What Is 802.11ax (Wi-Fi 6)

Before 802.11ax, the OFDM mode was used for data transmission, and users were distinguished based on time segments. In each time segment, one user occupied all subcarriers and sent a complete data packet, as shown in the following figure.

Figure 1-3 OFDM working mode

OFDMA is a more efficient data transmission mode introduced by Wi-Fi 6. It is also referred to as MU-OFDMA since Wi-Fi 6 supports uplink and downlink MU modes. This technology enables multiple users to reuse channel resources by allocating subcarriers to various users and adding multiple access in the OFDM system. To date, OFDMA has been utilized in 3GPP Long Term Evolution (LTE) among numerous other technologies. In addition, Wi-Fi 6 defines the smallest subcarrier as a resource unit (RU), which includes at least 26 subcarriers and uniquely identifies a user. The resources of the entire channel are divided into small RUs with fixed sizes. In this mode, user data is carried on each RU; therefore, on the total time-frequency resources, multiple users can send data in a time segment simultaneously.

Figure 1-4 OFDMA working mode

Compared with OFDM, the following improvements have been made in OFDMA:

• More refined channel resource allocation

  Transmit power can be allocated based on channel quality, especially when the channel status of certain nodes is below standard. This can help allocate channel time-frequency resources in a more delicate manner. The following figure shows how 802.11ax selects optimal RU resources based on channel quality to transmit data when channel quality greatly differs in frequency domains of different subcarriers.

  Figure 1-5 Channel quality in frequency domains of different subcarriers
  
• Enhanced QoS

  In IEEE 802.11ac and all earlier Wi-Fi standards, to transmit data, users occupy the entire channel. Therefore, a QoS data frame can be sent only after the current transmitter releases the entire channel, which leads to long latency. In OFDMA mode, one transmitter occupies only some of the entire channel's resources. Therefore, data of multiple users can be sent simultaneously, thereby reducing network access latency of QoS nodes.

• More concurrent users and higher user bandwidth

  OFDMA divides the entire channel's resources into multiple subcarriers, which are then divided into several groups based on RU type. Each user may occupy one or more groups of RUs to meet various bandwidth requirements. In Wi-Fi 6, the minimum RU size and minimum subcarrier bandwidth are 2 MHz and 78.125 kHz, respectively. Therefore, the minimum RU type is 26-subcarrier RU. By analogy, RU types include: 52-subcarrier, 106-subcarrier, 242-subcarrier, 484-subcarrier, and 996-subcarrier RUs. The more the number of RUs, the higher efficiency of multi-user processing and the higher throughput.

  Table 1-7 Number of RUs at different frequency bandwidths

  RU Type	CBW20	CBW40	CBW80	CBW160 and CBW80+80
  26-subcarrier RU	9	18	37	74
  52-subcarrier RU	4	8	16	32
  106-subcarrier RU	2	4	8	16
  242-subcarrier RU	1-SU/MU-MIMO	2	4	8
  484-subcarrier RU	-	1-SU/MU-MIMO	2	4
  996-subcarrier RU	-	-	1-SU/MU-MIMO	2
  2×996-subcarrier RU	-	-	-	1-SU/MU-MIMO

  Figure 1-6 Positions of RUs in the 20 MHz frequency bandwidth
  
  A large number of RUs indicates higher efficiency of multi-user processing and higher throughput. The following figure shows the simulation benefits.

  Figure 1-7 Multi-user throughput simulation in OFDMA and OFDM modes
  

MU-MIMO uses spatial diversity of channels to transmit independent data streams on the same bandwidth. Unlike OFDMA, all users use all bandwidths, which brings multiplexing gains. Limited by the size of the antenna, a terminal typically supports only one or two spatial streams (antennas), which is less than the number of spatial streams (antennas) on an AP. Therefore, MU-MIMO technology is introduced to enable an AP to transmit data with multiple terminals at the same time, which greatly improves the throughput.

Figure 1-8 Throughput difference between SU-MIMO and MU-MIMO

• DL MU-MIMO technology

  MU-MIMO has been introduced since 802.11ac, but only DL 4x4 MU-MIMO is supported. In 802.11ax, the number of MU-MIMO is further increased, and DL 8x8 MU-MIMO is supported. DL OFDMA technology can be used to simultaneously perform MU-MIMO transmission and allocate different RUs for multi-user multiple-access transmission, which increases the concurrent access capacity of the system and balances the throughput.

  Figure 1-9 8x8 MU-MIMO AP scheduling sequence in the downlink multi-user mode
  
• UL MU-MIMO technology

  UL MU-MIMO is an important feature introduced in 802.11ax. Similar to UL SU-MIMO, UL MU-MIMO uses the same channel resources to transmit data on multiple spatial streams by using multi-antenna technology of the transmitter and receiver. The only difference is that multiple data streams of UL MU-MIMO are from multiple users. 802.11ac and earlier 802.11 standards use UL SU-MIMO, that is, a user can receive data from only one user, which is inefficient in multi-user concurrent scenarios. After 802.11ax supports UL MU-MIMO, UL OFDMA technology is leveraged to allow MU-MIMO transmission and multi-user multiple-access transmission at the same time. This improves the transmission efficiency in multi-user concurrent scenarios and greatly reduces the application delay.

  Figure 1-10 Uplink scheduling sequence in multi-user mode
  

Although 802.11ax allows OFDMA and MU-MIMO to work at the same time, they are different. OFDMA allows multiple users to subdivide channels (subchannels) to improve the concurrency efficiency. MU-MIMO allows multiple users to use different spatial streams to increase the throughput. The following table lists the comparison between OFDMA and MU-MIMO.

The 802.11ax standard aims to increase the system capacity, reduce the latency, and improve efficiency in multi-user high-density scenarios. However, high efficiency is not mutually exclusive with the fast speed. 802.11ac uses 256-QAM, and each symbol transmits 8-bit data (2^⁸ = 256). 802.11ax uses 1024-QAM quadrature amplitude modulation, and each symbol bit transmits 10-bit data (2^¹⁰ = 1024). Therefore, compared with 802.11ac, 802.11ax increases data throughput of a single spatial stream by 25%.

Figure 1-11 Constellation maps of 256-QAM and 1024-QAM

The successful application of 1024-QAM modulation in 802.11ax depends on channel conditions. Dense constellation points require great error vector magnitude (EVM) (used to quantize the performance of the radio receiver or transmitter in modulation precision) and receiver sensitivity. In addition, the channel quality must be higher than that in other modulation types.

A channel allows only one user to transmit data within a specified time. If a Wi-Fi AP and a STA detect transmission of another 802.11 radio on the same channel, they automatically avoid conflicts and wait for the channel to become idle for transmission. Therefore, each user uses channel resources in turn. Therefore, channels are valuable resources on wireless networks. In high-density scenarios, channel allocation and utilization greatly affect the capacity and stability of the entire wireless network. 802.11ax can run on the 2.4 GHz or 5 GHz frequency band (unlike 802.11ac, which can run only on the 5 GHz frequency band). In high-density deployment scenarios, the number of available channels may be too small (especially on the 2.4 GHz frequency band). The system throughput can be increased by improving the channel multiplexing capability.

In 802.11ac and earlier standards, the mechanism of dynamically adjusting the clear channel assessment (CCA) threshold is used to reduce co-channel interference. The system identifies the co-channel interference strength, dynamicaly adjusts the CCA threshold, ignores co-channel weak interference signals, and implements co-channel concurrent transmission. This increases the system throughput.

Figure 1-12 Default CCA threshold of 802.11

For example, as shown in the following figure, STA1 on AP1 is transmitting data. If AP2 wants to send data to STA2, AP2 needs to detect whether the channel is idle. The default CCA threshold is –82 dBm. When finding that the channel is occupied by STA1, AP2 delays the transmission because the parallel transmission cannot be performed. In fact, all the STAs associated with AP2 are delayed to send. The dynamic CCA threshold adjustment mechanism is introduced. When AP2 detects that the co-frequency channel is occupied, it can adjust the CCA threshold listening range (for example, from –82 dBm to –72 dBm) based on the interference strength to avoid interference impact. In this way, co-frequency concurrent transmission can be implemented.

Figure 1-13 Dynamic adjustment of the CCA threshold

Due to the mobility of Wi-Fi STAs, co-channel interference detected on the Wi-Fi network is not static but changes with the movement of the STAs. Therefore, the dynamic CCA mechanism is effective.

802.11ax introduces a new co-frequency transmission identification mechanism called BSS coloring. The BSS color field is added to the PHY packet header to color data from different BSS sand allocate a color to each channel. The color identifies a BSS that should not be interfered. The receiver can identify co-channel interference signals and stop receiving them at an early stage, thereby avoiding waste of transceiver time. If the colors are the same, the interference signals are considered to be in the same BSS, and signal transmission is delayed. If the colors are different, no interference exists between the two Wi-Fi devices. They can then transmit data on the same channel and at the same frequency. In this mode, the channels with the same color are kept far away from each other. The dynamic CCA mechanism is used to set such signals to be insensitive. In fact, they are unlikely to interfere with each other.

Figure 1-14 How the BSS color mechanism reduces interference

The 2.4 GHz frequency band is narrow, and only three 20 MHz channels (1,6 and 11) do not interfere with each other. The 2.4 GHz frequency band has been abandoned in the 802.11ac standard. However, it is undeniable that 2.4 GHz is still an available Wi-Fi frequency band. It is still widely used in many scenarios. Therefore, in the 802.11ax standard, the 2.4 GHz frequency band is supported to make full use of the advantages of this frequency band.

Advantage 1: Coverage

In a wireless communications system, signals with a relatively high frequency are more likely to penetrate obstacles than those with a relatively low frequency. A lower frequency indicates a longer wavelength, stronger diffraction capability, poorer penetration capability, smaller signal loss attenuation, and longer transmission distance. Although the 5 GHz frequency band can bring a higher transmission speed, the signal attenuation is larger. Therefore, the transmission distance is shorter than that of the 2.4 GHz frequency band. When deploying a high-density wireless network, the 2.4 GHz frequency band is not only used to be compatible with old devices but also to fill coverage holes in edge areas.

Advantage 2: Low cost

At present, hundreds of millions of 2.4 GHz devices are used online. Even IoT devices use the 2.4 GHz frequency band. In some scenarios with a low traffic volume (such as geo-fence and asset management), there are a large number of STAs. In this case, STAs in compliance only with the 2.4 GHz frequency band terminal are more cost-effective.

Referencing from 802.11ah, the Target Wake Time (TWT) is an important resource scheduling function supported by 802.11ax. It allows STAs to negotiate with APs for the waking schedule and then send or receive data. APs can group STAs into different TWT periods to reduce the number of devices that simultaneously compete for the wireless medium after wakeup. In addition, the TWT increases the device sleep time. For battery-powered STAs, battery life has significantly improved.

An 802.11ax AP can negotiate with the participating STAs the use of the TWT function to define a specific time or set of times for individual STAs to access the medium. The STAs and the AP exchange information that includes an expected activity duration. This avoids contention and overlapping between STAs. 802.11ax STAs may use TWT to reduce energy consumption, entering a sleep state until their TWT arrives. In addition, an AP can provide schedules and deliver TWT values to STAs without individual TWT agreements between them. The standard calls this procedure "Broadcast TWT" operation.

Figure 1-16 Broadcast TWT operation