WLAN 802.11ax Tutorial: WiFi 6 Basics

IEEE 802.11ax, commonly known as WiFi 6, is a next-generation wireless standard designed to improve efficiency, speed, and capacity in dense environments. It operates in both the 2.4 GHz and 5 GHz bands, with theoretical speeds up to 9.6 Gbps. WiFi 6 introduces advanced features like Orthogonal Frequency Division Multiple Access (OFDMA), MU-MIMO, and Target Wake Time (TWT), which enhance performance, reduce congestion, and optimize power consumption, making it ideal for high-density networks such as stadiums, airports, and smart homes.

This WLAN 802.11ax tutorial covers WiFi 6 basics including 802.11ax features, frame, modes, physical layer, MAC layer, advantages, and disadvantages.

What is 802.11ax (WiFi 6)? Introduction

The 802.11ax standard is the latest in the series of IEEE 802.11 WLAN standards. The 11ax devices are backward compatible with legacy WiFi devices as per 11a/g/n/ac standards operating in the same band.

Unlike 802.11ac, 802.11ax operates in both 2.4GHz and 5GHz bands. The increase in WLAN users leads to a reduction in data throughput due to packet collisions over the air and shorter free time periods for them to transmit. Moreover, closer WLAN APs (or routers) lead to interference, especially in apartment buildings or offices.

The previous WLAN standards (802.11g/n/ac) have introduced larger bandwidth, higher modulation schemes, and MIMO in order to increase the data throughput. The new features have been introduced in 802.11ax to make existing transmission capacities more efficient.

WLAN 802.11ax technology helps in performance improvement in extensively used WiFi networks at airports, train stations, stadiums, halls, apartments, public transportations, etc. It supports a coverage range of about 100 meters based on the modulation scheme used in addition to MIMO/Beamforming.

Image Courtesy: Ruckus Networks

Following are the major changes made in the 802.11ax technology:

• Guard interval and Symbol duration have been increased by a factor of 4 (subcarrier offset is reduced by a factor of 4). Hence, the WiFi signal becomes more robust for outdoor applications.
• 1024QAM has been added, hence it helps in achieving higher maximum data rates.
• Spatial multiplexing is extended to support 8x8 MIMO
• OFDMA has been introduced in both downlink and uplink to share OFDM subcarriers among multiple users, unlike OFDM used in previous standards where all the subcarriers are allocated for a single user. Due to OFDMA, a single channel can be shared by multiple users in parallel.
• MU (Multi-user) MIMO has been added in the uplink also, whereas in previous standards it was available only in the downlink part. Hence, higher throughput is achieved in the uplink through SDM (Spatial Division Multiplexing).
• An extended range preamble is used in the frame structure.

Following are silent features of 802.11ax WiFi technology which is also known as WiFi-6.

WLAN 802.11ax Frame Structure

Figure-1 depicts WLAN 802.11ax frame Structure. As shown, it consists of preamble, signal header, data payload, and packet extension.

WLAN 802.11ax Physical Layer (PHY)

Following are the features introduced in the 802.11ax physical layer. Let us understand PHY features as part of 802.11ax tutorial.

• OFDMA has been introduced in both the downlink and uplink. Hence multiple users can simultaneously transmit and share available bandwidth. Using OFDMA, each user is allotted a portion of spectrum by allocating contiguous set of subcarriers (known as “RU (Resource Unit)”). In 802.11ax, RU can take up a set of subcarriers (min. 26 and max. 996). For 20 MHz bandwidth, there are various RU options available. For example, RU26 allows up to 9 users to transmit simultaneously. Similarly for 160 MHz, there are about 74 simultaneous users.

• 1024-QAM modulation scheme has been introduced in addition to BPSK, 16-QAM, 64-QAM and 256-QAM. It helps in increasing data rates.
• The other major change is an increase in guard and symbol interval by four times.
• Downlink multi-user MIMO (DL MU-MIMO) has been introduced in 802.11ac itself. The same has been continued and in addition beamforming is added. This helps in directing beam towards each individual users more precisely. This has extended support for parallel data streams from 4 to 8 from previous release of 802.11ac. The figure depicts multi-user MIMO in downlink direction (from AP to Stations(STAs)).

• Uplink multi-user MIMO (UL MU-MIMO) has been implemented for the first time in WLAN 802.11ax. Hence multiple users can transmit simultaneously using additional antennas which increases effective data rate in the WiFi network. The figure depicts multi-user MIMO in uplink direction (From Stations(STAs) to AP).

• BSS coloring concept is added in 802.11ax to eliminate interference from interfering AP (Access Point). For example, as shown in the figure, STA2 receives a signal from its own network using AP1 and from a neighbor interfering AP (i.e. AP2). Due to BSS coloring, STA2 ignores the signal from AP2 and remains connected with AP1. The figure depicts BSS coloring concept used in 802.11ax.

• FEC schemes: IEEE 802.11ax proposes to use LDPC for large bandwidth (i.e. channel bonding) and BCC for narrower bandwidth scenarios.

WLAN 802.11ax MAC Layer

• Only one packet type has been used in legacy WLAN standards and there was no distinguishing feature to segregate downlink and uplink directions. IEEE 802.11ax introduces four packet types viz. HE_SU, HE_MU, HE_TRIG and HE_EXT_SU. Each of these packets uses different preamble followed by payload.
• Single User mode (HE_SU): It is used to establish communication between AP and single user. Both uplink and downlink use the same packet structure. The single user is allocated the entire spectrum with maximum size RU. This eliminates overhead usually found in the multi-user mode.
• Multi-User mode (HE_MU): It is used in the downlink direction (from AP to STAs) along with OFDMA. The packet contains HE_SIG_B field which carries information about RU allocation for all the individual users i.e. STATIONs.
• Multi-User uplink (HE_TRIG): All the users (i.e. STAs) transmit simultaneously over the uplink using OFDMA technique when they are triggered by AP. In order to avoid collisions, each users are pre-assigned with their share of allocations in the spectrum in units of RUs. Moreover they have been assigned with power levels and how much data to be transported in the packet. To achieve this, AP first sends out trigger frame to all the STAs. This trigger frame contains configuration parameters including payload length, BW, RU allocation, modulation mode etc. All the stations start their PPDU transmissions after a predefined time interval (i.e. SIFS).
• Extended range PPDU (HE_EXT_SU): This mode is designed for outdoor WiFi usage over long distances where poor SNR is usually found. 20 MHz BW is used along with the most robust modulation techniques such as BPSK/QPSK. Preamble part in the packet is transmitted with 3 dB higher power compared to the rest of the part.

Following MAC layer enhancements are possible to be introduced by IEEE in the 802.11ax standard.

• Improving spatial reuse (PHYCCA- Physical Clear Channel Assessment).
• Improving Spatial reuse (Transmit Power Control-TPC)
• Improving Spatial reuse (BSS color)
• Improving Spatial reuse (Multiple NAVs for spatial reuse)
• Interference management
• Multi-user aggregation scheme, that will allow a single access to send frames to multiple recipients.
• MU-MIMO concepts in downlink and uplink as explained above.
• TWT technique has been introduced to reduce power consumption.

Conclusion

WiFi 6 (802.11ax) and WiFi 6E together mark a significant advancement in wireless technology, providing greater capacity, efficiency, and performance, especially in crowded environments. With the addition of the 6 GHz band in WiFi 6E, networks can now deliver even higher speeds and reduced latency, enabling better support for bandwidth-intensive applications like augmented reality, 4K streaming, and IoT devices. These technologies are set to play a crucial role in meeting the growing demands for seamless, high-speed wireless connectivity in both consumer and enterprise networks.

This 802.11ax tutorial is very informative for beginners who want to gain knowledge in WiFi 6 basics including its frame and protocol layers (Physical and MAC layers).