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