What is MU-MIMO, and why is it essential for Wi-Fi 6 and 6E?

The only thing techies love more than creating acronyms is the chance to create even longer ones. Such is the case with wireless acronym MIMO (multiple input, multiple output), which got some additional letters with the release of MU-MIMO a few years ago.

As wireless standards evolved from 802.11ac (Wi-Fi 5) to 802.11ax (Wi-Fi 6), new features were added to MU-MIMO as well to improve speeds and efficiency, specifically in the number of streams it can support, as well as bidirectional functionality (uplink and downlink).

What is MU-MIMO?

 MU-MIMO stands for multi-user, multiple input, multiple output, and represents a significant advance over single-user MIMO (SU-MIMO), which is generally referred to as MIMO. MIMO technology was created to help increase the number of simultaneous users a single access point can support. This was initially achieved by increasing the number of antennas on a wireless router.

MU-MIMO technology is now recognized as a major part of the Wi-Fi 6 (802.11ax) protocol, which emerged from the 802.11ac protocol. Standards older than 802.11ac (such as 802.11b, g and n) do not support MU-MIMO.

With MU-MIMO supported in 802.11ac (Wi-Fi 5), many endpoints and access points now come equipped with MU-MIMO technology. It’s still in the early days of Wi-Fi 6 and 6E (which extends the wireless spectrum available in the 6GHz band), but you can check with the Wi-Fi Alliance to see if a specific product includes MU-MIMO and Wi-Fi 6. Chip shortages and supply chain delays due to the pandemic slowed the adoption of Wi-Fi 6, but many are hopeful that the technology will take off in 2022.

With 802.11ax (Wi-Fi 6), basic MU-MIMO was updated to include uplink MU-MIMO, which means that an access point transmitting concurrently to multiple receivers can now also simultaneously receive from multiple transmitters (that also include uplink MU-MIMO). With 802.11ac (Wi-Fi 5), MU-MIMO was limited to downlink transmissions (from an access point to an endpoint).

How MU-MIMO works with wireless devices

 MU-MIMO was created to support environments where multiple users are trying to access the wireless network at the same time. The nature of the 802.11 protocol is that users are served on a first-come, first-serve basis.

When multiple users begin accessing the router at or near the same time, congestion can be introduced as the router services the first user’s request while the second (and third, fourth, etc.) wait. While these wait times can be minuscule, they can add up with more devices (smartphones, tablets, computers, etc.) and users asking for resources. MU-MIMO helps this by allowing multiple users to access router functions without the congestion.

Types of MU-MIMO implementations in wireless routers

 MU-MIMO technology breaks up the available bandwidth into separate, individual streams that share the connection equally. A MU-MIMO router can come in 2×2, 3×3, 4×4 variations, and even 8×8, which refer to the number of streams that are created by the router.

In simple terms, imagine you’re in line at the school cafeteria – you get served after the people in front of you. With MU-MIMO, instead of one lunch lady, you now get two, three, four or eight lunch ladies, which reduces the time you need to wait.

The jump from 802.11ac to 802.11ax cuts the wait time even more by borrowing technology from the cell phone base station world – Orthogonal Frequency Division Multiple Access (OFDMA). This splits each MU-MIMO stream into four additional streams, boosting the effective bandwidth per user by four times. So with a 4-lunch-lady scenario, we have 4 lunch ladies serving four lines, and each lunch lady is serving four hungry students at the same time.

With uplink MU-MIMO, while this is all happening, students can give (transmit) the lunch lady money at the same time. In the earlier versions of MU-MIMO, the lunch ladies could only transmit to the students, anyone wanting to “pay” would need to wait their turn once the serving was done.

Summing up, the addition of OFDMA and uplink MU-MIMO means that Wi-Fi 6 and 6E are a whole lot faster than Wi-Fi 5 systems, which is particularly helpful for very dense environments, such as stadiums, airports, or even in office and home environments in which multiple users within the same relative space are competing for bandwidth.

One thing to note – the streams are spatial, which means if two devices are close to each other, they still have to share the same stream. Imagine that cafeteria scenario again, but this time the four lines that were created are now points on a compass. If you’re physically located in the south line, you have to wait with everyone else unless you move to the east, north or west. In an office setting, if your adjacent coworker is streaming video while you’re trying to download a super-large sales presentation, you’ll have to wait, unless you move to the other side of the office. This scenario assumes that the router/access point has enabled MU-MIMO and beamforming support.

In addition, in order for users to take advantage of uplink MU-MIMO, their devices will require multiple antennas, which could require more power and space, increasing the cost of endpoint systems.

Yet the future remains bright for MU-MIMO as it remains core to the Wi-Fi 6 and 6E marketplace. Let’s just hope we don’t need to add any more letters to this already large-enough acronym.

Keith Shaw is a former Network World senior editor and writer of the Cool Tools column. He is now a freelance writer and editor from Worcester, Mass.