The 802.11ax specification introduces significant changes to the physical layer of the standard. However, it maintains backward compatibility with 802.11a/b/g/n and /ac devices, such that an 802.11ax STA can send and receive data to legacy STAs. These legacy clients will also be able to demodulate and decode 802.11ax packet headers – though not whole 802.11ax packets – and backoff when an 802.11ax STA is transmitting.
The following table highlights the most important changes to this revision of the standard, in contrast to the current 802.11ac implementation:
2.4 GHz and 5 GHz
20 MHz, 40 MHz, 80 MHz, 80+80 MHz & 160 MHz
20 MHz, 40 MHz, 80 MHz, 80+80 MHz & 160 MHz
64, 128, 256, 512
256, 512, 1024, 2048
OFDM SYMBOL DURATION
3.2 us + 0.8/0.4 us CP
12.8 us + 0.8/1.6/3.2 us CP
433 Mbps (80 MHz, 1 SS)
6933 Mbps (160 MHz, 8 SS)
600.4 Mbps (80 MHz, 1 SS)
9607.8 Mbps (160 MHz, 8 SS)
Table 1. 802.11ac vs. 802.11ax
Notice that the 802.11ax standard will operate in both the 2.4 GHz and 5 GHz bands. The specification defines a four times larger FFT, multiplying the number of subcarriers. However, one critical change with 802.11ax is that the subcarrier spacing has been reduced to one fourth the subcarriers spacing of previous 802.11 revisions, preserving the existing channel bandwidths.
Figure 5. Narrower sub-carrier spacing
The OFDM symbol duration and cyclic prefix also increased 4X, keeping the raw link data rate the same as 802.11ac, but improving efficiency and robustness in indoor/outdoor and mixed environments. Nevertheless, the standard does specify 1024-QAM and smaller cyclic prefix ratios for indoor environment, which will increase the maximum data rate.
802.11ax will employ an explicit beamforming procedure, similar to that of 802.11ac. Under this procedure, the beamformer initiates a channel sounding procedure with a Null Data Packet. The beamformee measures the channel and responds with a beamforming feedback frame, containing a compressed feedback matrix. The beamformer uses this information to compute the channel matrix, H. The beamformer can then use this channel matrix to focus the RF energy toward each user.
Multi-User Operation: MU-MIMO and OFDMA
The 802.11ax standard has two modes of operation:
Single User: in this sequential mode the wireless STAs send and receive data one at a time once they secure access to the medium, as this paper has described above.
Multi-User: this mode allows for simultaneous operation of multiple non-AP STAs. The standard divides this mode further into Downlink and Uplink Multi-user.
- Downlink multi-user refers to data that the AP serves to multiple associated wireless STAs at the same time. The existing 802.11ac standard already specifies this feature.
- Uplink multi-user involves simultaneous transmission of data from multiple STAs to the AP. This is new functionality of the 802.11ax standard, which did not exist in any of the previous versions of the Wi-Fi standard.
Under the Multi-User mode of operation, the standard also specifies two different ways of multiplexing more users within a certain area: Multi-User MIMO and Orthogonal Frequency Division Multiple Access (OFDMA). For both of these methods, the AP acts as the central controller of all aspects of multi-user operation, similar to how an LTE cellular base station controls the multiplexing of many users. An 802.11ax AP can also combine MU-MIMO with OFDMA operation.
Borrowing from the 802.11ac implementation, 802.11ax devices will use beamforming techniques to direct packets simultaneously to spatially diverse users. That is, the AP will calculate a channel matrix for each user and steer simultaneous beams to different users, each beam containing specific packets for its target user. 802.11ax supports sending up to eight multi-user MIMO transmissions at a time, up from four for 802.11ac. Also, each MU-MIMO transmission may have its own Modulation and Coding Set (MCS) and a different number of spatial streams. By way of analogy, when using MU-MIMO spatial multiplexing, the AP could be compared to an Ethernet switch that reduces the collision domain from a large computer network to a single port.
As a new feature in the MU-MIMO Uplink direction, the AP will initiate a simultaneous uplink transmission from each of the STAs by means of a trigger frame. When the multiple users respond in unison with their own packets, the AP applies the channel matrix to the received beams and separates the information that each uplink beam contains. The AP may also initiate Uplink multi-user transmissions to receive beamforming feedback information from all participating STAs as shown in Figure 7.
Figure 6. AP using MU-MIMO beamforming to serve multiple users located in spatially diverse positions
Figure 7. A beamformer (AP) requesting channel information for MU-MIMO operation
The 802.11ax standard borrows a technological improvement from 4G cellular technology to multiplex more users in the same channel bandwidth: Orthogonal Frequency-Division Multiple Access (OFDMA). Building on the existing orthogonal frequency-division multiplexing (OFDM) digital modulation scheme that 802.11ac already uses, the 802.11ax standard further assigns specific sets of subcarriers to individual users. That is, it divides the existing 802.11 channels (20, 40, 80 and 160 MHz wide) into smaller sub-channels with a predefined number of subcarriers. Also borrowing from modern LTE terminology, the 802.11ax standard calls the smallest subchannel a Resource Unit (RU), with a minimum size of 26 subcarriers.
Based on multi-user traffic needs, the AP decides how to allocate the channel, always assigning all available RUs on the downlink. It may allocate the whole channel to only one user at a time – just as 802.11ac currently does – or it may partition it to serve multiple users simultaneously (see Figure 8).
Figure 8. A single user using the channel Vs. multiplexing various users in the same channel using OFDMA
In dense user environments where many users would normally contend inefficiently for their turn to use the channel, this OFDMA mechanism now serves them simultaneously with a smaller – but dedicated – subchannel, thus improving the average throughput per user. Figure 9illustrates how an 802.11ax system may multiplex the channel using different RU sizes. Note that the smallest division of the channel accommodates up to 9 users for every 20MHz of bandwidth. 
Figure 9. Subdividing Wi-Fi channels using various Resource Unit sizes
The following table shows the number of users that can now get frequency-multiplexed access when the 802.11ax AP and STAs coordinate for MU-OFDMA operation.
CBW160 and CBW80+80
2x996 subcarrier RU
Table 2. Total number of RUs by channel bandwidth
Multi-User Uplink Operation
To coordinate uplink MU-MIMO or uplink OFDMA transmissions the AP sends a trigger frame to all users. This frame indicates the number of spatial streams and/or the OFDMA allocations (frequency and RU sizes) of each user. It also contains power control information, such that individual users can increase or reduce their transmitted power, in an effort to equalize the power that the AP receives from all uplink users and improve reception of frames from nodes farther away. The AP also instructs all users when to start and stop transmitting. As Figure 10depicts, the AP sends a multi-user uplink trigger frame that indicates to all users the exact moment at which they all start transmitting, and the exact duration of their frame, to ensure that they all finish transmitting simultaneously as well. Once the AP receives the frames from all users, it sends them back a block ACK to finish the operation.
Figure 10. Coordinating uplink multi-user operation
One of the main goals of the 802.11ax is to support 4X higher average per-user throughput in dense user environments. With that goal in mind, the standard designers have specified that 802.11ax devices support Downlink and Uplink MU-MIMO operation, MU-OFDMA operation, or both for an even larger number of simultaneous users.