Today's widely deployed 11n access points, routers, and clients all use MIMO, with most access points supporting three spatial streams. But if there are three clients — laptops, smartphones or tablets — associated to a given three-stream 11n access point, only one can send and receive with the access point at a time: in effect, all three spatial streams are "aimed" at only one client. A time slicing algorithm allows fair access by all the clients associated to the access point. Depending on the number of antennas in the client (three for a laptop, one for a smartphone, for example), and hence on the maximum throughput it can support, most of the access point's capacity may be idle at any given moment.
"A common misconception is that multiuser MIMO makes the network 'go faster,'" says Matthew Gast, director of emerging technologies at Aerohive Networks and author of "802.11ac: A Survival Guide". "It really doesn't. It creates multiple logical connections with devices at the same time. This increases network efficiency." That means by being able to transmit to as many as four clients at once, the 11ac access point can make full use of its available "gigabit" capacity at every moment.
Currently, Quantenna Communications offers a commercially available 11ac chip, the QSR1000, with four transmit and four receive antennas, supporting four spatial streams and multi-user MIMO, so far the only chip to do so. Rivals Qualcomm and Broadcom, and others, have pledged to bring MU-MIMO to market. Quantenna says it will introduce an 8 x 8 MIMO 11ac chip in 2015.
The Quantenna chip can talk with up to four clients at the same time. Or it can group clients into as many as four groups, again making four simultaneous transmissions, according to Bahador Amir, technical marketing director for the chipmaker. Within each group of, say, five or six or more clients, the Quantenna chip then uses the traditional Wi-Fi convention of talking to each client one at a time in sequence.
An access point in this case could group together the clients furthest away; as distance from the access point increases, the clients have to step down to lower speeds, which takes more network time, "hogging" the bandwidth inefficiently, according to Amir. But with multi-user MIMO, these more distant clients can be grouped, sharing that capacity, while other closer clients are served at higher speeds, at the same time.
Transmit beamforming, which was optional in 11n and is mandatory in 11ac, is a key companion technology, says Aerohive's Matthew Gast. Transmit beamforming in effect lets the access point "concentrate" or "steer" the RF energy in a specific direction, for a specific client. It does this, first by constantly talking with each client to glean specific channel measurements, which the access point uses to slightly change the signal phasing. That change allows the signal to that client to propagate at a higher effective power level. With MU-MIMO, the access point can thus optimize its signal for up to four clients at the same time.
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