The IEEE 802.11ax standard (marketed as Wi-Fi 6 by the Wi-Fi Alliance) was published in 2021 and provided an important step forward for wireless connectivity.

Wi-Fi 6 retains the same bandwidth as Wi-Fi 5, adds back the 2.4 GHz frequency band (Wi-Fi 6E further adds the 6 GHz frequency band) and leverages innovative features to boost the maximum data rate to 9.6 Gbps[i]. The data in the table below shows the headline performance figures.

Being backward-compatible, Wi-Fi 6 devices can communicate with previous generation stations (STAs), though they are limited to the highest speed afforded by that generation.

[i] The maximum rate is achieved when data is transmitted at the highest HE-MCS11 with a code rate of 5/6 in a 160 MHz or 80+80 MHz channel with 8 spatial streams and a GI of 0.8 μs.

Table 1. Derived from Wi-Fi Alliance document entitled Wi-Fi CERTIFIED 6™ A new era for Wi-Fi®. Maximum data rate depends upon number of spatial streams and channel used.

An IEEE 802.11ax compliant device is defined as High Efficiency (HE) STA. The name indicates the essence of the Wi-Fi 6 standard lies in enhancing spectrum efficiency, which is distinct from Wi-Fi 5 (802.11ac), which focused on Very High Throughput.

The focus of the Wi-Fi 6 standard is not (only) about boosting the data transmission rate of the individual devices. Rather, it addresses the fact that Wi-Fi usage is so pervasive that network performance can be degraded in areas of dense Wi-Fi traffic, such as sports stadiums, concert halls, public transportation hubs, and increasingly even in our homes, where routers must communicate with a growing number of digital gadgets simultaneously [1].

The Wi-Fi 6 standard improves spectrum utilization and Wi-Fi performance in high-density scenarios by means of the key features shown in Figure 1 and detailed in the following sections.

Orthogonal Frequency-Division Multiple Access (OFDMA)

Wi-Fi 5 employed OFDM, in which a transmitting device occupies all the subcarriers of the entire channel at a given time.

Wi-Fi 6 introduces OFDMA, which is a multiple access technology based on OFDM. OFDMA subdivides the channel subcarriers and allocates them to one or several Wi-Fi stations concurrently. This allows STAs to transmit and receive data simultaneously without causing mutual interference [3].

OFDM is a single user transmission technology. This means that each time data is sent, each of the Wi-Fi 5 STAs occupies the entire channel when transmitting. Therefore, only one STA is allowed to transmit at any one time, even if it is transmitting a small data frame. Let’s imagine Wi-Fi communication is express delivery, and information represents the goods to be transported to the receiver. In OFDM, the van delivers one package per trip, regardless of its size. Consequently, some of the space in the van is usually wasted, as shown in Figure 2.

To make better use of the van’s space, Wi-Fi 6 introduces OFDMA, which is essentially a multiple access technique. Wi-Fi 6 STAs are allowed to transmit data at the same time, each occupying a different frequency range of the channel. In more detail, OFDMA divides channel resources into multiple Resource Units, RUs.

Different users are allocated these RUs, which carry their respective data. In this way, data of multiple users can be sent on one channel simultaneously.

Let’s review the delivery van analogy. With OFDMA, the van is divided into several compartments to simultaneously carry different packages. As such, it can deliver several packages to different receivers on a single trip, as shown in Figure 3.

In high-density Wi-Fi networks, the adoption of OFDMA-based uplink and downlink channel access methods enables concurrent transmissions among multiple devices. This mechanism not only minimizes contention between Access Points (APs) and STAs for the shared channel but also effectively decreases the likelihood of packet collisions. As a result, these enhancements lead to a significant boost in channel utilisation efficiency and, consequently, a marked improvement in overall network performance.

Multi-User Multiple-Input Multiple-Output (MU-MIMO)

In a MU-MIMO communication, the transmitter simultaneously sends data streams to multiple receivers through multiple antennas.

While OFDMA allows multi-user multiplexing in the frequency domain, MU-MIMO allows multiple users to use different spatial streams, thereby increasing the throughput.

By referring to the delivery van example, a MU-MIMO transmitter corresponds to a fleet of vans which deliver goods to multiple receivers, as shown in Figure 4.

MU-MIMO was introduced in Wi-Fi 5 but was limited to downlink (DL) traffic only. The Wi-Fi 5 standard defines DL MU-MIMO support for up to four users and a maximum of eight spatial streams.

In contrast, Wi-Fi 6 MU-MIMO extends this capability by defining support up to eight users and eight spatial streams for both uplink and downlink traffic, as shown in Figure 5. This means that Wi-Fi 6 APs can transmit to and receive data from multiple STAs simultaneously, enabling bidirectional concurrent data transmission and further improving channel utilisation [3].

The Wi-Fi 6 standard also facilitates the combination of the OFDMA and MU-MIMO technologies, providing a more flexible and efficient method to support multiple user scenarios.

1024-QAM

By way of background, quadrature amplitude modulation (QAM) is a technique used to transmit information.

Wi-Fi 5 uses 256-QAM which carries 8 data bits per symbol, while Wi-Fi 6 uses 1024-QAM, which carries 10 bits of data per symbol. This translates to a 25% increase in the data throughput for Wi-Fi 6 devices.

An easy-to-understand analogy is a road optimisation in which the radio frequency channel is represented by the road and the data being transmitted by the traffic on that road [4].

The 256-QAM modulation in Wi-Fi 5 is represented by a road with wider, fewer lanes.

The introduction of 1024-QAM in Wi-Fi 6 provides for a greater number of narrower lanes within the same total road width (spectrum).

BSS Colouring

A Basic Service Set (BSS) is a group of wireless devices that communicate with each other using a single AP in an infrastructure mode. Within a BSS, multiple devices contend for access to the wireless channel at any given time.

In a high-density scenario multiple APs may operate simultaneously in a confined space, leading to Overlapping Basic Service Sets (OBSS). Each BSS operates on its own dedicated channel, enabling APs and STAs from different BSSs to communicate on their respective channels. However, due to the restricted number of channels in the operating frequency bands, nearby APs may inadvertently select the same operating channel. This results in mutual interference between them, causing devices within different BSSs to defer their transmissions whenever they detect activity from neighbouring BSSs [3].

Wi-Fi 6 introduces Spatial Reuse (SR) technology and BSS colouring to enhance performance and reduce interference in OBSS scenarios.

SR and BSS colouring features enable APs and STAs within overlapping BSSs to discern interference from their own BSS or neighbouring BSS, thereby allowing them to transmit efficiently without impacting other BSSs.

To determine which BSS is the originator of a frame without decoding the entire frame, 802.11ax uses the non-unique ID of the BSS, called the BSS colour, which is transmitted in the frame preamble. Initially, the BSS colour field of 3 bits length appeared in 802.11ah to reduce power consumption. To decrease the BSS colour collision probability, the IEEE 802.11ax standard increases the length of the BSS colour field to 6 bits [5].

When a Wi-Fi device receives data frames, it determines their origin by examining the BSS Colour field. If the data originates from another BSS and the signal strength is below a certain threshold, the device considers the interference negligible for the OBSS signals.

Figure 6 shows how BSS colouring helps to eliminate interference in high-density AP deployments. APs on the same channel will not interfere with each other as long as their channels are marked with different colours, for example, an AP on black channel 36 and an AP on grey channel 36 can transmit data at the same time.

SR reuse and BSS Colouring promote operational efficiency within each BSS and more efficient use of the available Wi-Fi channels. In a stadium, trade-show floor, airport or similarly crowded environment with multiple routers and other access points, BSS Colouring can dramatically increase throughput and decrease latency, improving overall Wi-Fi efficiency for all users.

Target Wake Time

For many Wi-Fi devices (e.g., laptops and smartphones) power consumption is an important issue. In 802.11 networks, power management is based on alternating between two states: awake and doze. In the awake state, a STA can transmit and receive frames, while in the doze state, its radio is switched off.

In the traditional Wi-Fi power-saving mode, a STA wakes up periodically, receives the Beacon frame broadcast by the AP, and determines whether there is any buffered data awaiting it. If there is an indication of buffered information, the STA sends a poll frame to the AP to request the downstream data, after which the AP sends the buffered data frames to the STA. Once the STA concludes the reception process, it returns to its doze state [3].
In typical 802.11ax scenarios with dense networks, the high traffic load and the large number of power-limited smartphones and laptops, legacy power-saving mechanisms are inefficient.

Moreover, the need to poll the AP to receive buffered traffic leads to a relatively huge overhead.

To minimize the contention between STAs and to reduce power consumption, TGax adopted the Target Wake Time (TWT) mechanism, which was initially conceived as part of IEEE 802.11ah, a Wi-Fi standard for the Internet of Things scenarios and requirements.

TWT allows a STA – called the TWT requesting STA – to negotiate TWT service periods (TWT SPs) with another STA or AP – called the TWT responding STA. In the negotiated TWT SPs, the TWT requesting STA wakes up for some time and exchanges frames with the TWT responding STA.
Thanks to this mechanism, the TWT requesting STA can doze continuously except during the TWT SP intervals [5].

TWT decreases power consumption for devices requiring infrequent, low-rate data transmission, such as IoT devices, thereby reducing the number of devices waking up simultaneously and competing for the wireless medium. TWT technology provides advanced management for power saving mode, extending the battery life of STAs.

Alessia Autolitano, Andrea Pezzoli and Roberto Ricci are Wi-Fi experts with Sisvel Technology based in None (Turin), Italy.