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Wi-Fi Pro Lab › Modulation and waveforms

OFDMA: sharing a channel between clients

OFDM gives the whole channel to one device at a time. OFDMA, new in Wi-Fi 6, splits the channel into slices so that several devices can use it in the same moment.

26 to 996tones in a resource unit (RU)
926-tone RUs in a 20 MHz channel
78.125 kHzsubcarrier spacing in 802.11ax
12.8 + 0.8 µssymbol plus shortest guard interval

1. DefinitionWhat is OFDMA?

Orthogonal frequency-division multiple access assigns groups of OFDM subcarriers to different users. A group is a resource unit (RU). 802.11ax defines RUs of 26, 52, 106, 242, 484 and 996 tones (with a 2 × 996 for 160 MHz and 4 × 996 for 320 MHz in 802.11be). Each RU carries data and pilot subcarriers: a 26-tone RU has 24 data subcarriers and 2 pilots; 52-tone, 48 and 4; 106-tone, 102 and 4; 242-tone, 234 and 8; 484-tone, 468 and 16.

  • Resource unit: a block of subcarriers assigned to one client.
  • Trigger frame: in uplink OFDMA, the access point sends a trigger frame that tells each client which RU to use and when, so all uplink frames start together.
  • Scheduled access: the access point decides who gets which RU, instead of every device contending separately.

2. PurposeWhy share a channel?

A modern network has many devices each sending small packets: chat, sensors, acknowledgements. In OFDM each one pays the full cost of getting the channel (waiting, backoff, preamble, acknowledgement) to send a few hundred bytes. OFDMA lets the access point serve several of them in one transmission and spreads that fixed cost between them, which cuts delay and wasted airtime in crowded places. It helps least when one client is sending a large file.

3. MechanismSlice the channel

Choose a channel width and how to divide it. The chart shows the resource units across the channel. The bar chart compares the time needed to send the same small amount of data to every client one at a time (OFDM) with sending to all of them together in resource units (OFDMA). The fixed cost per transmission is yours to change.

Try this. (1) Choose 9 x 26 and a small payload of about 100 to 200 bytes: OFDMA wins by a large margin. (2) Raise the payload towards 1500 bytes: the gap narrows, because each client now needs the data time of a narrow slice. (3) Choose 1 x 242: with one client there is nothing to share.

4. VisualWhere each client sits

A 20 MHz channel has 256 subcarrier positions at 78.125 kHz spacing (the 802.11ax OFDM uses 4 times as many subcarriers as 802.11a/g, each 4 times narrower). A 242-tone RU covers the whole usable channel; two 106-tone RUs and a 26-tone RU, four 52-tone RUs and a 26-tone RU, or nine 26-tone RUs cover it in smaller pieces. A 40 MHz channel holds 18 of the 26-tone RUs. Compare the layouts in the model above.

5. MathematicsThe rate of a resource unit

  • Rate of an RU = data subcarriers × bits per subcarrier × coding rate × spatial streams / symbol time.
  • Symbol time in 802.11ax is 12.8 µs plus a guard interval of 0.8, 1.6 or 3.2 µs. With 0.8 µs that is 13.6 µs.
  • Example: a 26-tone RU at MCS 7 (64-QAM, 5/6), one stream: 24 × 6 × 5/6 / 13.6 µs = 8.8 Mbit/s. The whole 20 MHz channel (242-tone RU) at MCS 11 (1024-QAM, 5/6): 234 × 10 × 5/6 / 13.6 µs = 143.4 Mbit/s.
  • Sharing: nine users on 26-tone RUs at MCS 7 get 8.8 Mbit/s each, 79.4 Mbit/s in total, which is a little lower than the 86.0 Mbit/s of one user on the whole channel at the same MCS (234 × 6 × 5/6 / 13.6 µs), because each resource unit carries its own pilots: sharing gives up a little peak speed in exchange for waiting less and for the saved overheads.

6. Real-world behaviourWhat to expect

  • Both sides must support it. OFDMA needs an 802.11ax or 802.11be access point and clients. Older clients still use ordinary OFDM and are served separately.
  • The benefit is for many small flows. Offices, classrooms, stadiums and homes full of small devices see it most; a single speed test does not.
  • Uplink is scheduled. Because clients wait for a trigger frame, uplink OFDMA can reduce collisions; but triggers also have their own overheads.
  • Wi-Fi 7 refines it with multi-RU allocation, where one client can be given several RUs, and with puncturing around interference.

Practical validationProve it in a capture and on a spectrum analyser

Reading about a band is one thing; seeing it in your own evidence is the skill. These checks work with any monitor-mode capture:

  • In a capture of an 802.11ax network, look for trigger frames sent by the access point. They are control frames; the Wireshark display filter reference lists the HE fields.
  • Compare airtime used by many small flows on an 802.11n versus an 802.11ax access point.
  • Question to answer: why does OFDMA not make a single large download faster?

Check your understandingQuick quiz

ReferencesSources and further reading

802.11ax numerology (resource units, 78.125 kHz spacing, 12.8 µs symbols) is arithmetically derived from the 256-point FFT and is defined in IEEE 802.11ax; it is not in the 2020 edition of the base standard. The time comparison in the model uses a fixed cost per transmission that you can change; it illustrates the effect and is not a measured result.