Figure 12: Diagram of downlink transmission of SU-MIMO and MU-MIMO
However, MU-MIMO cannot solve all problems of Wi-Fi slowness. As mentioned, it only optimizes multi-
user transmission efficiency. Below are some limitations:
1) MU-MIMO requires protocol support on both wireless access points and wireless clients. However,
there still a great number of user device without 802.11ac support. So, in most deployment
scenario, access points are always facing hybrid terminals. Most of the time, multiple standard
802.11a/n/ac terminals work at the same time. In this case, AP must provide service on their
supported protocol too, which makes it hard to fully utilize the features of 802.11ac Wave-2 and the
AP cannot work in an optimal state.
2) MU-MIMO can only improve the efficiency of downlink multi-user data transmission, and it is
ineffective for the uplink direction.
3) The number of clients supporting MU-MIMO for simultaneous downlink transmission cannot be
greater than the number of antennas in the AP;
Below is a diagram of a SU-MIMO simple omnidirectional antenna comparing to a MU-MIMO beamforming
(Figure 13).
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