We are already in the sixth generation of WiFi that is expected to be ratified by the IEEE by the end of 2019. Even though the amendment has not yet been ratified, 802.11ax chipset and WiFi AP vendors have already accepted and committed to support some key aspects of the 802.11ax standard such as ‘OFDMA, Target Wake Time or BSS coloring’.
Although most manufacturers of WiFi access points have announced their latest version, WiFi end user devices remain few but will multiply in 2019. The new Galaxy S10 phone includes the 802.11ax chip and two ‘Gaming laptop’ Asus and HP, integrates Intel’s WiFi6 chip.
The IEEE is currently scheduled to ratify 802.11ax amendment in the third quarter of 2019. The Wi-Fi Alliance has a similar schedule for an 802.11ax certification with tests scheduled for August 2019.The 802.11 standard is promoted by IEEE, while Wi-Fi 6 is the name of the 802.11ax standard promoted by the Wi-Fi alliance.
The standard offers tremendous performance in the order of 4 times higher than the previous 802.11ac standard, but let’s be careful with these marketing comparisons, which are often unrealistic. There will obviously be improved efficiency, since as its scientific name indicates: HE for ‘High Efficiency’, will be at the rendezvous but it will take a few years to have real impact on WiFi networks, like the previous standards.
The speed of connection or ‘data rate’ could potentially reach 10 Gbps according to the standard but there is no guarantee that this can be achieved as in the past when the 802.11n had to reach 600 Mbps of connection speed but only 450 Mbps were possible in the best conditions. The secret recipe for this new standard lies in the OFDMA modulation borrowed from the cellular world which designates ‘orthogonal frequency division multiplexing access’ allowing to have several WiFi clients transmitting in parallel in the same 20 MHz channel or multiple of 20 MHz This is possible by subdividing subcarriers to a minimum of 2MHZ per WiFi devices.
Prior to the 802.11ax standard, the 20 MHz channel could only be used by one WiFi client for download or upload even though it may had used only a portion of the frequency. Thanks to the sub-carrier of 2 MHz, henceforth called RU, the efficiency will be increased, since up to 9 customers will be able to transmit simultaneously in a 20 MHz channel for 2,4 and 5 GHz frequency band. Since most WiFi frames are small, less than 512 bytes, it becomes productive to coordinate these parallel transmissions, thus reducing the cumbersome ‘overhead’ for multiple single transmission as opposed to multiple simultaneous transmissions.
A successor at 802.11ac
Of course, the 802.11ac standard (wave 2) had certified a form of parallel transmission called ‘multi-user mimo’ (MU-MIMO) but this version did not have much success because WiFi client devices had not implemented this feature. Moreover, the way to realize these transmissions in parallel required wireless access points ideally 4×4: 4 allowing 4 1×1: 1 device to transmit simultaneously only for download. This approach required constant ‘null data packet’ communication to check the condition of the clients and their relative positioning of the access point to create beamforming. In addition, to be reliable a minimum distance between WiFi client devices was required, and the constant sounding frames added overhead to the protocol making this type of communication ineffective.
As always, all WiFi generations have one thing in common: transmissions are always half-duplex, so you cannot transmit and receive at the same time. In addition, it has been proven in the past that combining multiple 20 MHz channels together, does not necessarily improve per-client throughput in a high-density area of 100 users or more. Combining several channels of 20 MHz ‘channel bonding’ can be interesting when you have only few WiFi clients and no nearby networks. The main reason for this inferior performance is because when combining several channels, the opportunity to transmit for a given WiFi client mobilizes a large part of the frequency. This leaves no room for other communications until the client has finished transmitting even though he does not use the whole frequency effectively to transmit. Added to this, is the fact that its signal level must be higher (RSSI) than the one using a smaller channel to transmit.
The use of smaller width channel is exactly what the new 802.11ax standard has introduced with the ‘OFDMA’. Allowing up to 9 parallel communication in a 20MHZ channel, in the same direction, download or upload, optimizes channel utilization and increases throughput per client every second. This approach is more efficient and reduces contention in a WiFi cell while optimizing the ‘airtime’ required for each client. WiFi6 brings a new nomenclature for these small subchannels, they are called RU for ‘resources units’ and we have RU-26, RU-52, RU-106, RU-242 in a 20 Mhz. This corresponds to subchannels of 2, 4, 8 and 20 MHz respectively. RU-484 in a 40 MHz channel and RU-996 in 80 and 160 MHz channels.
These RUs can be scheduled simultaneously and in different combinations depending on the state of the network and available resources. Also, having RUs that require a smaller bandwidth will allow WiFi clients to operate further away from the access point since the RSSI level required to transmit will be less restrictive. The introduction of « Orthogonal Frequency Division Multiple Access (OFDMA) », a technique developed in the cellular world that divides a channel into subcarriers called resource units and dedicated to specific end-users as demonstrated in the picture below:
Figure: OFDMA transmissions over time
OFDMA is designed to deal with many WiFi clients, a proven technology from cellular technology was also part of the 802.16 standard (WiMAX). Regardless of the number of streams, all access points will support the same number of OFDMA 802.11ax clients. OFDMA technology makes better use of the available frequency space by dividing channels into resource units for simultaneous downlink and uplink multi-user transmissions. Even a 20 MHz channel, can simultaneously talk to 9 client devices and theoretically up to 37 client devices using 80 MHz channel (although enterprise customers can rarely use more than 20 or 40 MHz channel.
Another important feature with 802.11ax for WiFi client devices, such as smart phones, is TWT (Target Wake Time) maximizing battery life. Already available in SamsungS10 smartphone autonomy is greatly improved allowing a full day of use without recharging. WiFi 6 promotes 3 modes of TWT for WiFi interface to sleep and save energy. These modes are: Individual, Broadcast or Opportunistic which can be negotiated with the access point for the most appropriate to increase the battery life and transmit in a timely manner. The TWT will surely be interesting for IoT devices requiring a very long battery life.
An 802.11ax access point can negotiate with the participating STAs to use the TWT (Target Wake Time) to transmit at a specific time so that each station has access. Stations and AP’s exchange information that includes a planned duration of activity. Thus, the AP controls the level of contention and overlap between the stations needing access. The 802.11ax STA can use TWT to reduce power consumption and go into standby until their TWT arrives. In addition, an AP can further design calendars and transmit the TWT values to the STAs without individual TWTs or agreements between them.
BSS (Basic Service Set) Coloring is another improvement that will help reduce the interference of wireless access points between them or commonly called CCI (co-channel interference) When a station actively listens for WiFi frames and detects a 802.11ax frame, it checks the color byte of the BSS and the MAC address in the MAC header. If the color BSS in the detected PPDU is the same as the AP with which it is associated, the STA considers this frame as an intra-BSS frame. However, if the detected frame has a BSS color different from its own, the STA considers this frame as an inter-BSS frame and determines if this WiFi BSS cell can overlap its own. The STA then considers the radio frequency as busy for Intra-BSS and will ignore the Inter-BSS to gain transmit opportunity by adjusting his signal detect capabilities.
The standard still needs to define some of the mechanisms for ignoring traffic from superimposed BSS (inter BSS). However, the implementation may allow to modify the threshold of detection of the radio signal of the inter-BSS WiFi cells, while maintaining a lower threshold for intra-BSS traffic, thus reducing the contention to have the opportunity to transmit in the same space physical. This feature will greatly improve the problems related to CCI (Co-channel interference) in high-density WiFi networks.
Another enhancement for 802.11ax standard will be to transmit more information per symbol than the previous 802.11ac wave 2 standard. This is possible by using a new type of modulation named 1024 QAM which allows 10 bits per symbol compared to 256 QAM using 8 bits/symbol.
Quadrature amplitude modulation QAM is a highly developed modulation scheme used in mobile communications industry where data is transmitted over radio frequencies. For wireless communications, QAM is a signal in which two carriers (two sine waves) shifted 90 degrees (one quarter phase) are modulated and the resulting output is composed of both amplitude and phase variations. These variations form the base of transmitted bits, atoms of the digital world, that produce the information we see on our devices.
By varying these sine waves in phase and amplitude, radio engineers can build signals that transmit an ever-increasing number of bits per hertz (information per signal). Systems designed to maximize spectral efficiency place high importance on bit / hertz efficiency and therefore always use techniques to build increasingly dense QAM constellations to increase data rates. In simple terms, higher QAM levels increase the throughput capabilities of wireless devices. By varying the amplitude of the signal as well as the phase, the Wi-Fi radios can build the following constellation diagram which shows the values associated with the different states for a 16 MAQ signal below:
Figure: 16-QAM constellation exemple
In real terms, 1024-QAM allows a 25% increase in the data rate of access points and Wi-Fi 6 devices. With more than 30 billion connected « things » expected by 2020, a higher wireless throughput facilitated by 1024-MAQ technology is essential to ensure quality of service (QoS) in high-density venues such as stadiums, convention centers, auditoriums and any place with a high density of WiFi users. Indeed, applications such as 4K video streaming (which is becoming the norm) should drive Internet traffic to 278,108 petabytes per month by 2021.
Also, WiFi outdoor networks being increasingly popular and whose distances can induce a delay in transmission and bouncing of radio waves (multipath effect), the new standard will help with ‘Long Symbol OFDM’ feature. Long Symbol will allow more time to transmit and receive data, from 3.2 to 12.8 microseconds, thus mitigating signal bouncing effects and greater distance.