WHAT IS MU-MIMO?
MU-MIMO, which stands for Multi-User, Multi-Input Multi-Output, exists to solve a problem faced by its predecessor, Single-User MIMO (SU-MIMO). MIMO itself means that a router has multiple antennas, each capable of emitting one spatial stream of data. The issue with this is that most devices do not have that many antennas themselves, and are thus not capable of taking in the full data output of a router. Add the fact that SU-MIMO does not allow for the multiple antennas to serve different client devices at a time, and the result is wastage of the router’s full capacity, called the MIMO gap.
Instead of pushing all the data to one device that can’t make use of it all, MUMIMO means that the spatial streams are split between the other devices that are connected to the network.
MU-MIMO closes this gap by splitting its individual spatial streams to serve one device each, resulting in, theoretically, an equally high speed enjoyed by up to four devices per router or access point. With MUMIMO and devices supporting this technology, the multiple streams are spread out among the clients, meaning each gets its own stream of data.Another result of this is the router is capable of sending more data out at a time, closing the MIMO gap.
With MU-MIMO, devices are each taking one spatial stream of data, meaning there is less wait time caused by the turn-taking of multiple devices sharing all spatial streams.
Still, MU-MIMO has some drawbacks of its own, chief among which being adoption. So far, there are few client devices out there that support MU-MIMO, and you will need such devices to truly take advantage of this feature. And while still faster than using a router that does not support MU-MIMO, if a device that does not support MU-MIMO connects to a MU-MIMO access point, other connected, MU-MIMO
supported devices will experience a dip in speed. This is because the router will have to serve the legacy device with data the old-fashioned way – by pushing all four spatial streams of data to the device, despite
the device being incapable to make full use of it. Other devices will also have to take turns in getting data from the router, to accommodate the device that doesn’t support MU-MIMO.
Of course, all that is in theory. To make sense of all this, we ran a test ourselves with the aforementioned Linksys EA8500, an AC2600 router that supports MU-MIMO. Also with us are two Acer Aspire E14 notebooks, one of the earliest adopters of MU-MIMO among notebooks. We plugged in a USB flash drive containing 4GB worth of video files to the router, and first copied it from the drive to one of the notebooks. The process took two minutes and 14 seconds, with data transfer averaging at 31MB/s. We then repeated the test, this time with both notebooks. Transfer speeds took a dip here, going down to an average of 23MB/s, with the file transfer completed in three minutes. We would attribute the drop in speed to processing bottlenecks on the router’s end because of one key factor: in other, non-MU-MIMO routers, the ordinary observable pattern is to see the transfer speed of one system dip while the other climbs. This wasn’t the case here: the speeds of both transfers rise and fall independent of each other, sometimes climbing and falling together,perhaps due to interference and processing limitations of the router. Also, with two devices going at 23MB/s simultaneously, this means a higher output from the router, going at a total of 46MB/s, compared to 31MB/s when serving a single notebook.
To push things further, we connected a Lenovo IdeaPad Z470 with an 802.11ac adapter to the router, and made it copy the same set of files together with the two Aspire E14s. The two MU-MIMO notebooks suffered another dip in performance, pulling down the average transfer rate to 18MB/s and completing the process in four minutes and 14 seconds. The IdeaPad, however, remained crawling at 1MB/s transfer rate until the two Aspire E14s were done, before going back up to averaging at 10MB/s.