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Key features of Wi-Fi 6

Wi-Fi 6 specifications

Some of the main Wi-Fi 6 specifications supported by Nordic’s nRF70 series are listed in the below table.

Feature	nRF70 Series support
IEEE standards supported	IEEE 802.11 a/b/g/n/ac/ax
Security protocols supported	WEP, WPA, WPA2-PSK, WPA2-PSK-SHA256, WPA3-SAE
Radio bands supported	2.4 GHz and 5 GHz
Maximum PHY throughput	86 Mbps
Channel bandwidths	20 MHz
Multi-user operation	OFDMA, BSS coloring, and spatial re-use
Power Save modes	Legacy, WMM, and TWT
Max spatial streams	1
Co-existence	Yes. With Bluetooth LE, Thread, and Zigbee

Wi-Fi 6 specifications supported by the nRF70 series

Wi-Fi: From start to Wi-Fi 6

Wi-Fi has evolved significantly since its inception. The early IEEE 802.11 standards introduced basic wireless connectivity in the 2.4 GHz band with speeds of 1 Mbps and 2 Mbps. Subsequent releases, such as IEEE 802.11a and 802.11b, aimed to improve data transfer speeds, offering 54 Mbps in the 5 GHz band and 11 Mbps in the 2.4 GHz band, respectively. IEEE 802.11g combined the best of both worlds by operating in the 2.4 GHz band and achieving speeds up to 54 Mbps.

The advent of IEEE 802.11n marked a milestone in the Wi-Fi evolution. This standard operated in both the 2.4 GHz and 5 GHz bands, delivering speeds up to 600 Mbps and enabling seamless multimedia streaming. IEEE 802.11ac (Wi-Fi 5) further enhanced performance, reaching speeds over 1000 Mbps primarily in the 5 GHz band and enabling 4K video streaming.

The latest standard, IEEE 802.11ax (Wi-Fi 6), introduced advancements in radio access, antennas, and power saving. It provides gigabit speeds even in crowded environments, wider coverage, and reduced power consumption. Wi-Fi 6 delivers such high performance at low power consumption figures and higher device density, which makes it a perfect candidate for low-power IoT adoption.

The unmatched radio performance Wi-Fi 6 provides is due to a variety of features enabled by the IEEE 802.11ax standard. The figure below provides an overview of the key features Wi-Fi 6 supports. However, since some of these features are mainly targeting non-power-constrained applications where data throughout is a priority over battery consumption, Nordic’s Wi-Fi solutions support only a subset of these features. Key Wi-Fi 6 features supported by Nordic’s Wi-Fi solutions are:

Orthogonal Frequency Division Multiple Access (OFDMA)
Beamforming
Longer Symbol Duration
Target Wake Time
BSS Coloring

Wi-Fi 6 features supported by Nordic’s Wi-Fi solution

OFDMA

Wi-Fi uses the popular modulation technique known as Orthogonal Frequency Division Multiplexing (OFDM), which divides the available frequency spectrum into subcarriers. The orthogonality of the subcarriers plays a pivotal role, allowing these subcarriers to overlap without causing interference, thus optimizing the use of the available spectrum. This orthogonality ensures that the peak of one subcarrier aligns with the null of the adjacent subcarrier, preventing them from interfering with each other.

However, per one-time segment, one user occupies all the available subcarriers in a specific channel. For example, in a 20 MHz Wi-Fi 5 channel, one user would use the 52 subcarriers available for data transmission.

With Wi-Fi 6, the focus shifted to more efficient use of the already available radio resources by introducing Orthogonal Frequency-Division Multiple Access (OFDMA). Wi-Fi 6, more specifically the 802.11ax standard, also brought an improvement in the number of available subcarriers, increasing it to four times as many as the previous version.

OFDMA brings a frequency multiplexing ability to the modulation scheme, enabling multiple users at the same time. This is done by grouping the subcarriers into Resource Units (RUs), with each user being allocated one RU. RUs can have variable lengths, with the smallest possible being 26 subcarriers.

The image below shows the utilization of the available frequency spectrum with OFDM in Wi-Fi 2-5 and OFDMA in Wi-Fi 6. As we can see, OFDMA allows multiple users to use the same time-segment, thereby utilizing much more of the spectrum.

The benefits of OFDMA in Wi-Fi 6 include:

Increased spectral efficiency: Multiple devices can transmit data simultaneously, optimizing bandwidth utilization and improving overall network performance.
Enhanced network capacity: Multiple users can access the same resource, allowing for a larger number of concurrent connections without compromising Quality of Service (QoS).
Reduced latency: Parallel transmission and reception enable faster response times, eliminating congestion and delays for time-sensitive applications.

Longer symbol guard intervals

As mentioned before, Wi-Fi 6 increases the number of subcarriers in the same channel bandwidth, making each subcarrier narrower. Since time is the reciprocal of frequency, narrower subcarriers lead to an increase in symbol duration. Specifically, it increased from 3.2 μs in Wi-Fi 5 to 12.8 μs in Wi-Fi 6.

Longer symbol duration allows Wi-Fi 6 to introduce longer symbol guard intervals. Guard intervals are the time periods in between consecutive symbols where no user data is sent. It acts as a buffer between symbols, reducing inter-symbol interference.

Such separation between symbols is key, especially in poor multipath propagation environments. Longer guard times mean receivers are less prone to bit errors caused by multipath propagation, which directly improves performance and coverage.

BSS coloring and spatial re-use

In previous sections, we explored how OFDMA serves as the multiple access scheme at the PHY layer in Wi-Fi 6, enabling simultaneous transmissions over different subcarriers. Now, let’s discuss the multiple access scheme used at the MAC sublayer in Wi-Fi, namely, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

Before transmission, the Wi-Fi radio checks if the channel is occupied. The channel could be occupied by a device in the same basic service set (BSS) or by a device from a neighboring/overlapping BSS. If the channel is occupied, the device waits for some time before retrying. While effective in avoiding packet collisions, this approach can lead to contention and under-utilization of available resources.

Wi-Fi 6 solves this by using BSS coloring and spatial reuse. In simple terms, BSS coloring refers to each BSS by adding a color (6-bit field) to the PHY header of all its Wi-Fi frames. Before a device transmits a packet, it will “sense” the channel first. If the channel is occupied by a transmission of the same color, the device recognizes that this transmission is within its same BSS, and it will yield and not transmit.

However, due to spatial reuse, if the device finds the channel to be occupied with a transmission of a different color, it understands that this transmission is from a neighboring/overlapping BSS. If the received power of this transmission is below a certain threshold, the device will make its transmission regardless. The benefits of spatial re-use, in terms of enhanced network efficiency and performance, outweigh the minor probability of packet loss due to interference from a relatively far away device.

In the figure below, we can see an overlapping BSS, i.e a service set that can be seen by nearby APs or STAs. If both Basic Service Sets are operating on the same channel, the CSMA/CA mechanism will affect transmission, which is where BSS coloring comes in to achieve better performance.

Beamforming

Beamforming is a signal processing technique where the transmitter’s antennas focus the radiated signal towards a specific user, forming a beam for that user. This is opposed to the conventional omnidirectional radiation pattern which looks more like a sphere, as shown in the below figure.

This brings numerous benefits to the network performance such as reduced interference, improved signal quality and range, and increased battery lifetime.

Target Wake Time

TWT (Target Wake Time) is one of the most important features of Wi-Fi 6, and the main reason why Wi-Fi 6 is an enabler for low-power applications. Target Wake Time allows a Wi-Fi device to negotiate sleep and wake-up times with the AP, which allows the device to sleep in idle time, thus, saving on power consumption.

This is opposed to the conventional power saving techniques used in legacy Wi-Fi, where the device has to wake up for a pre-determined interval and poll the AP for messages.

We will take a closer look at Target Wake Time and other power saving modes in Lesson 6.

Wi-Fi Fundamentals