The big deal about 802.11ac, or "gigabit Wi-Fi," is the promise of much higher speeds for Wi-Fi clients compared to the maximums under 802.11n today. But 11ac is actually several big deals, including something called multiuser MIMO [my-moe], which is part of the "Wave 2" 11ac features expected later this year.
The IEEE 11ac standard adds or extends a number of technologies that, first, give a big boost to the overall data rate, and the accompanying throughput; and second that "protect" and sustain those gains, with a more reliable signal that's able to support higher throughput over longer distances and gradually and gracefully step down to lower throughput in order to maintain a reliable connection.
With 11ac, channel width jumps to 80 MHz wide (compared to 20 MHz for 11n), and then to 160 MHz wide in Wave 2; supports up to four spatial streams in Wave 1, and up to eight in Wave 2; a much higher density modulation scheme, 256 QAM vs 64 QAM for 11n; and in Wave 2, multiuser MIMO with transmit beamforming.
The basic idea of MU-MIMO is simple: An 11ac access point radio can send data to up to four client radios at the same time (though the clients also have to support MU-MIMO), directing a separate "spatial stream" to each one. By contrast, today's three-stream 11n access points can only talk to one client at a time, one after the other. Basically, MU-MIMO will make the Wi-Fi network much more efficient; and ensure that, as more users connect to the access point, they will have an overall better "wireless experience." A few chips for access points and wireless routers are available now and vendors expect the first network and client products to be available starting late this year.
Technically, multi-user MIMO is an 11ac option; vendors are not required to support it. But most observers think that, in short order, it will be a standard feature of almost all 11ac products.
MIMO was first introduced in the 802.11n standard. It took what had been a problem, and turned it into a way to vastly increase Wi-Fi data rates. The problem is called multi-path, which describes what happens when a radio signal is transmitted and then is reflected and bounced around by objects around it, arriving at the receiver in multiple paths at slightly different times and angles. The result is interference and a slower signal.
MIMO exploits the characteristics of multipath, by adding antennas on both the transmit end and the receiving end of a signal. The multiple antennas, via spatial diversity, improve the transmission's signal-to-noise ratio and improve the reliability of the system by combatting fading. They also, through spatial multiplexing, use the different paths between the antenna groupings to carry vastly more data via two or three or four spatial streams.
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