As 802.11ac becomes more widely deployed in environments I find myself looking to the next wave and the promise it brings.  802.11ac Wave 1 for me really isn’t that groundbreaking.  It’s an incremental improvement on 802.11n.  Wave 1 really only serves to wake up the manufacturers to the fact that 5 GHz radios are needed on devices now.  The real interesting stuff comes in Wave 2.  Wider channels, more spatial streams, and a host of other improvements are on the way.  But the most important one for me is MU-MIMO.

Me Mi Mo Mum

Multi-user Multiple-Input Multiple-Output (MU-MIMO) is a huge upgrade over the MIMO specification in 802.11n.  MIMO allowed access points to multiplex signals on different channels into one data stream.  It accomplished this via Spatial Division Multiplexing (SDM).  This means that more antennas on an access point are a very good thing.  It increases the throughput above and beyond what could be accomplished with just a single antenna.  But it does have a drawback.

Single-user MIMO can only talk to one client at a time.  All the work necessary to multiplex those data streams require the full attention of a single access point for the period in time that the client is transmitting.  That means that crowded wireless networks can see reduced throughput because of shorter transmit windows.  And what wireless network isn’t crowded today?

MU-MIMO solves this problem by utilizing additional antenna capacity to transmit multiple data streams at once.  If you have spare antennas, you can send another data stream.  The AP then takes the multiple streams and stitches them back together.  This means an effective increase in throughput for certain devices even though the signal strength isn’t as high (thanks to FCC power limits).  Here’s a great video from Wireless Field Day 7 that explains the whole process:

What I found most interesting in this video is two-fold.  First, MU-MIMO is of great benefit to client devices that don’t have the maximum number of spatial streams.  Laptops are going to have three stream and four stream cards, so their MU-MIMO benefit is minimal.  However, the majority of devices on the market using wireless are mobile.  Tablets and phones don’t have multiple spatial streams, usually just one (or in some cases two).  They do this to improve battery life.  MU-MIMO will help them out considerably.

The second takeaway is that devices without a high number of receive chains will make the AP do more work.  That’s because the AP has to process the transmit stream and prevent the extra streams from being transmitted toward the wrong client.  That’s going to incur processing power.  That means you’ll need an AP with a lot of processing power.  Or a control system that can crunch those numbers for you.

When you consider that a large number of APs in a given system are sitting idle for a portion of the time it would be nice to be able to use that spare capacity for MU-MIMO processing.  In addition, having those extra antennas available to help with MU-MIMO sounding would be nice too.  There’s already been some work done on the research side of things.  Maybe we’ll soon see the ability to take the idle processing power of a wireless network and use it to boost the client throughput as needed.

Tom’s Take

Wireless never ceases to amaze me.  When I started writing this article, I thought I knew how MU-MIMO worked.  Thankfully, Cisco set me straight at Wireless Field Day 7.  MU-MIMO is going to help clients that can’t run high-performance networking cards.  The kinds of clients that are being sold as fast as possible today.  That means that the wireless system is already being developed to support a new kind of wireless device.

A device that doesn’t have access to limitless power from a wall socket or a battery that lasts forever.  There’s been talk of tablets with increased spatial streams for a while, but the cruel mistress of battery life will always win in the end.  That’s why MU-MIMO matters the most. Because if the wireless device can’t get more powerful, maybe it’s time for the wireless network to do the heavy lifting.