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How does 802.11ac get to 6.9 Gigabits per second?

You know from a previous post how 802.11n gets to 600 megabits per second. 802.11ac does just three things to increase that by 1,056%:

  1. It adds a new Modulation and Coding Scheme (MCS) called 256-QAM. This increases the number of bits transmitted per symbol from 6 to 8, a factor of 1.33.
  2. It increases the maximum channel width from 40 MHz to 160 MHz (160 MHz is optional, but 80 MHz support is mandatory.) This increases the number of subcarriers from 108 to 468, a factor of 4.33.
  3. It increases the maximum MIMO configuration from 4×4 to 8×8, increasing the number of spatial streams by a factor of 2. Multi-User MIMO (MU-MIMO) with beamforming means that these spatial streams can be directed to particular clients, so while the AP may have 8 antennas, the clients can have less, for example 8 clients each with one antenna.

Put those factors together and you have 1.33 x 4.33 x 2 = 11.56. Multiply the 600 megabits per second of 802.11n by that factor and you get 600 x 11.56 = 6,933 megabits per second for 802.11ac.

Note that nobody does this yet, and 160 MHz channels and 8×8 MIMO are likely to remain unimplemented for a long time. For example Broadcom’s recently announced BCM4360 and Qualcomm’s QCA9860 do 80 MHz channels, not 160 MHz, and 3 x 3 MIMO, so they claim maximum raw bit-rates of 1.3 gigabits per second. Which is still impressive.

Maximum theoretical raw bit-rate is a fun number to talk about, but of course in the real world that will (almost) never happen. What’s more important is the useful throughput (raw bit-rate minus MAC overhead) and rate at range, the throughput you are likely to get at useful distances. This is very difficult, and it is where the manufacturers can differentiate with superior technology. For phone chips power efficiency is also an important differentiator.

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