New generation of 802.11ac devices keeps networks fast even with lots of users

MU-MIMO will be hitting the market over the next few months.
by Peter Bright - Jan 7 2015, 7:15pm PST  ars technica

LAS VEGAS—Wi-Fi networks promise to get faster still this year with the release of hardware supporting an important optional feature of the 802.11ac wireless specification, MU-MIMO. At CES this week, companies, including D-Link and Netgear, announced MU-MIMO-compatible devices powered by Qualcomm's range of Wi-Fi chipsets.

Traditional Wi-Fi is a broadcast technology. If a base station is talking to a device on the network, every other device on the same network has to wait its turn. So while typical 802.11ac offers fast headline speeds of 1.3Gbps or more, that bandwidth ends up being shared. If there are three computers connected to the base station, then on average they each get only 325Mbps of bandwidth.

MU-MIMO ("multi-user multiple input multiple output") means that those three computers could each get close to the 1.3Gbps of bandwidth simultaneously, through a process called beamforming.

A MU-MIMO router measures the time it takes for a client to receive data on the network, and by comparing the different timings from the different antennas, the base station can figure out which direction the client lies in. Using this information, the base station can then modulate the signals transmitted from its antennas so that the signal is strongest in the direction of the client and reduced everywhere else.

This means that instead of broadcasting the signal equally in all directions, the base station is only sending it in the client's direction. Here's where the "multi user" aspect kicks in: the base station cansimultaneously send a signal beam to a second client, provided that the client isn't in the same direction as the first client.

MU-MIMO only works in the downstream direction, from the base station to the client. That's because it needs a bunch of spatially separated antennas and complex signal processing; most clients, especially small ones such as smartphones, don't have space.

802.11ac MU-MIMO supports transmission in up to four different directions simultaneously. While devices located close together, in the same direction, will still have to share bandwidth, devices that are spatially separated should be able to receive at close to the maximum speed possible.

Both the clients and access points need to support MU-MIMO. The changes on the client side are small (such that a driver or firmware update may be sufficient to upgrade existing 802.11ac adapters to support MU-MIMO), the greater complexity on the access point end means that new hardware will be required. The alien-looking D-Link AC3100 and AC5300 routers, shipping later this year, will support the specification, as will some forthcoming Netgear devices.