Multi-User Wi-Fi: OFDMA vs MU-MIMO

Wi-Fi 6 can serve several clients in one transmission, in two different ways: OFDMA gives each client a slice of the channel's frequencies, MU-MIMO gives each client its own beam through the whole channel. See which wins, and why it depends on the traffic and on where clients are.

Split the frequencies, or split the space?

Without multi-user features an AP serves one client at a time, and every turn pays for waiting, a preamble and an acknowledgement. OFDMA cuts the channel into resource units (groups of subcarriers) and sends to several clients at once, each on its own slice: great when many clients each have a little data. MU-MIMO uses the AP's antennas to form a separate beam toward each client, so several clients share the whole channel at the same moment: great for big downloads, but only if the clients are far enough apart for their beams not to overlap, and only after the AP has measured every client's channel (sounding).

Drag the clients around the room, click one to switch its traffic (small packets or a big download) or its antennas (1×1 or 2×2), change the channel width, and compare the three methods.

Method shown in detail

Scenario

Channel width

AP antennas

Clicking a client switches its

Clients

1 · The room

● phone = big download○ phone = small packets1×1 / 2×2: the client's antennasDrag to move · click to switch traffic or antennasShaded wedges: MU-MIMO beams (red where two overlap)

2 · Who gets what

Waiting for the channel Sounding (MU-MIMO) Preamble Coloured: each client's data Padding (finished early, waiting for the others) Block Acks

3 · Scorecard

Sending every client's pending data once (one "round"), with each method, on the same clients.

4 · Side by side

	OFDMA	MU-MIMO
What it divides	Frequency: each client gets its own resource unit (group of subcarriers)	Space: each client gets its own beam through the whole channel
Each client's rate	Lower (a narrower slice), but everyone starts at once	Up to the full channel width, minus the streams it gives up and any interference
Best for	Many clients with small packets: IoT, voice, gaming, chat; cutting latency	A few clients with big, steady downloads, spread out around the AP
Needs	Wi-Fi 6 AP and clients. No extra antennas, no sounding	More AP antennas than streams per client; clients well apart; sounding before sending
Overhead	A longer multi-user preamble and a trigger for the acknowledgements; padding when clients finish at different times	Sounding (announcement, NDP and a report from every client), repeated every few tens of milliseconds as clients move
Users at once	Up to 9 per 20 MHz, 37 per 80 MHz, 74 per 160 MHz	Up to 8 in Wi-Fi 6 (4 in Wi-Fi 5), and never more streams than AP antennas
By generation	Wi-Fi 6: downlink and uplink. Wi-Fi 7: several RUs per client (multi-RU)	Wi-Fi 5: downlink only. Wi-Fi 6: downlink and uplink. Wi-Fi 7: same, still up to 8 streams in total (16 was proposed but dropped from 802.11be)

They're not rivals: Wi-Fi 6 and 7 can also run MU-MIMO inside large resource units (106 tones or more in Wi-Fi 6, 242 or more in Wi-Fi 7) of an OFDMA transmission, though few APs do it often.

Notes and simplifications

Radio: Wi-Fi 6, 0.8 µs guard interval. Each client's signal-to-noise ratio falls with distance from the ceiling AP (3 m up), and by 3 dB each time the channel width doubles (the same power spread over more noise); the MCS is the highest the SNR supports.
Client antennas: a 2×2 client can take 2 streams, a 1×1 client only 1. Single-user and OFDMA send each client as many streams as it can take, so with a 1×1 client most of a 4- or 8-antenna AP sits idle during its turn. MU-MIMO gives each client 1 stream, or 2 when the AP has antennas to spare, and fills the AP's other antennas with other clients; the AP's power is shared by all streams.
Channel width: OFDMA picks the largest equal resource unit that fits every client (20 MHz: 242 / 106 / 52 / 26 tones; each doubling of width doubles the choices). Sounding reports grow with width, because each client reports its channel for every group of subcarriers.
Beams: MU-MIMO interference is modelled from the angle between clients as seen from the AP: with 4 antennas clients need roughly 90° between them for clean beams, with 8 antennas about 45°. Real separation also depends on reflections and distance, and precoding is never perfect.
Timing: each transmission waits AIFS + average backoff (110 µs). Preambles: single-user 36 µs + 7.2 µs per training field; multi-user adds HE-SIG-B (grows with users). Block Acks: 40 µs single-user, a 60 µs trigger-based response for multi-user. Transmissions are limited to about 5.3 ms, so long data is split. Sounding (NDPA, NDP, trigger and every client's report) is estimated at about 180 µs + 8 µs per AP antenna + (200 µs + 30 µs per client) × width ÷ 80 MHz, done once per round here; real APs repeat it every 10–50 ms.
Traffic: "small" is one 300-byte packet, "download" is a 64 KB burst. Only downlink is shown. OFDMA resource units are equal-sized; real schedulers size them to each client's queue.
Background reading (Thai): OFDMA vs. MU-MIMO: ความเหมือนที่แตกต่าง.