  Next-generation 802.11ax WLANs will make extensive use of multi-user
communications in both downlink (DL) and uplink (UL) directions to achieve high
and efficient spectrum utilization in scenarios with many user stations per
access point. It will become possible with the support of multi-user (MU)
multiple input, multiple output (MIMO) and orthogonal frequency division
multiple access (OFDMA) transmissions. In this paper, we first overview the
novel characteristics introduced by IEEE 802.11ax to implement AP-initiated
OFDMA and MU-MIMO transmissions in both downlink and uplink directions. Namely,
we describe the changes made at the physical layer and at the medium access
control layer to support OFDMA, the use of \emph{trigger frames} to schedule
uplink multi-user transmissions, and the new \emph{multi-user RTS/CTS
mechanism} to protect large multi-user transmissions from collisions. Then, in
order to study the achievable throughput of an 802.11ax network, we use both
mathematical analysis and simulations to numerically quantify the benefits of
MU transmissions and the impact of 802.11ax overheads on the WLAN saturation
throughput. Results show the advantages of MU transmissions in scenarios with
many user stations, also providing some novel insights on the conditions in
which 802.11ax WLANs are able to maximize their performance, such as the
existence of an optimal number of active user stations in terms of throughput,
or the need to provide strict prioritization to AP-initiated MU transmissions
to avoid collisions with user stations.
