Question about MIMO

Im gonna ramble a bit but here goes....

MIMO can use multiple streams to send data to a client. It relies on multipath / bouncing off walls to create a slight delay in each stream. At the receiving end, some fancy processing splits out the streams and assembles the data in the original order again.

To understand how a client capable of a smaller number of streams affects total throughput, its best to think of airtime.

If each stream ran at 54mbits, then a 1x client can communicate at 54mbits per second under perfect conditions.
I use some older common speed numbers but its just to illustrate the example.
A 2x stream client can therefore communicate at 108mbits per second.

But consider the airtime impact.

We will need to remove the concept of collision collapse, more on that later, and assume the clients use TDMA in a perfect world for this example....

Example 1 - 1x stream client1 + 2x stream client2

If Client1 is communicating, the AP will in fact slow down to accommodate that client.

A 2x stream AP which is capable of 108mbits per second, can divide its transmit time between the two clients.
If we assume each client is communicating at the same time, then in our perfect world example, the time is divided in half equally between the two clients.

The impact therefore is during single second of airtime, client1 gets 50% of a single streams throughput = 27mbits since it only has half the airtime to communicate.

Client2 also gets half of the second to communicate but due to it having two streams it has a higher starting point so 108mbits of a full second equivalent = 54mbits during its half second of airtime.

The total throughput of the AP therefore becomes 81mbits during that full second of airtime.

Once client1 stops requiring airtime, the AP can spend more airtime transmitting packets using both streams to client2.

There are a couple of caveats....

CSMA - Collision Sense Multiple Access.
This is the multi-user mechanism used by Wifi up until 802.11ac generation 2.
Each packet sent has some extra error correction wrapped around it. Hence why you only get 50-60% TCP throughput compared to the advertised air-rates of the wifi equipment. A CRC check is performed on each packet and an acknowledgement sent between the wireless nodes at the wireless protocol layer. This is in addition to the CRC checking that the TCP protocol performs at the TCP layer. If a packet fails, it causes the CSMA recovery to take place.

If an AP and client1 are talking, and client2 wants to communicate, it will wait until the air is clear to transmit. If it takes too long, or it cant hear client1 because its too far away at the opposite end of the house (called the hidden node issue) then it will transmit anyway.
This causes the packets to fail their CRC check due to the interference or a "collision", and the CSMA recovery will take place.

The recovery involves each station choosing a random wait time, waiting out the chosen amount of time, then retransmitting with the hope that its packet will get through. Repeat as necessary. This can waste a lot of airtime.

So this explains why two devices on a wifi network streaming video will often have problems. Due to CSMA collisions, the throughput capability of the access point will "collapse" and having multiple devices on the network does not mean that each will get 50% of the throughput - the random wait times will often drop throughput capability to 20% or less of what the AP is capable of when communicating with only a single client.

TDMA - Time Division Multiple Access
This is the multi-user mechanism used in new versions of the Wifi protocol. It was introduced as an optional extra in what is often called the second generation of 802.11ac but not all manufacturers use it and if used, the AP and all stations must support it.
I understand it is universally adopted by 802.11ax or future standards coming after 802.11ac.

When using TDMA, the AP will assign specific airtime slots to each client. The clients will only transmit during their allocated time slots. As a client starts transferring data, the AP will allocate more time slots to the client and announce a revised timing plan at regular intervals.
This removes the dead-airtime caused by a packet collision under the old CSMA standard and thus enables much higher throughput when multiple clients are communicating at the same time.

MU-MIMO or Multiple User- MIMO

With multiple streams being transmitted from the AP, its possible to go further and dynamically reallocate the streams to specific clients.
For example, our client1 is a single stream device, and client2 is a dual stream device.
Before we learned how the throughput of the AP would drop from 108mbits to 81mbits because of how client1 would use extra airtime per megabit of throughput by not using the second stream.

With some fancy processing, its possible that stream 1 is allocated to client 1, while stream 2 is allocated to client 2.
The AP is able to send data to both clients at the same time. Client 2 just ignores the first stream, and client 1 ignores the second stream.

So Client 1 doesnt have to share its airtime, and gets the full 54mbits of throughput.
Client 2 meanwhile will indeed drop in speed from its 108mbits using 2 streams to 54mbits using 1 stream.
However the total throughput of the AP is back up at 108mbits (if we ignore the possibility of collision collapse)
By reallocating the streams and not airtime, its much more efficient.

MU-MIMO is another feature of the latest revision of 802.11ac and isnt backward compatible with older clients.
To make use of it, the AP and clients must support it.

Like TDMA, MU-MIMO is moving to the core feature set of the newer wifi standards such as in 802.11ax. Currently its optional and only some manufacturers support it in 802.11ac.

So to summarize...

If you have the latest "spider-style" AP and multi-stream capable clients with the latest generation 802.11ac then you can achieve some very high throughput rates - especially noticable when multiple clients are transferring large amounts of data compared to the same setup on an 802.11n network.

But if you have an 802.11g/n device on the 802.11ac network then assume everything will slow down to accommodate that device, and anything faster than dual stream 802.11n and mu-mimo will be disabled for the duration of that legacy device being connected to the AP.

In any case, if you have a device that has a low signal usually due to distance from the AP, or interference, then the AP will waste a lot of airtime having to re transmit packets to that device so everything will slow down.

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