OFDMA in 802.11ax:
- [x] The format of RU in 802.11ax
- [x] RU allocation
- [x] Uplink/Downlink OFDMA

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:::warning
Resources:
[Wi-Fi6 book - Extreme Network](https://www.extremenetworks.com/resources/ebook/wi-fi-6-for-dummies/)
[802.11 Frame Types and Formats](https://howiwifi.com/2020/07/13/802-11-frame-types-and-formats/)
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## Resource Unit (RU) format in 802.11ax


In 802.11ax, a 20MHz channel bandwidth is separated to 256 of 78.125 Khz subcarriers.

They are divided into RU tones:

- 9 tones of 26 subcarriers (with one 26-tone being separated in the middle);
- (or) 4 tones of 52 subcarriers;
- (or) 2 tones of 106 subcarriers;
- (or) 1 tone of 242 subcarriers.


Each tone can be used by an user to allocate RUs.

Therefore, we can have maximum 9 users, each of them use a 26-tone RU channel (~2MHz) to transmit data simutaneously.

Similarly, there are 40, 80 and 160 MHz channel bandwidth in 802.11ax with the corresponding number of tones.


## RU allocation in OFDMA

As mentioned, there are up to 9 clients (users/STAs) per a 20MHz bandwidth channel.

There are multiple users in one HE PPDU and its sizes could be different (26, 52, 106 or 242-tone RU).


In the diagram, AP triggers users to occupy RU slots. It called **RU allocation**.

So, each RU has its Subcarrier range and RU allocation bits, which helps AP determine the location and which user (STA) the RU belongs to.


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**Important note:**

The Wi-Fi CERTIFIED 6 certification program from the Wi-Fi Alliance currently validates up to four resource units.

Which means so far there are maximum four RUs can be allocated using OFDMA in Wi-Fi 6 standard.
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## Downlink OFDMA


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Downlink OFDMA Frame Exchange Process:

- MU-RTS (Extended trigger frame) is sent across the whole bandwidth so legacy clients understand
- AP wins TxOP to control the entire frame exchange
- CTS responses from the clients (on assigned RUs simutaneously)
- DL MU PPDU data transmissions from the AP to the OFDMA STAs
- After receiving their data via their assigned RUs, STA send a Block ACK to the AP.
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## Uplink OFDMA


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Uplink OFDMA Frame Exchange Process:

- AP sends a Buffer Status Report Poll (BSRP)
- STAs reply With synchronized Buffer Status Report (BSR) frames
- AP builds RU schedule with following information
- AP may send a trigger frame variant: MU-RTS
- STAs send CTS responses in parallel RUs
- AP sends a basic Trigger frame to allocate the RUs and time-sync
- STAs send uplink data on their assigned RUs
- Multi-User Block ACK from the AP
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### Key frame in 802.11ax OFDMA

From UL/DL OFDMA, we basically know that:
- Key frames sent from the AP to STAs include: Beacon, trigger (MU-RTS or BSRP), DL-PPDU (SU or MU), 802.11 Block ACK
- Key frames received in the AP (from STAs) include: trigger (BSR, CTS, RTS, UL-PPDU.

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To filter out trigger frame, we use :

wlan.trigger.he.trigger_type == <number of trigger type>

The possible number for the wlan.trigger.he.trigger_type filter:

0 – Basic
1 – Beamforming Report Poll (BRP)
2 – Multi-User Block ACK Request (MU-BAR)
3 – Multi-User Request To Send (MU-RTS)
4 – Buffer Status Report Poll (BSRP)
5 – Group Cast Retries (GCR MU-BAR)
6 – Bandwidth Query Report Poll (BQRP)
7 – NDP Feedback Report Poll (NFRP)
8-15 – Reserved
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