With OFDMA in 802.11ax, the size of the subcarriers has been divided by 4. Going from 312.5KHz wide with OFDM to 78.125KHz wide.
The symbol duration has been increased by 4 times in the meantime. Going from 3.2 microseconds with OFDM to 12.8 microseconds.
Allow OFDMA to extend to small sub-channels. Each sub-channel requires at least one (usually two) pilot subcarriers, and with a 2 MHz minimum sub-channel size, a smaller subcarrier spacing loses a much smaller percentage of the overall bandwidth to pilots.The number of guard and null subcarriers across a channel can be reduced as a percentage of the number of usable subcarriers, again increasing the effective data rate in a given channel. The figures above show a ~10% increase in usable subcarriers compared to 802.11ac, after allowing for the 4x factor. Example: OFDM: 64 subcarriers, 12 GuardNull subcarriers = 18.75%, OFDMA: 256 subcarriers. 22 GuardNull subcarriers = 8.5%.The longer OFDM symbol allows for an increase in the cyclic prefix length without sacrificing spectral efficiency, which in turn enables increased immunity to long delay spreads, especially in outdoor conditions. The cyclic prefix can be reduced to a smaller percentage of the symbol time, increasing spectral efficiency even while more robust to multipath conditions. And it reduces the jitter-sensitivity of uplink multi-user modes.
The smallest sub-channel is composed of 26 subcarriers.
Type of subcarriers:
Data subcarriersPilot subcarriersDC subcarriersGuard subcarriersNull subcarriers
A 26-tone RU consists of 24 data subcarriers and 2 pilot subcarriers.
A 52-tone RU consists of 48 data subcarriers and 4 pilot subcarriers.
A 106-tone RU consists of 102 data subcarriers and 4 pilot subcarriers.
A 242-tone RU consists of 234 data subcarriers and 8 pilot subcarriers.
A 484-tone RU consists of 468 data subcarriers and 16 pilot subcarriers.
A 996-tone RU consists of 980 data subcarriers and 16 pilot subcarriers.
DC (Direct Current) subcarriers are used for the subcarriers located in the center of the channel. Depending on the channel width and the number of tone used, the number of DC subcarriers can vary (Ex: 3 or 7 for a 20MHz wide channel). Most of the time it will be 7 for the 20MHz and 80MHz wide channels and 5 for the 40MHz wide channels.
A 20MHz wide channels has 11 guard interval: the first 6 and the last 5 of the channel.
Here are the diagrams extracted from the 802.11ax draft document detailing the structure of the subcarriers for each channel width using different RUs sizes.