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MU-MIMO technology boosts system capacity for WLANs

MU-MIMO technology might not be magic, but it upgrades WLAN performance and system capacity so well that some network engineers might call it magical.

It was legendary science-fiction writer Arthur C. Clarke who wrote, "Any sufficiently advanced technology is indistinguishable from magic."

Many would put wireless LAN in the magical category. And if that's the case, multiple input, multiple output (MIMO) and, most recently, multiuser MIMO (MU-MIMO) technology would really have to be the rabbits in the hat. Even those of us who've spent the majority of our careers in wireless networking are amazed at what those technologies can do.

MIMO's been with us since 802.11n, and it unlocked the amazing performance improvements that extend into the current era. Yet, MIMO is still little understood, even among many network engineers. No surprise there, as MIMO seems to violate many of the laws that govern communications theory.

In a nutshell, MIMO technology takes two-dimensional radio waves -- you can think of them as having just frequency and amplitude, which are the variables that we use to modulate a carrier signal, thus embossing the information we wish to transmit on this simple structure -- and makes them three-dimensional. The third dimension is space, and that's why MIMO is often referred to as spatial multiplexing. More dimensions result in a greater capacity to carry information. That's why MIMO yields such amazing results, without violating any physical laws whatsoever.

Overcoming the wireless interference problem

But there's more to the performance of wireless communications than simply cramming more bits into a given transmission cycle. Wireless has historically been a serial-access medium: Only one transmitter can be active in any given location at a particular frequency at any given moment in time. Two transmitters in close proximity attempting simultaneous transmission will likely cause mutual interference and at least some degradation to overall system capacity. While licensed radio services, like cellular, can schedule transmissions to avoid this problem, the unlicensed bands have no such controls; stations cannot coordinate with one another and must accept as reality any interference they encounter.

Now, several stations can receive the data they need with less overall waiting. System capacity goes up.

In response, the wireless industry developed many techniques to deal with the potential damage inherent in interference, mostly related to how information is modulated and coded before it's sent over the air. But a large problem remains: Only one station can transmit at any moment in time. Until recently, this has meant only one receiving client station could be served -- again, at any moment in time -- and any others desiring communication would have to wait. The result: Overall system capacity was limited.

And that's just the problem MU-MIMO technology solves. In yet another incarnation of magic, MU-MIMO enables an access point to transmit -- simultaneously -- to multiple clients in one transmit cycle, with each client receiving a unique data stream from the others also sent at the same time. Now, several stations can receive the data they need with less overall waiting. System capacity goes up. And more users go home -- or, better said, stay at work -- happy.

Single-user MIMO vs. multiuser MIMO

802.11ax standard introduces new flavor of MU-MIMO technology

Now, how this technique is actually implemented is so complex that the math involved would make great bedtime reading. Suffice it to say, MU-MIMO technology works very, very well -- Farpoint Group's own testing shows performance gains close to the theoretical maximum. Your mileage will likely vary, but the potential here is enormous. The upcoming 802.11ax standard is expected to add bi-directional MU-MIMO, meaning stations will be able to transmit simultaneously to an access point. Now, we're talking real magic.

Farpoint Group recommends new Wi-Fi infrastructure and client purchases specify support for MU-MIMO -- yes, you'll need new gear on both ends; field upgrades are not possible in most cases. But you'll be glad you specified this requirement. Of course, MU-MIMO technology isn't really magic, but it certainly looks that way.

This was last published in July 2018

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What expectations do you have for MU-MIMO and the 802.11ax standard?
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