  As wireless devices boom, and bandwidth-hungry applications (e.g., video and
cloud uploading) get popular, today's Wireless Local Area Networks (WLANs)
become not only crowded but also stressed at throughput. Multi-user
Multiple-Input and Multiple-Output (MU-MIMO), an advanced form of MIMO, has
gained attention due to its huge potential in improving the performance of
WLANs.
  This paper surveys random access based MAC protocols for MU-MIMO enabled
WLANs. It first provides background information about the evolution and the
fundamental MAC schemes of IEEE 802.11 Standards and Amendments, and then
identifies the key requirements of designing MU-MIMO MAC protocols for WLANs.
After that, the most representative MU-MIMO MAC proposals in the literature are
overviewed by benchmarking their MAC procedures and examining the key
components, such as the channel state information acquisition, de/pre-coding
and scheduling schemes. Classifications and discussions on important findings
of the surveyed MAC protocols are provided, based on which, the research
challenges for designing effective MU-MIMO MAC protocols, as well as the
envisaged MAC's role in the future heterogeneous networks, are highlighted.
