MIMO has been used in the wireless standard since 802.11n but I want to use this post to be able to describe what MIMO is and how it has evolved over time.
What is MIMO and why is it used?
MIMO is used to solve one of the biggest inefficiency we have had with Wi-Fi, on how do we utilize multiple antennas to be able to transmit and receive to increase performance.
Before MIMO, we had SISO (single input single output) where even if client and the AP have multiple antennas, only one could be activate at the same time. The use for the other antenna was to be able to determine which antenna received a better signal so the client would prefer one signal over another.
With MIMO, it allows clients and the AP to be able to use multiple antennas at the same time for transmit (TX) and receive (RX), resulting in overall better performance. To determine if a client or the AP supports MIMO is to look at its configuration, which is typically designated as (transmit x receive : number of streams supported). For example, if a client has the specification of 2x2: 2, it would mean it can support up to 2 TX and 2 RX Ant configuration and has support up to 2 streams.
What is MU-MIMO?
With MIMO, it allowed wireless devices to be able to support simultaneous transmit and receive capability, but it still did not allow where multiple devices can communicate at the same time.
To provide an example, let's say you have an access point that supports a 4x4:4 configuration. If there are two clients that are 2x2:2 configuration, only one of the user, based on their TXOP, can communicate to the AP, fully utilizing the 2 radio chain that client can offer. This is still inefficient because the AP is capable to transmit with its 4 radio chains.
Downlink MIMO (AP -> STA)
802.11ac/Wi-Fi 5 introduced the feature of downlink MU-MIMO, where it allowed multiple users to be able to communicate at the same time for up to the number of spatial streams the AP was capable of. Some of the APs support up to 8x8:8 configuration, making wireless medium a lot more efficient in terms of supporting large number of clients connecting to the AP at the same time.
While making the AP to STA communication efficient, the clients would still have to take turns to ACK since clients were not able to communicate at the same time to the AP with this method.
Uplink MIMO (STA -> AP)
802.11ax/Wi-Fi 6 introduced the concept of uplink MIMO, where clients are now able to communicate to the AP at the same time. With this method, the AP would be able to send a trigger frame to all clients that it's communicating with its available radio chains, allocating clients with OFDMA resource units, allowing clients to communicate back to the AP at the same time, making the communication much more efficient.
In summary, MIMO technology over different wifi standard has improved overall efficiency, making overall wireless connectivity experience much more better for WiFi clients, and solving one of the biggest inefficiency of a shared medium.