Three keys enabling technologies in 802.11ax

Hello, everyone!

CURRENT Wi-Fi CHALLENGES

A Wi-Fi network needs to be designed to address the growing demand for the high volume and diversity of connected devices and services.

So the problem is: does Wi-Fi network have enough capacity to handle the expanding population of devices as well as users with diverse networking needs?

The changes to the 802.11ax standard will improve the way Wi-Fi networks operate by leveraging technology that greatly improves capacity, provides better coverage, and even reduces congestion.

802.11ax offers the ability for many of devices to simultaneously communicate with each AP radio. The 802.11ax standard is designed to increase capacity by up to four times. This will provide benefits in both the 2.4 GHz and 5 GHz bands in a variance of environments such as enterprises, retail businesses, airports ...etc.

THE 802.11ax

• Enhancing both indoor and outdoor environments by improving the efficiency of traffic management.

• Enhancing operation in the 2.4 GHz and 5 GHz band.

• Improving power efficiency in stations.

• Increasing average throughput per station by at least four times.

To reach the goals, 802.11ax focuses on enhancing the below components:

• 5 GHz and 2.4 GHz support;

• new PHY headers;

• enhanced outdoor robustness;

• OFDMA;

• OFDM;

• TWT - power saving;

• MU-MIMO 8×8  and UL/DL;

• spatial reuse, also referred to as BSS Coloring;

• 1024-QAM.

802.11ax, 802.11ac AND 802.11n COMPARISON

THE 802.11ax TECHNOLOGIES

There are three key enabling technologies in 802.11ax:

• MU-OFDMA;

• MU-MIMO;

• spatial reuse (basic service set [BSS] coloring).

MU-OFDMA

MU-OFDMA is a multi-user version of the orthogonal frequency division multiplexing (OFDM) digital modulation technology currently used for single-user transmissions in 802.11a/g/n/ac radios. For backward compatibility, 802.11ax radios also support OFDM.

OFDMA subdivides a Wi-Fi channel into smaller frequency allocations, called resource units , thereby enabling an access point (AP) to synchronize communication (uplink and downlink) with multiple individual clients assigned to specific RUs.

By subdividing the channel, small frames (such as streaming video) can be simultaneously transmitted to multiple users in parallel.

The simultaneous transmission cuts down on excessive overhead at the medium access control (MAC) sublayer, as well as medium contention overhead.

The AP can allocate the whole channel to a single user or partition it to serve multiple users simultaneously, based on client traffic needs.

So (OFDMA) is the most important new capability in 802.11ax, allowing multiple users with varying bandwidth needs to be served simultaneously.

Source: https://support.huawei.com/enterprise/en/doc/EDOC1100102755

OFDM divides a channel into subcarriers through a mathematical function (IFFT).

The spacing of the subcarriers is orthogonal, so they do not interfere with one another despite the lack of guard bands between them.

This creates signal nulls in the adjacent subcarrier frequencies and prevents intercarrier interference (ICI).

802.11ax introduces a longer OFDM time of 12.8 microseconds, which is four times the legacy time of 3.2 microseconds.

As a result of the longer time, the subcarrier size and spacing decreases from 312.5 kHz to 78.125 kHz. The narrower subcarrier spacing allows better equalization and enhanced channel robustness. Because of the 78.125 kHz spacing, an OFDMA 20 MHz channel consists of a total of 256 subcarriers.

There are three types of subcarriers for 802.11ax:

• pilot subcarriers - these subcarriers are used for synchronization between the transmitter and receiver and do not carry any modulated data;

• data subcarriers - these subcarriers will use the same modulation and coding schemes (MCSs) as 802.11ac as well as two new MCSs with the addition of 1024 quadrature amplitude modulation (1024-QAM);

• unused subcarriers - the remaining unused subcarriers are mainly used as guard carriers or null subcarriers against interference from adjacent channels or sub channels.

Source: https://support.huawei.com/enterprise/en/doc/EDOC1100102755

An OFDMA channel consists of a total of 256 subcarriers. These subcarriers can be grouped into smaller subchannels know as resource units (RUs). When subdividing a 20 MHz channel, an 802.11ax AP can designate 26, 52, 106, and 242 subcarrier RUs, which roughly equates to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels, respectively.

Source: https://support.huawei.com/enterprise/en/doc/EDOC1100102755

MU-MIMO

Multi-user, multiple-input multiple output (MU-MIMO) technology theoretically allows multiple frames to be transmitted to different receivers at the same time and on the same channel, using multiple spatial streams to provide greater efficiency.

MU-MIMO provides spatial diversity by transmitting unique modulated data streams to multiple clients simultaneously.

MU-MIMO would theoretically be a favorable option in very low client density, high-bandwidth application environments where large packets are transmitted.

802.11ax allows for simultaneous use of both MU-OFDMA and MU-MIMO.

OFDMA with MU-MIMO - OFDMA enables multiuser access by subdividing a channel. MU-MIMO enables multiuser access by using different spatial streams.

Spatial reuse (BSS Coloring)

802.11ax was tasked with addressing the OBSS challenge by improving spatial reuse, which is often referred to as BSS coloring.

BSS coloring is a mechanism, originally introduced in 802.11ah, to address medium contention overhead due to OBSS by assigning a different 'color' to each BSS in an environment 'a number between 0 and 7 that is added to the PHY header of the 802.11ax frame'.

BSS coloring detects a color bit in the PHY header of an 802.11ax frame transmission. This means that legacy 802.11a/b/g/n clients will not be able to interpret the color bits because they use a different PHY header format.

When an 802.11ax radio is listening to the medium and hears the PHY header of an 802.11ax frame sent by another 802.11ax radio, the listening radio will check the BSS color bit of the transmitting radio. Channel access is dependent on the color detected.

Source: https://support.huawei.com/enterprise/en/doc/EDOC1100102755

802.11ax radios can adjust the carrier sense operation based on the color of the BSS to improve spatial reuse efficiency and performance.

Depending on the BSS from which the traffic is generated, the station can use different sensitivity thresholds to transmit or defer. This results in higher overall performance.

Using adaptive clear channel assessment (CCA), an 802.11ax radio can raise the SD threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS frames.

BSS coloring can thus potentially decrease the channel contention problem that is symptomatic of existing low SD thresholds.

 
REFERENCES

• Huawei Documents;

• IEEE.