OFDMA is a key enhancement in Wi-Fi 6 and Wi-Fi 7, enabling efficient multi-user transmissions by dividing a Wi-Fi channel into smaller resource units (RUs). This allows an access point (AP) to coordinate uplink and downlink communication with multiple clients simultaneously, improving efficiency and reducing latency. OFDMA enhances network performance, supports better frequency reuse, and reduces contention for the medium. In this blog, we will learn how DL OFDMA will work, how subcarriers are allocated to each client, and how to check the DL OFDMA in the PCAP.
Terminologies
1. Subcarriers
A subcarrier is a smaller frequency channel within a larger wireless communication channel.
In Simple Terms:
-
- Think of the total frequency bandwidth (e.g., 20 MHz) as a highway.
- This highway is divided into many lanes — these are the subcarriers.
- Each subcarrier can carry its own data stream.
- Multiple subcarriers are used together to send data efficiently and simultaneously.
Example: In a 20 MHz Wi-Fi channel:
- There are typically 256 subcarriers.
- Some are used for actual data.
- Others are for control, synchronization, or left unused as guard bands.
So, subcarriers are like tiny, parallel data lanes that together make wireless communication faster and more efficient.
2. Resource Units (RUs)
In 802.11n/ac, when an AP transmits downstream to clients over an OFDM channel, the entire channel frequency space is utilized for each individual transmission.
OFDMA channel consists of a total of 256 subcarriers (tones). These tones can be grouped into smaller subchannels known as Resource Units (RUs).
Medium contention rules still apply, requiring the AP to compete with legacy clients for a transmission opportunity (TXOP). Once the AP secures a TXOP, it controls multiple Wi-Fi 6 and WiFi 7 clients for downlink or uplink transmissions. The number of resource units (RUs) allocated can vary per TXOP, with the AP dynamically assigning RUs to maximize transmission efficiency.
3. Physical Protocol Data Unit (PPDU)
It contains both PLCP and PSDU (Control and data)
4. High Efficiency [HE]
High Efficiency” in the context of 802.11ax
5. Extremely High Throughput [EHT]
EHT in the context of 802.11be
6. MU-RTS Trigger and CTS Frame
The MU-RTS Trigger/CTS frame exchange procedure allows an AP to initiate a TXOP and protect the TXOP frame exchanges. An AP may transmit an MU-RTS Trigger frame to solicit simultaneous CTS frame responses from one or more non-AP STAs.
7. MU-RTS Trigger Frame Transmission
In each 20 MHz channel occupied by the PPDU that contains an MU-RTS Trigger frame, the transmitter of the MU-RTS Trigger frame shall request at least one client to send a CTS frame response that occupies the 20 MHz channel.
What is OFDMA?
OFDMA is a multi-user version of OFDM (Orthogonal Frequency Division Multiplexing). It divides the wireless channel into multiple smaller sub-channels (called Resource Units or RUs) and assigns them to different users at the same time, allowing for parallel data transmission. Whereas Legacy 802.11a/g/n/ ac radios currently use orthogonal frequency division multiplexing (OFDM) for single-user transmissions on any given channel.
How Does DL OFDMA Work?
OFDMA is exclusively used for 802.11 data frame exchanges between APs and clients. It is not utilised for management or control frames.
AP must first contend for the medium and secure a TXOP before initiating a DL-OFDMA frame exchange. The AP may transmit a multi-user request-to-send (MU-RTS) frame to optimize medium access and reduce interference. The AP dynamically divides the channel into RUs of varying sizes (e.g., 26-tone, 52-tone, 106-tone, 242-tone, 484-tone, 996-tone, and 2×996-tone RUs) based on data demand. Wi-Fi 7 enables clients to be assigned multiple RUs, allowing them to receive data over non-contiguous RUs, increasing data throughput. Following the simultaneous CTS responses from the clients, the AP initiates multi-user DL-PPDU transmissions to the OFDMA-capable clients. Note that the AP has decided how to allocate the 20 MHz channel into multiple RUs. After receiving their data through the assigned RUs, the clients must send a Block ACK to the AP. The AP first transmits a Block ACK Request (BAR) frame, prompting clients to respond with Block ACKs simultaneously. Alternatively, clients may send an automatic Block ACK in parallel.
Downlink OFDMA.
Analyzing RU allocation in HE MU PPDU:-
Note:- HE-MU: MU-PPDU is used for DL MU-OFDMA transmission.
1. HE-SIG-A Field:
HE-SIG-A field carries information necessary to interpret HE PPDU’s. HE-SIG-A has different content depending on the type of frame.
For example, HE SU PPDU, HE ER SU PPDU and HE MU PPDU’s will have different content in HE-SIG-A field. We will only concentrate on MU PPDU’s which are part of MU-OFDMA transmission.
2. HE-SIG-B Field:
SIG-B field provides necessary information regarding OFDMA and DL MU-MIMO resource allocation.This field only exists in multi-user transmission.
So, it is safe to say most of HE information data in the PCAP belongs to SIG-A.
Single-user transmission has only HE information field in the PPDU as shown in the figure below.
Single user PPDU
HE-MU information, which is only available in MU-PPDU. So, most of this data represents SIG-B as shown in the figure below.
Multi user PPDU
Rules governing HE SIG-B fields related to OFDMA:-
An AP that transmits an HE MU PPDU shall set the UL/DL field in the HE-SIG-A field to 0.
The HE-SIG-B field consists of one or two HE-SIG-B content channels, with each HE-SIG-B content channel conveying user allocation for one or more 20 MHz subchannels. A 20 MHz HE MU PPDU has one HE-SIG-B content channel, while an HE MU PPDU with greater than 20 MHz PPDU bandwidth has two HE-SIG-B content channels.
If the value of the HE-SIG-B Compression field in the HE-SIG-A field is 0, then the RU Allocation subfield in the Common field in each HE-SIG-B content channel indicates the combination of RUs in the current PPDU and the number of User fields included in the corresponding HE-SIG-B content channel for each RU.
The RU allocation for the above figure is 484 RU and we can also see the supported bandwidth, ie,. bandwidth from Bandwidth field in SIG-A: 3 if it sets the value 3 means 160Mhz
Multi Resource Unit (MRU) Support in EHT PPDUs: Small and Large MRU Configurations in OFDMA
Now, let’s explore Wi-Fi 7’s MRU (Multiple Resource Unit) feature in OFDMA, a newly introduced enhancement in Wi-Fi 7
The EHT PHY allows the use of MRUs in an EHT PPDU. An MRU is formed by combining multiple RUs, including 26-tone, 52-tone, 106-tone, 242-tone, 484-tone, 996-tone, and 2×996-tone RUs.
RUs that are the same size as or larger than 242-tone RUs are defined as large size RUs and RUs that are smaller than 242-tone RUs are defined as small size RUs.
1. Small Size MRUs
The small size MRUs defined for DL and UL OFDMA transmissions are as follows: 52+26-tone MRU and 106+26-tone MRU.
The 52+26-tone MRU is formed by combining a 52-tone RU and an adjacent 26-tone RU within the same 20 MHz channel. Its data subcarriers are the combined data subcarriers of both RUs, while its pilot subcarriers are the union of the pilot subcarriers from the 52-tone and 26-tone RUs.
Allowed 52+26-tone MRUs in an OFDMA 20 MHz EHT PPDU
The 106+26-tone MRU is formed by combining a 106-tone RU and an adjacent 26-tone RU within the same 20 MHz channel. Its data subcarriers are the combined data subcarriers of both RUs, while its pilot subcarriers are the union of the pilot subcarriers from the 106-tone and 26-tone RUs.
Allowed 106+26-tone MRUs in an OFDMA 20 MHz EHT PPDU
2. Large Size MRUs
The large size MRU defined for DL and UL OFDMA transmissions are as follows: 484+242-tone MRU,996+484-tone MRU, 2×996+484-tone MRU, 3×996-tone MRU, and 3×996+484-tone MRU.
The 484+242-tone MRU is permitted in OFDMA 80 MHz, 160 MHz, and 320 MHz EHT PPDUs. It is formed by combining a 484-tone RU and a 242-tone RU within an 80 MHz frequency subblock. Its data subcarriers are the combined data subcarriers of both RUs, while its pilot subcarriers are the union of the pilot subcarriers from the 484-tone and 242-tone RUs.
Allowed 484+242-tone MRUs in a non-OFDMA 80 MHz EHT PPDU
Analyzing RU allocations in EHT PPDU:-
-
- Rules governing the allocation in 11be:-
If the UL/DL subfield in the U-SIG is set to 0, then it indicates the PPDU is sent to a STA. If a value of 1 indicates the PPDU is sent to an AP.
And also in the EHT MU PPDU, if the PPDU Type and Compression Mode are set the value to 0, then it indicates a DL OFDMA transmission.
Note:- The below tables explain the bits needed to set in EHT MU PPDU’s for OFDMA, MUMIMO etc..
U-SIG field of an EHT MU PPDU
U-SIG field of an EHT MU PPDU
Combination of UL/DL and PPDU type and compression mode field
The figures below illustrate the bit configurations used to enable DL-OFDMA transmissions
- RU allocation for DL PPDU data frame
In the RU Allocation B7-B1: Bites are assigned as 0x41 this is in hexadecimal value convert the hexadecimal value to decimal value ie, 0x41 = 65.
Refer the RU ALLocation for the User to check the RU Allocation for the client, the assigned RU Allocation B7-B1 65 is 484 tones.
The figures below illustrate RU allocations for a STA.
In this blog, we explored the key differences between OFDM and OFDMA, gained an understanding of how DL OFDMA operates in WiFi 7, and learned how to verify the assigned RU allocation in a PCAP for a specific WiFi 7 client. Stay tuned for our next blog, where we will dive into how Uplink OFDMA (UL OFDMA) works.
Appendix:-
Understanding The RU Allocation In WiFi 7
The EHT variant User Info field is defined for all Trigger frame variants except the NFRP Trigger frame and the MU-RTS TXS Trigger frame.
EHT variant user info field format
In a Trigger frame that is not an MU-RTS Trigger frame, the RU Allocation subfield in the EHT variant User Info field, along with the UL BW subfield in the Common Info field, the UL BW Extension subfield in the Special User Info field, and the PS160 subfield in the EHT variant User Info field, determines the size and location of the RU or MRU. The mapping of B7–B1 of the RU Allocation subfield along with the settings of B0 of the RU Allocation subfield and PS160 subfield in the EHT variant User Info field are defined in below table, where the bandwidth is determined by combining the UL BW subfield and the UL Bandwidth Extension subfield.
| PS160 subfield | B0 of the RU Allocation subfield | B7–B1 of the RU Allocation Subfield | Bandwidth (MHz) | RU or MRU Size | RU or MRU Index | PHY RU or MRU Index |
|---|---|---|---|---|---|---|
| 0–3: | -- | 0–8 | 20, 40, 80, 160, or 320 | 26 | RU1 to RU9, respectively | -- |
| 80 MHz frequency subblock where the RU is located | -- | 9–17 | 40, 80, 160, or 320 | 26 | RU10 to RU18, respectively | 37×N+RU index |
| (See NOTE 1) | -- | 18 | 80, 160, or 320 | 26 | Reserved | -- |
| -- | -- | 19–36 | 80, 160, or 320 | 26 | RU20 to RU37, respectively | -- |
| -- | -- | 37–40 | 20, 40, 80, 160, or 320 | 52 | RU1 to RU4, respectively | -- |
| -- | -- | 41–44 | 40, 80, 160, or 320 | 52 | RU5 to RU8, respectively | 16×N*+RU index |
| -- | -- | 45–52 | 80, 160, or 320 | 52 | RU9 to RU16, respectively | -- |
| -- | -- | 53, 54 | 20, 40, 80, 160, or 320 | 106 | RU1 and RU2, respectively | -- |
| -- | -- | 55, 56 | 40, 80, 160, or 320 | 106 | RU3 and RU4, respectively | 8×N*+RU index |
| -- | -- | 57–60 | 80, 160, or 320 | 106 | RU5 to RU8, respectively | -- |
| -- | -- | 61 | 20, 40, 80, 160, or 320 | 242 | RU1 | -- |
| -- | -- | 62 | 40, 80, 160, or 320 | 242 | RU2 | 4×N*+RU index |
| -- | -- | 63, 64 | 80, 160, or 320 | 242 | RU3 and RU4, respectively | -- |
| -- | -- | 65 | 40, 80, 160, or 320 | 484 | RU1 | 2×N*+RU index |
| -- | -- | 66 | 80, 160, or 320 | 484 | RU2 | -- |
| -- | -- | 67 | 80, 160, or 320 | 996 | RU1 | N*+RU index |
| 0–1: | 0 | -- | 20, 40, 80, 160, or 320 | Reserved | Reserved | Reserved |
| 160 MHz segment where the RU is located (See NOTE 3) | 1 | 68 | 160 or 320 | 2u996 | RU1 | X1 + RU index |
| 0 | 0 | -- | -- | -- | -- | -- |
| 0 | 1 | 69 | 20, 40, 80,160, or 320 | Reserved | Reserved | Reserved |
| 1 | 0 | -- | -- | -- | -- | -- |
| 1 | 1 | -- | 320 | 4x996 | RU1 | RU1 |
| 0–3: | -- | 70 | 20, 40 | 52+26 | MRU1 | -- |
| 80 MHz frequency subblock where the MRU is located (See NOTE 1) | -- | -- | 80, 60, or 320 | Reserved | Reserved | -- |
| -- | -- | 71, 72 | 20, 40, 80,160, or 320 | 52+26 | RU1 to RU9, respectively | -- |
| -- | -- | 73-74 | 40, 80, 160, or 320 | 52+26 | MRU4 and MRU5, respectively | 12xN+MRU index |
| -- | -- | 75 | 40 | 52+26 | MRU6 | -- |
| -- | -- | -- | 80, 160, or 320 | Reserved | Reserved | -- |
| -- | -- | 76 | 20, 40, 80,160, or 32 | Reserved | Reserved | -- |
| -- | -- | 77–80 | 80, 160, or 320 | 52+26 | MRU8 to MRU11, respectively | -- |
| -- | -- | 81 | 20, 40, 80, 160, or 320 | Reserved | Reserved | -- |
| -- | -- | 82 | 20, 40, 80, 160, or 320 | 106+26 | MRU1 | -- |
| -- | -- | 83 | 20, 40 | 106+26 | MRU2 | -- |
| -- | -- | -- | 80, 160, or 320 | Reserved | Reserved | -- |
| -- | -- | 84 | 40 | 106 + 26 | MRU3 | 8xN+MRU index |
| -- | -- | -- | 80, 160, or 320 | Reserved | Reserved | -- |
| -- | -- | 85 | 40, 80, 160, or 320 | 106+26 | MRU4 | -- |
| -- | -- | 86 | 80, 160, or 320 | 106+26 | MRU5 | -- |
| -- | -- | 87–88 | 20, 40, 80, 160, or 320 | Reserved | Reserved | -- |
| -- | -- | 89 | 80, 160, or 320 | 106+26 | MRU8 | -- |
| -- | -- | 90–93 | 80, 160, or 320 | 484+242 | MRU1 to MRU4, respectively | 4xN+MRU index |
| 0-1: | 0 | -- | -- | -- | MRU1 and MRU2, respectively | -- |
| 160 MHz segment where the MRU is located (See NOTE 3) | 1 | 94, 95 | 160 or 320 | 996+484 | MRU3 and MRU4, respectively | 4xX1 + MRU index |
| 0: | 0 | -- | 160 | 996+484+242 | MRU1 to MRU4, respectively | MRU index |
| MRU is located in the primary 160 MHz | 1 | Any | 20, 40, 80, 160, or 320 | Reserved | Reserved | Reserved |
| 0 | 0 | 100-103 | 320 | 2x996+484 | MRU1 to MRU4, respectively | -- |
| 0 | 1 | 100-101 | -- | -- | MRU5 and MRU6, respectively | -- |
| 0 | 1 | 102-103 | 20, 40, 80, 160, or 320 | Reserved | Reserved | MRU index |
| 1 | 0 | 100-101 | 20, 40, 80, 160, or 320 | Reserved | Reserved | -- |
| 1 | 0 | 102-103 | 320 | 2x996+484 | MRU7 and MRU8, respectively | -- |
| 1 | 1 | 100-103 | -- | -- | MRU9 to MRU12, respectively | -- |
| 0 | 0 | -- | -- | MRU1 | -- | -- |
| 0 | 1 | 104 | 320 | 3x996 | MRU2 | MRU index |
| 1 | 0 | -- | -- | -- | MRU3 | -- |
| 1 | 1 | -- | -- | -- | MRU4 | -- |
| 0 | 0 | -- | -- | -- | MRU1 and MRU2, respectively | -- |
| 0 | 1 | 105, 106 | 320 | 3x996+484 | MRU3 and MRU4, respectively | MRU index |
| 1 | 0 | -- | -- | -- | MRU5 and MRU6, respectively | -- |
| 1 | 1 | -- | -- | -- | MRU7 and MRU8, respectively | -- |
| Any | Any | 107–127 | 20, 40, 80,160 and 320 | Reserved | Reserved | Reserved |
Note 1— B0 of the RU Allocation subfield is set to 0 to indicate that the RU or MRU allocation applies to the primary 80 MHz channel and set to 1 to indicate that the RU allocation applies to the secondary 80 MHz channel in the primary 160 MHz, if PS160 subfield is equal to 0 and the RU or MRU size is smaller than or equal to 996 tones. B0 of the RU Allocation subfield is set to 0 to indicate that the RU or MRU allocation applies to the lower 80 MHz in the secondary 160 MHz and is set to 1 to indicate that the RU or MRU allocation applies to upper 80 MHz in the secondary 160 MHz, if PS160 subfield is equal to 1 and the RU or MRU size is smaller than or equal to 996 tones.
Note 2— The PHY MRU index of a 52+26-tone MRU is not defined in the case of the MRU index equal to 1, 6, 7, or 12, if the bandwidth indicates 80, 160, or 320 MHz. The PHY MRU index of a 106+26-tone MRU is not defined in the case of the MRU index equal to 2, 3, 6, or 7, if the bandwidth indicates 80, 160, or 320 MHz. Refer to Small size MRUs.
Note 3— If the size of RU or MRU is smaller than or equal to 2u996 tone, then the PS160 subfield is set to 0 to indicate the RU or MRU allocation applies to the primary 160 MHz channel and set to 1 to indicate the RU or MRU allocation applies to the secondary 160 MHz channel. Otherwise, the PS160 subfield is used to indicate the RU or MRU index along with the RU Allocation subfield.
Note 4— The PHY RU or MRU index in this table indicates the allocated RU or MRU index defined in Subcarrier and resource allocation.
