What is Multiple User Input Multiple User Output (MIMO)?

So what is MIMO?  Most non-technical individuals often come across variations of the term MIMO when looking to purchase Wi-Fi Access points / Routers or browsing at the specifications of different Wi-Fi access points. some times indicated as (SU-MIMO) on some Wi-Fi Access Points and as recent as 2017 on newer Wi-Fi standards starting from Wi-Fi 5 wave 2 indicated as (MU-MIMO).

To Understand MIMO and the different evolutions of MIMO technology i.e (SU-MIMO & MU-MIMO),

you need to understand Wi-Fi spatial streaming.

Wi-Fi Spatial Streams:

Multiple end-user Wireless devices connect to your Wi-Fi network on a daily basis. Most of them include a variety of user smart phones, tablets, laptops, watches, and more! 

One of the universal conclusions that network engineers or people responsible for the I.T and network requirements usually come to, is that they will always be a few complaints from the Wi-Fi users regarding the Wi-Fi network performance, while some user devices will perform better than others, even while they are located in the same place. Why is that? Well, one of the reasons some end-user devices perform better than others is “Wi-Fi Spatial Streams”.

Wi-Fi spatial streaming is a transmission technique used in wireless communications to transmit or receive independent and separately coded data signals, Called streams, by way of each of the multiple antennas available on end-user devices.

 Each spatial stream can carry a certain amount of data, and much like adding additional lanes to a road, multiple spatial streams allow the wireless device to transmit or receive more data simultaneously.

Spatial streams and MIMO Representations and what they mean?

The number of spatial streams supported by an end-user device is normally represented using something like 1×1:1 , 2x2:2, 3x3:3 or 4×4:4. As for what each of these numbers represents, let us take one of them apart.

 For example, on a device that supports 4×4:4: 

  1. The first 4 represents the numbers of antennas used for transmitting (TX).
  2. The second 4 represents the number of antennas used for receiving (RX).
  3. The last 4 represents the number of spatial streams the device can handle at the same time. 

I know you may be wondering how the number of spatial streams supported by an end-user device can affect Wi-Fi performance. Is there really a big performance difference between a device that supports 1×1:1 and a device that supports 4×4:4? 

Much as we all hate to admit it, gone are the days when you could use a phone & Laptop for 3yrs and still enjoy the benefits of not dashing out cash to buy new devices..

Take an example of Wi-Fi 6: Wi-Fi 6 (802.11ax) came out in 2019, and Wi-Fi 6E  (802.11axe on 6GHz) came out in 2021. less than 3yrs apart.   At the rate at which Wi-Fi technology is advancing, getting better with higher bandwidth and throughputs faster than we can buy newer devices, we are bound to stay behind with older devices that cannot support higher speeds and are not able to understand Wi-Fi network signals broadcasted by the newer Wi-Fi Access points /Routers.

 hence we are always bound to gets complaints from users who are still connecting to Wi-Fi networks with older devices.  

So lets take an example of a new wireless end user device with support for 4x4:4 MU-MIMO such as the iPhone 14 or the Samsung S21

and compare it to a 1x1:1 end user device such as  the  Oneplus 9.

  • The more antennas an end-user device has, the better it can hear Wi-Fi signals in the air. Which in very oversimplified terms means higher transmit and receive signal strengths. For example, let us say that you use an iPhone 13 Pro Max or iPhone 14 that supports 4×4:4 to measure the signal strength in a conference room and you see a signal of -58 dBm. Well, if you try using the Oneplus 9 that supports 1×1:1 in the same conference room as the iPhone 14 to perform the same measurements your signal strength would be closer to -77 dBm, which is a lot weaker.

  • More antennas translate into higher upload and download speeds. Of course, the main reason for higher speeds is being able to use multiple antennas to transmit or receive in parallel (more data streams at the same time). Still, that is not the only reason, as mentioned above more antennas improve signal strength. This means the end-user device can connect at higher data rates, and thus achieve higher upload and download speeds.

To put in contrast SU-MIMO and MU-MIMO, Kindly refer to image below from research gate:

It's important to remember that, unlike SU-MIMO, MU-MIMO with Wi-Fi 5 works only with downlink wireless connections. Only wireless routers and APs are able to simultaneously send data to multiple users, whether it's one or more streams of data to each. The wireless devices themselves, such as smartphones, tablets or laptops, still must take turns sending data to the wireless router or AP, although they can individually utilize SU-MIMO to send multiple streams when it's their turn.

With the Wi-Fi 6 standard, wireless client devices can also take part in sending simultaneous streams. So multiple clients get data from a wireless AP and send data back at the same time. This includes sending acknowledgements, which seems like a small feat but can really help speed up communications.

It's worth noting that MIMO technology works together Frequency Division and Multiplexing technology (FDM). In our next article in this category I will cover how MU-MIMO and OFDM in Wi-Fi 5 (802.11ac wave 1&2) has been improves in Wi-Fi 6 (802.11ax) MU-MIMO and introduction of OFDMA.

I will also talk about new developments in Wi-Fi 6 such as 1024 QAM, Simultaneous Dual-Band, WPA3 and the 6GHz Frequency Band.

I hope this article was helpful.

Kind Regards,

Abdulaziz Musa

Ray Support