WLAN 802.11ax Tutorial-Basic features, frame, modes, PHY, MAC

This WLAN 802.11ax tutorial covers basic 11ax features, 11ax frame, 11ax modes, 11ax physical layer, MAC layer etc. The 802.11ax tutorial also covers benefits and drawbacks of 802.11ax wifi system.

What is 802.11ax ?

Introduction:
802.11ax standard is 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.

Increase in WLAN users lead to 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 specially in the apartment buildings or offices. The previous WLAN standards (802.11g/n/ac) have introduced larger bandwidth, higher modulation scheme 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. The standard is going to be ratified in the year 2019.

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

802.11ax features

Image Courtesy: Ruckus Networks

Following are the major changes made in the 802.11ax technology.
➨Guard interval and Symbol duration have been increased by factor of 4 (subcarrier offset is reduced by factor of 4). Hence 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 in all the subcarriers are allocated for single user. Due to OFDMA, single channel can be shared by multiple users in parallel.
➨MU (Multi-user) MIMO has been added in the uplink also where as in previous standards it was available only in the downlink part. Hence higher throughput is achieved in the uplink through SDM (Spatial Division Multiplexing).
➨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.

Specifications	802.11ax support
Frequency range	2.4 GHz and 5 GHz
Modulation Scheme	Max. upto 1024-QAM, legacy modulation schemes (BPSK, 16-QAM, 64-QAM, 256-QAM) are supported.
Channel Bandwidth	Upto 40 MHz at 2.4 GHz, Upto 160 MHz at 5 GHz
FEC Coding	LDPC (Low Density Parity Check) , BCC (Binary Convolutional Coding)
Symbol Time	12.8 µs
Subcarrier spacing	78.125 KHz (legacy used 312.5 KHz)
Guard interval	0.8, 1.6, 3.2 µs which incurs 5%, 10% and 20% overhead respectively.
FFT size	256 (20 MHz), 512 (40 MHz), 1024 (80 MHz), 2048 (160 MHz)
Max. Data rate	600.4 Mbits/sec (with 80 MHz, 1 SS) , 9.6078 Gbits/sec (with 160 MHz, 8 SS)
MIMO configuration	MU-MIMO is used both in the downlink and uplinl
Beamforming	Supported only in downlink
Basic Channel Access	OFDMA on top of CSMA/CA
Random channel access	UL OFDMA based random access on top of CSMA/CA
Contention free access	Trigger based UL OFDMA
Spatial reuse	BSS coloring
Power Management	Enhanced TWT

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.
READ MORE➤.

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 set of subcarriers (min. 26 and max. 996). For 20 MHz bandwidth, there are various RU options available. For example, RU26 allows upto 9 users to transmit simulneously. Similarly for 160 MHz, there are about 74 simultaneous users. Refer RU in 802.11ax >>.
RUs in WLAN 802.11ax
• 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 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)).
WLAN 802.11ax multi user downlink
• Uplink multi-user MIMO (UL MU-MIMO) has been 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).
WLAN 802.11ax multi user uplink
• 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 signal from its own network using AP1 and from neighbour interfering AP (i.e. AP2). Due to BSS coloring, STA2 ignores signal from AP2 and remains connected with AP1. The figure depicts BSS coloring concept used in 802.11ax.
Refer BSS coloring in 11ax >>.
WLAN 802.11ax BSS Color
• FEC schemes: IEEE 802.11ax proposes to use LDPC for large bandwidth (i.e. channel bonding) and BCC for narrower bandwidth scenarios. Refer IEEE 802.11ac physical layer >> which describes all the modules except the above enhancements which are introduced in the WLAN 802.11ax physical layer.

WLAN 802.11ax MAC Layer

• Only one packet type has been used in legacy WLAN standards and there was no distinguing 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 use 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 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 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 send 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 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 most robust modulation techniques such as BPSK/QPSK. Preamble part in the packet is transmitted with 3 dB higher power compare to rest of the part.

Following MAC layer enhancement are possible to be introduced by IEEE in 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 accessto send frames to multiple recipients.
• MU-MIMO concepts in downlink and uplink as explained above.
• TWT technique has been introduced to reduce power consumption.
Refer WLAN MAC Layer Protocol Part#1 >> and WLAN MAC Part#2 >> which describes legacy WLAN MAC layer protocol and various messages exchanged between AP and STAs.

Conclusion: This WLAN 802.11ax tutorial is useful for beginners having knowledge on legacy WLAN standards such as 802.11g/n/ac. Refer Advantages and disadvantages of 802.11ax >> which describes benefits and drawbacks of 802.11ax technology in depth.

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