13 things you need to know about MU-MIMO Wi-Fi

It’s a big breakthrough in wireless connectivity, but don’t let MU-MIMO’s limitations catch you off-guard.

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It's been nearly 20 years since Wi-Fi made its debut, and one of its most significant innovations to date is now beginning to spread: MU-MIMO, an optional feature in Wave 2 of the emerging 802.11ac wireless standard.

MU-MIMO stands for multi-user multiple input and multiple output. It builds on single-user MIMO (SU-MIMO), which was introduced close to a decade ago with the 802.11n standard.

SU-MIMO increases Wi-Fi speeds by allowing a pair of wireless devices to simultaneously send or receive multiple data streams. MU-MIMO ups the ante, allowing multiple devices to simultaneously receive the multiple data streams. It is one of the most complex Wi-Fi features to date.

MU-MIMO vs. SU-MIMO Qualcomm

SU-MIMO technology allows multiple input and output streams with one device at a time, whereas MU-MIMO allows simultaneous communication with multiple devices.

As 802.11ac Wave 2 wireless routers and access points (APs) hit the market, anyone who supports a Wi-Fi network will need to get educated about MU-MIMO. Read on for 13 facts that will help get you up to speed.

1. MU-MIMO applies only to downlink connections

It's important to remember that, unlike SU-MIMO, MU-MIMO currently 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.

For administrators, that means MU-MIMO is more useful on networks where users download more than they upload.

We might see the uplink version of MU-MIMO added to the next big Wi-Fi standard: 802.11ax.

2. MU-MIMO works only in the 5GHz Wi-Fi band

SU-MIMO works in both the 2.4GHz and 5GHz Wi-Fi frequency bands, but MU-MIMO doesn't. Wireless routers and APs can simultaneously serve multiple users only in the higher band. It's unfortunate that the smaller and more congested lower band can't utilize the new technology. But we'll keep seeing more and more dual-band devices that can utilize MU-MIMO, making the effort to deploy the new technology worth it.

3. 'Beamforming' helps direct signals

MU-MIMO uses what's known as 'beamforming,' a separate feature of 802.11ac that directs signals toward the intended wireless device(s) instead of randomly in all directions. Since the signal is more efficiently used, the technology helps increase Wi-Fi ranges and speeds.

Although beamforming was optionally available with 802.11n, most vendors implemented only proprietary versions of it. Those vendors can still include proprietary versions, but they now must include the simplified and standardized version if they choose to support beamforming or MU-MIMO in their 802.11ac products.

4. MU-MIMO doesn't support unlimited simultaneous streams and devices

Unfortunately, a MU-MIMO router or AP can't simultaneously serve unlimited streams and devices. A router or AP has a certain number of streams it supports, often three or four, and that's also the limit on streams it can simultaneously serve. However, a MU-MIMO router or AP can choose to use those streams across multiple devices. For instance, a four-stream AP may choose to simultaneously serve four different one-stream devices or a single one-stream device and a single three-stream device.

5. User devices don't require multiple antennas

As with SU-MIMO technology, only wireless devices with built-in support for MU-MIMO can receive MU-MIMO signals. But unlike with SU-MIMO technology, wireless devices aren't required to have multiple antennas to receive MU-MIMO streams from wireless routers and APs. If the wireless device has only one antenna, it still can receive one MU-MIMO data stream from an AP.

More antennas would allow a device to accept more simultaneous data streams (typically one stream per antenna), which would be good for the device's Wi-Fi performance. However, including multiple antennas in a device requires more power and space, and adds to its cost. Since MU-MIMO has less taxing hardware requirements than SU-MIMO, device manufacturers are more likely to adopt the new technology.

6. Access points do the heavy processing

Further simplifying the requirements of end-user devices, MU-MIMO was designed to have the APs do the majority of the signal processing. This is another improvement over SU-MIMO, where the burden of signal processing is on the devices. Again, this can help device manufacturers save power, space and costs in the MU-MIMO devices.

7. Simpler devices have the most to gain

You'll see the biggest advantages of MU-MIMO when there are simpler devices on the network that support just one or two streams vs. those supporting three or four streams. That's because the technology doesn't speed up individual connections, but rather increases total network throughput by serving multiple devices at once.

For instance, say an AP supports three streams. If you have only three-stream devices on the network, MU-MIMO won't help increase the total network throughput over what you'd see using SU-MIMO. However, if you have all one-stream devices, MU-MIMO can serve up to three at the same time instead of each device having to wait in line to get transmissions from the AP. You can expect to see this on most Wi-Fi networks, as most wireless devices support fewer streams.

MU-MIMO throughput vs. SU-MIMO throughput Qualcomm

MU-MIMO can send three times the amount of data as SU-MIMO in the same amount of time, more than doubling the throughput of each device.

8. Some devices have had hidden support for MU-MIMO

Although there are currently few routers, APs or mobile devices that support MU-MIMO, Wi-Fi chipmaker Qualcomm says some device manufacturers have included the hardware requirements in select end-user devices for the past few years. For those devices, a relatively simple software update could add support for the new technology, and that in turn could help spread the technology more quickly as networks are upgraded with 802.11ac equipment.

9. Non-MIMO devices will see a benefit

Although Wi-Fi devices must specifically support MU-MIMO in order to utilize the technology, even those that don't could indirectly benefit on networks where the router or APs support the technology. Remember, Wi-Fi is all about airtime. If MU-MIMO can help serve supported devices quicker, there's more airtime for other devices.

10. MU-MIMO helps increase network capacity

When you increase Wi-Fi speeds, you also increase the network's capacity. As devices are served more quickly, there's more airtime to serve even more devices. Thus, MU-MIMO can help alleviate congestion on busy or dense networks, such as Wi-Fi hotspots — a real boon since the number of smartphones and other mobile devices is likely to keep growing.

11. Any channel width is supported

One way to increase throughput on a Wi-Fi channel is through channel bonding, which combines two adjacent channels to create a single channel that's twice as wide, effectively doubling the Wi-Fi speed. The 802.11n standard supported 40MHz-wide channels, and the first wave of 802.11ac added support for 80MHz-wide channels. Now the second wave of 802.11ac products supports channels as wide as 160MHz.

5GHz Wi-Fi channels Security Uncorked

The 802.11ac standard supports channel widths up to 160MHz in the 5GHz band.

"How to configure Wi-Fi channels for top network performance" explains how using larger channel widths cuts down the number of available channels, increasing the chances of causing co-channel interference. That means it might not be the right approach for every network. However, MU-MIMO can be used with any of the channel widths.

Thus, even if your network uses narrower 20MHz- or 40MHz-wide channels, MU-MIMO can help it run faster. How much faster depends on how many supported devices are on the network and how many streams each supports. But using MU-MIMO even without implementing wide channels could more than double the downlink throughput of each device.

12. Signal processing increases security

An interesting side effect of MU-MIMO is that the router or AP scrambles data before sending it via the airwaves. By default, only the intended receiving device can decode the data sent with MU-MIMO. Although tools might be developed that allow others to process the traffic, for now, MU-MIMO does mask the data from any nearby eavesdroppers. In that way, the technology helps boosts Wi-Fi security, especially on open networks such as hotspots or those secured only with the personal, or pre-shared key, mode of WPA or WPA2.

13. It's best for stationary Wi-Fi devices

One caveat about MU-MIMO: It doesn't work well with rapidly moving devices, as the beamforming process becomes more difficult and less effective. Thus, the technology might not provide much benefit in networks with constantly roaming devices. However, these problem devices shouldn't affect the MU-MIMO transmissions to devices that are more stationary, nor their performance.

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