IEEE 802.11ac

The specification has multi-station throughput of at least 1.1 gigabit per second (1.1 Gbit/s) and single-link throughput of at least 500 megabits per second (0.5 Gbit/s). This is accomplished by extending the air-interface concepts embraced by 802.11n: wider RF bandwidth (up to 160 MHz), more MIMO spatial streams (up to eight), downlink multi-user MIMO (up to four clients), and high-density modulation (up to 256-QAM).

The Wi-Fi Alliance separated the introduction of 802.11ac wireless products into two phases ("waves"), named "Wave 1" and "Wave 2". From mid-2013, the alliance started certifying Wave 1 802.11ac products shipped by manufacturers, based on the IEEE 802.11ac Draft 3.0 (the IEEE standard was not finalized until later that year). Subsequently, in 2016, Wi-Fi Alliance introduced the Wave 2 certification, which includes additional features like MU-MIMO (downlink only), 160 MHz channel width support, support for more 5 GHz channels, and four spatial streams (with four antennas; compared to three in Wave 1 and 802.11n, and eight in IEEE's 802.11ax specification). It meant Wave 2 products would have higher bandwidth and capacity than Wave 1 products.

New technologies

New technologies introduced with 802.11ac include the following:

Extended channel binding
- Optional 160 MHz and mandatory 80 MHz channel bandwidth for stations; cf. 40 MHz maximum in 802.11n.
More MIMO spatial streams
- Support for up to eight spatial streams (vs. four in 802.11n)
Downlink multi-user MIMO (MU-MIMO, allows up to four simultaneous downlink MU-MIMO clients)
- Multiple STAs, each with one or more antennas, transmit or receive independent data streams simultaneously.
  - Space-division multiple access (SDMA): streams not separated by frequency, but instead resolved spatially, analogous to 11n-style MIMO.
- Downlink MU-MIMO (one transmitting device, multiple receiving devices) included as an optional mode.
Modulation
- 256-QAM, rate 3/4 and 5/6, added as optional modes (vs. 64-QAM, rate 5/6 maximum in 802.11n).
- Some vendors offer a non-standard 1024-QAM mode, providing 25% higher data rate compared to 256-QAM
Other elements/features
- Beamforming with standardized sounding and feedback for compatibility between vendors (non-standard in 802.11n made it hard for beamforming to work effectively between different vendor products)
- MAC modifications (mostly to support above changes)
- Coexistence mechanisms for 20, 40, 80, and 160 MHz channels, 11ac and 11a/n devices
- Adds four new fields to the PPDU header identifying the frame as a very high throughput (VHT) frame as opposed to 802.11n's high throughput (HT) or earlier. The first three fields in the header are readable by legacy devices to allow coexistence
- DFS was mandated between channels 52 and 144 for 5 GHz to reduce interference with weather radar systems using the same frequency band.

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Features

Mandatory

Borrowed from the 802.11a/802.11g specifications:
- 800 ns regular guard interval
- Binary convolutional coding (BCC)
- Single spatial stream
Newly introduced by the 802.11ac specification:
- 80 MHz channel bandwidths

Optional

Borrowed from the 802.11n specification:
- Two to four spatial streams
- Low-density parity-check code (LDPC)
- Space–time block coding (STBC)
- Transmit beamforming (TxBF)
- 400 ns short guard interval (SGI)
Newly introduced by the 802.11ac specification:
- five to eight spatial streams
- 160 MHz channel bandwidths (contiguous 80+80)
- 80+80 MHz channel bonding (discontiguous 80+80)
- MCS 8/9 (256-QAM)

New scenarios and configurations

The single-link and multi-station enhancements supported by 802.11ac enable several new WLAN usage scenarios, such as simultaneous streaming of HD video to multiple clients throughout the home, rapid synchronization and backup of large data files, wireless display, large campus/auditorium deployments, and manufacturing floor automation.

To fully utilize their WLAN capacities, 802.11ac access points and routers have sufficient throughput to require the inclusion of a USB 3.0 interface to provide various services such as video streaming, FTP servers, and personal cloud services. With storage locally attached through USB 2.0, filling the bandwidth made available by 802.11ac was not easily accomplished.

Example configurations

All rates assume 256-QAM, rate 5/6:

Wave 1 vs. Wave 2

Wave 2, referring to products introduced in 2016, offers a higher throughput than legacy Wave 1 products, those introduced starting in 2013. The maximum physical layer theoretical rate for Wave 1 is 1.3 Gbit/s, while Wave 2 can reach 2.34 Gbit/s. Wave 2 can therefore achieve 1 Gbit/s even if the real world throughput turns out to be only 50% of the theoretical rate. Wave 2 also supports a higher number of connected devices.

Data rates and speed

Several companies are currently offering 802.11ac chipsets with higher modulation rates: MCS-10 and MCS-11 (1024-QAM), supported by Quantenna and Broadcom. Although technically not part of 802.11ac, these new MCS indices became official in the 802.11ax standard, ratified in 2021.

160 MHz channels are unavailable in some countries due to regulatory issues that allocated some frequencies for other purposes.

Advertised speeds

802.11ac-class device wireless speeds are often advertised as AC followed by a number, that number being the highest link rates in Mbit/s of all the simultaneously usable radios in the device added up. For example, an AC1900 access point might have 600 Mbit/s capability on its 2.4 GHz radio and 1300 Mbit/s capability on its 5 GHz radio. No single client device could connect and achieve 1900 Mbit/s of throughput, but separate devices each connecting to the 2.4 GHz and 5 GHz radios could achieve combined throughput approaching 1900 Mbit/s. Different possible stream configurations can add up to the same AC number.