Wireless Communications Basics -- Spatial Streams

(1) Background
The spectrum of an electromagnetic wave can be seen as a land and 2.4 GHz band is a road on the land. The frequency range of 2.4GHz–2.4835 GHz is the width of the road. The road can be further divided into lanes (channels) and each lane has a valid width of 20 M Hz. Vehicles can only run on one of these lanes. The number of vehicles running on the road simultaneously is limited because the road is narrow.
(2) MIMO in Wi-Fi 4
In Wi-Fi 4, MIMO (Multiple-Input Multiple-Output) is like an overpass above a narrow road. This is called spatial reuse. In this case, with one more road, the number of vehicles running simultaneously is doubled, greatly improving the transmission rate.

As you may find in advertisements, some routers feature 4X4 MIMO. That means, the router has four Tx/Rx antennas. If a phone or computer supports 4X4 MIMO (having four Tx/Rx antennas), there will be four spatial streams between the router and phone or computer. If a phone supports only 2X2 MIMO, the router and phone will negotiate and agree to use two spatial streams.
Therefore, you may find that the transmission rate is 1200 Mbps although the router supports 4x4 MIMO (up to 2400 Mbps). That is because most phones support only 2 spatial streams. Very few phones support three or four spatial streams because the more the antennas, the higher the power consumption and faster the battery is drained.
(3) MU-MIMO
MIMO in Wi-Fi 4 is also called SU-MIMO (Single-User MIMO) because the router can communicate with only one terminal simultaneously although spatial streams are increased. The router can communicate with another terminal only when it finishes communication with the current one.
MU-MIMO (Multi-User MIMO) in Wi-Fi 5 supports simultaneous data transmission between a router and multiple terminals. However, it supports simultaneous transmission only for downlink data like file download and TV broadcast. Uplink data needs to queue up for transmission.
In Wi-Fi 6, MU-MIMO supports simultaneous transmission of both uplink and downlink data.