10 things to know about MU-MIMO Wi-Fi

Multi-User MIMO allows multiple Wi-Fi devices to receive multiple data streams simultaneously. For example, a wireless access point (AP) can send data to four different Wi-Fi devices at the same time. MU-MIMO can greatly improve the throughput of the network and is an important indicator in high-density scenarios.

MIMO, short for Multiple Input Multiple Output, has been around for many years, since the single-user mode (SU-MIMO) of the 802.11n wireless standard was introduced a decade ago.

The 802.11ac (Wi-Fi 5) standard introduced optional MU-MIMO in Wave 2 products. Now with the 802.11ax (Wi-Fi 6) standard, we are seeing even more improvements to MU-MIMO. As 802.11ax wireless routers and access points (APs) come to market, anyone supporting Wi-Fi networks needs to get accustomed to MU-MIMO.

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1. According to the Wi-Fi standard, MU-MIMO can be unidirectional or bidirectional

It's important to remember that, unlike SU-MIMO, MU-MIMO with 802.11ac only applies to downlink wireless connections. Only wireless routers and APs are capable of sending data to multiple users simultaneously, either one or multiple data streams per user. The wireless devices themselves (such as smartphones, tablets, or laptops) still have to take turns sending data to the wireless router or AP, although they can individually take advantage of SU-MIMO to send multiple streams when it's their turn.

When Wave 2 of 802.11ax arrives, wireless devices (like smartphones, tablets, or laptops) will also be able to participate in simultaneous streams when transmitting. In addition to multiple wireless devices being able to get data from a wireless router or AP, they will also be able to send data at the same time. This includes devices sending back acknowledgments via MU-MIMO, which may seem like a small feat but can really help speed up the communication process.

2. OFDMA technology is a supplement to MU-MIMO

Orthogonal Frequency Division Multiple Access (OFDMA) technology is part of 11AX, just like the technology used by LTE networks. It basically divides the channel into smaller segments and allows multiple devices to communicate at the same time, each in their own channel segment, technically called a resource unit (RU). While it doesn't directly increase data rates, it allows devices to coordinate better and faster when they can talk, resulting in more efficient use of the channel overall.

While both OFDMA and MU-MIMO are technologies that allow multiple devices to transmit simultaneously, they are very different. OFDMA can help in high-density environments with low-throughput or small-packet applications (such as IoT sensors), while MU-MIMO can help with high-throughput applications.

3. 802.11ax (Wi-Fi 6) supports more simultaneous MU-MIMO streams

The 802.11ac standard only allows up to four clients in a MU-MIMO group, so the AP or wireless router is limited to sending to four devices at the same time. With 802.11ax, now up to eight clients can be in a group. This feature should help speed up connections and increase throughput.

4. MU-MIMO with 802.11ax (Wi-Fi 6) for both 2.4GHz and 5GHz bands

Back with 802.11n and 802.11ac, SU-MIMO worked on both the 2.4GHz and 5GHz bands. But MU-MIMO with 802.11ac only supported the 5GHz band. However, with 802.11ax coming to both bands, we’ll have MU-MIMO (and OFDMA) on both bands. This is one of the biggest improvements to the crowded 2.4GHz band we’ve seen in years. Keep in mind that this band can only support up to three non-overlapping channels at a time and uses smaller, traditional channel widths. MU-MIMO with 802.11ax can help save this lower band by speeding it up and making it more usable in dense environments.

5. Beamforming helps steer the signal

MU-MIMO uses what's called beamforming, a separate feature of 802.11ac and 802.11ax that directs the signal toward the intended wireless device rather than in random directions. The technology helps improve Wi-Fi range and speed because the signal is used more efficiently.

While 802.11n has the option to use beamforming, most vendors have only implemented a proprietary version of it. Having a standardized version helps with beamforming and MU-MIMO in 802.11ac or 802.11ax products.

6. User devices need multiple antennas to achieve MU-MIMO uplink

As with 802.11ac, wireless devices do not need to have multiple antennas to receive MU-MIMO streams from wireless routers and APs. If a wireless device only has one antenna, it can still receive one MU-MIMO data stream from the AP. However, for uplink MU-MIMO, the wireless device needs to have at least two antennas to send MU-MIMO back to the AP or wireless router, even for a one-stream connection.

More antennas will allow the device to support more simultaneous data streams (typically one per antenna), which will benefit the device's Wi-Fi performance. However, including multiple antennas in a device requires more power and space, and increases its cost. Eight antennas are required to fully utilize 802.11ax features.

7. Traditional equipment will also benefit

While legacy 802.11n and 802.11ac devices won’t directly see any range or performance improvements from their connections to 11ax APs or wireless routers, they can see indirect benefits. Remember, Wi-Fi is all about airtime: The faster any device is served, the longer other devices will have airtime. If new technologies like MU-MIMO and OFDMA can serve supported devices faster, other devices (including legacy devices) will have longer airtime, too.

8. MU-MIMO helps improve network capacity

When you increase the speed of a wireless network, you also increase the capacity of the network. With faster service speeds for devices, more airtime is available to serve more devices. As a result, MU-MIMO can help relieve congestion on busy or dense networks, such as Wi-Fi hotspots.

9. Support any channel width

One way to increase the throughput of a Wi-Fi channel is through channel bonding, which combines two adjacent channels to create a single channel that is twice as wide, effectively doubling Wi-Fi speeds. The 802.11n standard supported 40MHz-wide channels, and the first wave of 802.11ac added support for 80MHz-wide channels. The second wave of 802.11ac introduced channel widths up to 160MHz, still with 802.11ax.

So even if your network uses narrower 20MHz or 40MHz wide channels, MU-MIMO can help it run faster by serving some clients at the same time. The faster speed depends on how many devices are supported on the network and how much traffic each supports. But even without implementing wide channels, using MU-MIMO can double the downlink throughput for each device.

10. Signal processing improves safety

An interesting side effect of MU-MIMO is that the router or AP scrambles the data before sending it over the airwaves, so that only the intended receiving device can scramble it. While tools may be developed to allow others to process the traffic, for now MU-MIMO does shield the data from nearby eavesdroppers. In this way, the technology helps improve the security of wireless networks, especially on open networks, such as hotspots, or those that are protected only with WPA or WPA2 modes with personal or pre-shared keys.

With 802.11ac, this side effect and data scrambling was only implemented on downlink connections, as that was the only direction MU-MIMO supported. Now, with 802.11ax, uplink connections can benefit as well.