802.11ax: Efficient use of RF spectrum to increase network capacity

Whereas 802.11ac made the network faster, 802.11ax is designed for higher channel efficiency in a dense networking environment.
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In my last Industry Insight, I outlined how 802.11ax is addressing the changing needs of WiFi networks. In this article, I look at how 802.11ax will provide efficient use of the RF spectrum to increase network capacity.

Whereas 802.11ac made the network faster, 802.11ax is designed for higher channel efficiency in a dense networking environment. The key technology behind 802.11ax − multiple access OFDMA (orthogonal frequency division multiple access) − comes from the 3GPP LTE cellular technology and 802.16e WiMax. OFDMA is a field-proven technology to support higher density wireless networks.

802.11ax anticipates higher density in several ways: scheduled transmission time, dynamic frequency allocation in 2MHz increments, spatial reuse and improved battery management mechanisms.

802.11a, 802.11g, 802.11n and 802.11ac use OFDM (orthogonal frequency division multiplexing). OFDM was a significant enhancement over 802.11b DSSS. OFDM splits the signal over multiple narrow band carriers. This results in reduced crosstalk interference and allowed higher RF rates.

The capacity and high throughput advantages make MU-MIMO another key technology for 802.11ax.

In an 802.11ac network with OFDM, the smallest frequency unit assigned to a transmission is 20MHz. The entire 20MHz frequency is used to transmit to and from any single client device. If that device is sending a small 64 Byte IOT packet or a 1 248 Byte video packet, it uses the same 20MHz (we acknowledge that narrowband industrial and IOT applications using 802.11 can be purpose-built to use 5MHz or 10MHz frequencies. This is not equivalent to OFDMA sub-channels).

OFDMA divides up the 20MHz channel into 256 subcarriers. The smallest unit is 26 subcarriers in 2MHz of frequency. These subcarriers can be grouped together into larger units; at 52 (4MHz), 106 (8MHz), and 242 (20MHz) to support higher bandwidth applications. Each grouping of subcarriers is called a resource unit (RU).

Any single client device is assigned one or more RU. Each RU can be transmitted at a unique QAM (quadrature amplitude modulation) level. Additionally, as the client device moves farther away from the access point, the number of RU and the QAM level can be adjusted to maintain a strong signal level even as throughput degrades. Dynamic RU allocation is a direct benefit of OFDMA use in LTE networks.

By splitting the transmissions into smaller frequency units, and dynamically assigning RUs, the network can use the entire 20MHz wide channel to support multiple devices running at different speeds and with different throughput requirements.

Consider an enterprise hotel with IOT sensors and conference attendees in the ballroom. IOT sensors can be allocated a single 2MHz RU to send sensor updates to a server, while attendees accessing the Internet can be assigned an 8MHz RU to support a high-capacity and high-density wireless network. The full 20MHz channel will be utilised at each transmission opportunity.

MU-MIMO is back

Well, it never really went away. In 802.11ac wave 2 access points, DL MU-MIMO provides some network throughput improvement by scheduling transmissions to multiple clients at the same time. In the fourth Industry Insight in this series I’ll provide a longer discussion of DL MU-MIMO in 802.11ac.

This technology is particularly well-suited to support multiple high bitrate clients such as video streaming that use a large frame size. As the 802.11ax technology evolves, MU-MIMO will work in both downstream and upstream directions. Another key advantage with 802.11ax is support for eight concurrent MU-MIMO transmissions concurrently, double the number supported by 802.11ac.

One of the limiting factors for MU-MIMO in 802.11ac is the high management overhead required to communicate subcarrier information to the access point. In 802.11ax, enhancements to MU-MIMO will include grouping multiple client updates together to improve protocol efficiency and remove excess management overhead.

The capacity and high throughput advantages make MU-MIMO another key technology for 802.11ax. Note that MU-MIMO and MU-OFDMA are inherently incompatible. Thus the access point will choose when and how to use each technology for maximum performance and efficiency.

Teresa Huysamen

Head of the wireless and surveillance business unit, Duxbury Networking.

Teresa Huysamen is head of the wireless and surveillance business unit at Duxbury Networking. She has been part of the IT sector for the past 26 years – 15 of which have been at Duxbury – and is passionate about her role in the industry. As part of the product, marketing and sales cycle, she is focused on meeting customer needs with innovative technology.

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