Summary: <br> 802.11ax (Wi-Fi 6) brings OFDMA to wireless. It’s an enhancement over OFDM which was a single-user transmission. When a signal is sent or received it is done with one device. In OFDMA, it allows multiple access which means simultaneous transmissions to/from multiple devices.<br> <br> <br> <br> There is a downlink multi-user operation and an uplink multi-user operation. <br> <br> <br> <br> In OFDMA, a channel is subdivided into smaller channels, or resource units. This is so there can be simultaneous transmissions to different devices. Most transmissions are small frames so it’s an efficient way to send data by using a smaller channel and by making it multiple access we can have more communications at the same time.<br> <br> <br> <br> These subcarriers (tones), the smaller channels of the main channel, are called resource units. An AP can allocate varying resource units for multi-user communications.<br> <br> <br> <br> For example, a 20 MHz channel has 242 resource units which can be further split into 2x 106 resource units, 4x 52 resource units, or 9x 26 resource units.<br> <br> <br> <br> Resource Units in a 20 MHz channel width<br> <br> <br> <br> OFDMA allows subcarriers to be allocated to different devices for simultaneous transmission to or from those devices.<br> <br> <br> <br> OFDMA transmissions in DL and UL allow different stations to occupy different RUs in a PPDU. Within that RU it could be SU-MIMO or MU-MIMO.<br> <br> <br> <br> Resource Units (RUs) are defined for DL and UL transmissions and labeled as different tones. RUs are defined as:<br> <br> <br> <br> * 26-tone RU* 52-tone RU* 106-tone RU* 242-tone RU* 484-tone RU* 996-tone RU* 2×996-tone RU<br> <br> <br> <br> Number of 802.11ax (Wi-Fi 6) OFDMA Resource Units per channel bandwidth:<br> <br> <br> <br> RU TypeCBW20CBW40CBW80CBW80+80 & CBW16026-tone RU918377452-tone RU481634106-tone RU24816242-tone RU1248484-tone RUn/a124996-tone RUn/an/a122×996-tone RUn/an/an/a1<br> <br> <br> <br> Type of subcarriers:<br> <br> <br> <br> * Data subcarriers* Pilot subcarriers* DC subcarriers* Guard subcarriers* Null subcarriers<br> <br> <br> <br> 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.<br> <br> <br> <br> DC 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.<br> <br> <br> <br> A 20MHz wide channels has 11 guard interval: the first 6 and the last 5 of the channel.<br> <br> <br> <br> Downlink OFDMA<br> <br> <br> <br> An AP can transmit frames to different devices by splitting a channel into subchannels or subcarriers or resource units.<br> <br> <br> <br> Devices tune their radios to the specific resource unit to receive their transmissions. The AP still has to contend for airtime but will allocate resource units for different devices.<br> <br> <br> <br> Uplink OFDMA<br> <br> <br> <br> Similar to DL OFDMA, except devices transmit at the same time on different subchannels within the same channel (RUs). The use of trigger frames by the AP must be used in order to coordinate transmissions. <br> <br> <br> <br> AP solicits simultaneous response frames from multiple HE devices. If a client does not support TRS Control,
