  Downlink (DL) Multi-User (MU) Multiple Input Multiple Output (MU-MIMO) is a
key technology that allows multiple concurrent data transmissions from an
Access Point (AP) to a selected sub-set of clients for higher network
efficiency in IEEE 802.11ax. However, DL MU-MIMO feature is typically turned
off as the default setting in AP vendors' products, that is, turning on the DL
MU-MIMO may not help increase the network efficiency, which is
counter-intuitive. In this article, we provide a sufficiently deep
understanding of the interplay between the various underlying factors, i.e.,
CSI overhead and spatial correlation, which result in negative results when
turning on the DL MU-MIMO. Furthermore, we provide a fundamental guideline as a
function of operational scenarios to address the fundamental question "when the
DL MU-MIMO should be turned on/off".
