Wifi 6 Core Technologies
Wifi 6 (802.11ax) inherits all the advanced MIMO features of wifi 5 (802.11ac) and adds many new features for high-density deployment scenarios. Here are the core new features of wifi 6:
OFDMA Frequency Division Multiplexing Technology
DL/UL MU-MIMO
Higher-order modulation (1024-QAM)
Spatial Division Multiplexing (SR) & BSS Coloring
Extended Coverage (ER)
These core new features are described in detail below.
OFDMA Frequency Division Multiplexing Technology
Before 802.11ax, data transmission was in OFDM mode, and users were distinguished by different time segments. In each time segment, a user completely occupies all sub-carriers and sends a complete data packet (as shown in the figure below).
OFDM working mode
802.11ax introduces a more efficient data transmission mode called OFDMA. (It is also called MU-OFDMA because 802.11ax supports uplink and downlink multi-user modes.) , which allocates subcarriers to different users, and multiple access methods are added to OFDM systems to implement multi-user multiplexing of channel resources. To date, it has been adopted by many wireless technologies, such as 3GPP LTE. In addition, the 802.11ax standard also mimics LTE, and refers to the smallest subchannel as a "resource unit (Resource Unit, RU for short)". Each RU includes at least 26 subcarriers, and a user is distinguished based on a time-frequency resource block RU. We first divide a resource of an entire channel into small fixed-size time-frequency resource blocks RUs. In this mode, UE data is carried on each RU. Therefore, in terms of total time-frequency resources, multiple UEs may transmit data at the same time in each time slice, as shown in the following figure.
OFDMA working mode
Compared with OFDM, OFDMA has the following advantages:
Finer channel resource allocation.
Especially when the channel status of some nodes is poor, transmit power may be allocated according to channel quality, so as to allocate channel time-frequency resources more delicately. The following figure shows that the channel quality varies greatly with subcarriers in the frequency domain. 802.11ax selects the optimal RU for data transmission based on the channel quality.
Channel quality on different subcarriers in the frequency domain
Provides better QoS.
802.11ac and earlier standards occupy an entire channel for data transmission. If a QoS data packet needs to be sent, the QoS data packet needs to be sent only after the previous sender releases the entire channel. Therefore, a long delay exists. In the OFDMA mode, because one sender occupies only some resources of an entire channel, data of a plurality of users can be sent at a time, a delay in accessing a QoS node can be reduced.
More concurrent users and higher user bandwidth
OFDMA divides an entire channel resource into multiple subcarriers (also referred to as subchannels). The subcarriers are divided into several groups based on different RU types. Each user may occupy one or more groups of RUs to meet services with different bandwidth requirements. In 802.11ax, the minimum RU size is 2 MHz, and the minimum subcarrier bandwidth is 78.125 kHz. Therefore, the minimum RU type is 26 subcarrier RUs. By analogy, there are 52 subcarrier RUs, 106 subcarrier RUs, 242 subcarrier RUs, 484 subcarrier RUs, and 996 subcarrier RUs. The following table lists the maximum number of RUs under different channel bandwidths.
Number of RUs under different bandwidths
Position of the RU in the 20 MHz frequency band
Multi-user throughput simulation in OFDMA and OFDM modes
DL/UL MU-MIMO
MU-MIMO uses channel spatial diversity to transmit independent data streams on the same bandwidth. Unlike OFDMA, all UEs use the full bandwidth, resulting in multiplexing gains. A terminal has only one or two spatial streams (antennas), which is less than an AP's spatial streams (antennas). Therefore, MU-MIMO technology is introduced in the AP, so that data can be simultaneously transmitted between the AP and multiple terminals at the same time. The throughput is greatly improved.
Throughput Difference Between SU-MIMO and MU-MIMO
DL MU-MIMO
MU-MIMO has been introduced in 802.11ac, but only DL 4x4 MU-MIMO (downlink) is supported. In 802.11ax, the number of MU-MIMO is further increased, and DL 8x8 MU-MIMO can be supported. With the DL OFDMA technology (downlink), MU-MIMO transmission can be performed simultaneously and different RUs can be allocated for multi-user multiple access transmission. This increases the number of concurrent accesses and balances the throughput.
Scheduling sequence in downlink multi-user mode for 8x8 MU-MIMO APs
UL MU-MIMO
UL MU-MIMO (UL) is an important feature introduced in 802.11ax. The concept of UL MU-MIMO is similar to that of UL SU-MIMO. Both transmit and receive multi-antenna technologies simultaneously transmit data on multiple spatial streams by using the same channel resources. The only difference is that multiple data streams in UL MU-MIMO are from multiple UEs. 802.11ac and earlier 802.11 standards support UL SU-MIMO. That is, the UL SU-MIMO can receive data from only one UE. In multi-user concurrency scenarios, the efficiency is low. After 802.11ac supports UL MU-MIMO, the UL OFDMA technology (in the uplink) is used. MU-MIMO transmission can be performed simultaneously and different RUs can be allocated for multi-user multiple access transmission, improving the efficiency in multi-user concurrency scenarios and greatly reducing the application delay.
Uplink scheduling sequence in multi-user mode
Although the 802.11ax standard allows OFDMA to be used together with MU-MIMO, OFDMA should not be confused with MU-MIMO. OFDMA allows multiple UEs to segment channels (sub-channels) to improve concurrency efficiency, and MU-MIMO allows multiple UEs to use different spatial flows to improve throughput. The following table shows the comparison between OFDMA and MU-MIMO.
Comparison Between OFDMA and MU-MIMO
Higher-order modulation (1024-QAM)
802.11ax aims to increase system capacity, reduce latency, and improve efficiency in multi-user high-density scenarios. However, better efficiency and faster speed are not mutually exclusive. 802.11ac uses 256-QAM quadrature amplitude modulation. Each symbol transmits 8 bits of data (2 ^ 8 = 256). 802.11ax uses 1024-QAM quadrature amplitude modulation. Each symbol transmits 10 bits of data (2 ^ 10 = 1024). The increase from 8 to 10 is 25%. That is, compared with 802.11ac, 802.11ax increases the throughput of a single spatial stream by 25%.
Constellation diagram comparison between 256-QAM and 1024-QAM
Note that the successful use of 1024-QAM modulation in 802.11ax depends on channel conditions. A denser constellation distance requires a stronger EVM. (Error Vector Magnitude, used to quantify the performance of a radio receiver or transmitter in terms of modulation accuracy) The and accept the sensitivity function and have higher channel quality requirements than other modulation types.
Spatial Division Multiplexing (SR) & BSS Coloring
The principle of Wi-Fi radio transmission is that only one user is allowed to transmit data on a channel at any given time. If the wifi AP and client detect other 802.11 radio transmissions on the same channel, collision avoidance is automatically performed and the transmission is delayed. So each user has to take turns. Therefore, channels are very valuable resources in wireless networks. In high-density scenarios, proper channel division and utilization will greatly affect the capacity and stability of the entire wireless network. 802.11ax can operate in the 2.4 GHz or 5 GHz band (Unlike 802.11ac, which only operates in the 5 GHz band) In high-density deployment, too few channels may be available, especially in the 2.4 GHz frequency band. If the channel multiplexing capability can be improved, the system throughput will be increased.
In 802.11ac and earlier standards, the CCA threshold is dynamically adjusted to reduce intra-channel interference. By identifying the co-channel interference strength, the CCA threshold is dynamically adjusted to ignore weak co-channel interference signals to implement co-channel concurrent transmission, improving system throughput.
802.11 default CCA threshold
For example, as shown in FIG. 12, STA1 on AP1 is transmitting data. In this case, AP2 also wants to send data to STA2. According to a Wi-Fi radio transmission principle, AP2 needs to first sense whether a channel is idle. The default CCA threshold is -82 dBm, and it is found that the channel is occupied by STA1. In this case, the AP2 delays sending because the AP2 cannot perform parallel transmission. Virtually all co-channel clients associated with AP2 will delay transmission. The dynamic CCA threshold adjustment mechanism is introduced. When AP2 detects that an intra-frequency channel is occupied, the CCA threshold can be adjusted (for example, from -82 dBm to -72 dBm) based on the interference strength to avoid the impact of interference. In this way, intra-frequency concurrent transmission can be implemented.
Dynamic CCA Threshold Adjustment
Due to the mobility of wifi client devices, co-channel interference detected in the wifi network is not static and changes with the movement of the client devices. Therefore, the dynamic CCA mechanism is effective.
802.11ax introduces a new intra-frequency transmission identification mechanism called BSS coloring. The BSS color field is added to the PHY packet header to color data from different BSSs and allocate a color to each channel. This color identifies a group of basic service sets (BSSs) that should not be interfered with. The receiving end can identify intra-frequency interference signals and stop receiving them. This avoids wasting the time of the transceiver. If the colors are the same, the signals are considered as interference signals in the same BSS and the transmission is delayed. If the colors are different, there is no interference between the two wifi devices, and the two wifi devices can transmit data on the same channel and the same frequency in parallel. In a network designed in this way, channels with the same color are far away from each other. In this case, we use the dynamic CCA mechanism to set such signals to be insensitive, and in fact, they are unlikely to interfere with each other.
Comparison Between Without BSS Color Mechanism and With BSS Color Mechanism
