With MIMO based on spatial division multiplexing (SDM) in 802.11n, one device transmits multiple data streams to another device. With the inclusion of downlink multi-user MIMO (DL MU-MIMO) in 802.11ac, an access point (AP) simultaneously transmits independent data streams to multiple client devices. Consider the example illustrated in Figure 14.1, with an AP and two client devices. In the example the AP has four antennas, one client device has two antennas (device A in Figure 14.1), and the other client device has one antenna (device B in Figure 14.1). An AP can simultaneously transmit two data streams to device A and one data stream to device B.

The primary advantage of DL MU-MIMO is that client devices with limited capability (few or one antenna) do not degrade the network capacity by occupying too much time on air due to their lower data rates. Consequenty, downlink capacity, which is based on the aggregate throughput of the clients that receive the simultaneous transmission using DL MU-MIMO, is improved over that of 802.11n. However, the benefits of DL MU-MIMO do come with increased cost and complexity.