Compare 802.11ax OFDMA against single-user transmissions: resource units per channel, users per TXOP and the airtime saved.
OFDMA splits a channel into resource units so several clients are served in one transmission, paying the preamble, contention and acknowledgement overhead once instead of once per client. Each client gets a narrower slice, so its payload takes proportionally longer, and the saving comes entirely from amortising the fixed overhead. OFDMA only pays off for small frames from many clients — the exact profile of voice, telemetry and IoT traffic — and for a handful of large downloads it makes things slightly worse.
OFDMA Efficiency
resource units = data subcarriers ÷ RU size; OFDMA airtime = ceil(clients ÷ RUs) × (fixed overhead + frame airtime ÷ RU's share of the channel).
resource units = data subcarriers ÷ RU size; OFDMA airtime = ceil(clients ÷ RUs) × (fixed overhead + frame airtime ÷ RU's share of the channel). OFDMA splits a channel into resource units so several clients are served in one transmission, paying the preamble, contention and acknowledgement overhead once instead of once per client. Each client gets a narrower slice, so its payload takes proportionally longer, and the saving comes entirely from amortising the fixed overhead.
OFDMA only pays off for small frames from many clients — the exact profile of voice, telemetry and IoT traffic — and for a handful of large downloads it makes things slightly worse.
This calculator takes 5 inputs: Channel width, Resource unit size, Clients with data queued, Airtime for one frame at full channel width, Fixed overhead per transmission. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.