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A 20 MHz OFDMA channel consists of a total of 256 subcarriers (tones). These tones are grouped into smaller sub-channels, known as resource units (RUs). As shown in Figure 1, when subdividing a 20 MHz channel, an 802.11ax access point designates 26, 52, 106, and 242 subcarrier resource units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels, respectively. The 802.11ax access point dictates how many RUs are used within a 20 MHz channel, and different combinations can be used.
Figure 1- OFDMA resource units – 20 MHz channel
An 802.11ax AP may allocate the whole channel to only one client at a time, or it may partition the OFDMA channel to serve multiple clients simultaneously. For example, an 802.11ax AP could simultaneously communicate with one 802.11ax client using 8 MHz of frequency space while communicating with three additional 802.11ax clients using 4 MHz sub-channels. These simultaneous communications can be either downlink or uplink.
Figure 2 – OFDMA transmissions over time
In the example shown in Figure 2, the 802.11ax AP first simultaneously transmits downlink to 802.11ax clients 1 and 2. The 20 MHz OFDMA channel is effectively partitioned into two sub-channels. Remember that an ODFMA 20 MHz channel has a total of 256 subcarriers; however, the AP simultaneously transmitted to clients 1 and 2 using two different 106-tone resource units. In the second transmission, the AP simultaneously transmits downlink to clients 3, 4, 5, and 6. In this case, the ODFMA channel had to be partitioned into four separate 52-tone sub-channels. In the third transmission, the AP uses a single 242-tone resource unit to transmit downlink to a single client (5). Using a single 242-tone resource unit is effectively using the entire 20 MHz channel. In the fourth transmission, the AP simultaneously transmits downlink to clients 4 and 6 using two 106-tone resource units. In the fifth transmission, the AP once again transmits only downlink to a single client, with a single 242-tone RU utilizing the entire 20 MHZ channel. In the sixth transmission, the AP simultaneously transmits downlink to clients 3, 4, and 6. In this instance, the 20 MHz channel is partitioned into three sub-channels; two 52-tone RUs are used for clients 3 and 4, and a 106-tone RU is used for client 6.
For backward compatibility, 802.11ax radios will still support OFDM. Keep in mind that 802.11 management and control frames will still be transmitted at a basic data rate using OFDM technology that 802.11a/g/n/ac radios can understand. Therefore, the transmission of management and control frames will be transmitted using the standard 64 OFDM subcarriers of an entire primary 20 MHz channel. OFDMA is only for 802.11 data frame exchanges between 802.11ax APs and 802.11ax clients. Please check back every week and read future 802.11ax blogs, where we will discuss in more detail the mechanisms of ODMA including resource unit allocation and trigger frames. We also discuss the differences between downlink OFDMA and uplink OFDMA.