As far as I know, 802.11ax supported OFDMA modulation, which can separate different amount of sub-carriers to each device in each time slot. I want to figure out if there is unsupported 802.11ax devices in the same wireless network, all others 802.11ax devices should use OFDM? Or some time slots use OFDM and other time slots use OFDMA?
Both 802.11ax and non-802.11ax (legacy) station can coexist in a BSS that is controlled by an 802.11ax access point. Of course, OFDMA would work among only the 802.11ax stations (all the other legacy stations will simply back off). Similarly, when a legacy station gains channel access, it will transmit/receive over all the subcarriers across the entire channel using OFDM. Hence, 802.11ax and legacy stations' transmissions would never overlap.