Resource Unit

In OFDMA, a symbol is constructed of subcarriers where the total number defines a Physical Layer PDU bandwidth.

Each user is assigned different subsets of subcarriers to achieve simultaneous data transmission in MU (Multi-Users) environment.

For example, in 20 MHz OFDMA bandwidth there is a total of 256 subcarriers (tones) which are grouped in sub-channels (or Resource Units).

This scheme increases efficiency, compared to older method (802.11ac) where only a single user can transmit (uplink transmission) at a time (it uses the whole allocated 20 MHz bandwidth), while the rest must wait their turn.

To achieve that, both management and control frames must still be transmitted over standard OFDM subcarriers of the entire primary channel as well by both stations.

Each non-AP station receiving the trigger frame sends its own HE TB (Trigger-Based) PPDU back to AP using the assigned RUs.