802.11ax introduce the use of Orthogonal Frequency Division Multiple Access (OFDMA) Allows the multiplexing of multiple users in the frequency domain. A departure from the use of the OFDM where all resources are assigned to a single user as in previous IEEE 802.11 amendments. Support of OFDMA is both for the Uplink (UL) and the Downlink (DL) Supporting Triggered UL MU MIMO DL MU MIMO support is already in IEEE 802.11ac. Allows multiplexing of multiple users in the spatial domain The use of 256 FFT (20 MHz Channel) for the data portion of the 802.11ax PPDU. A departure from the 64 FFT used in previous IEEE 802.11 amendments. Pre-defined resource unit (RU) sizes Four frame formats Allows Spatial Reuse MCS 10 and MCS 11 introducing 1024 QAM
Listen Interval indicates how long the station will be "sleeping" without listen to any Beacon transmitted by the AP's when the station enter in power save mode, those beacons includes the DTIM information. The relevant point here is that if we don't tuned properly these 2 attributes it could happen that when the AP transmit the DTIM letting the Station know that is some information in the buffers to be transmitted the Station could be not listening because the Listen Interval is so high for instance.