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By Powered By SmileMbb | 11 November 2021 | 0 Comments

Wi-Fi 6 Core Technology introduction


Part One

Wi-Fi 6 (802.11ax) inherits all the advanced MIMO features of Wi-Fi 5 (802.11ac), and adds many new features for high-density deployment scenarios. The following are the core new features of Wi-Fi 6:

OFDMA frequency division multiplexing technology
OFDMA frequency division multiplexing technology
DL/UL MU-MIMO technology
More advanced modulation technology (1024-QAM)
Space Division Multiplexing Technology (SR) & BSS Coloring coloring mechanism
♦ 
Extended Range(ER)
These core new features are described in detail below.

1.  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).

 

Figure 1-1 OFDM working mode
 
802.11ax introduces a more efficient data transmission mode called OFDMA (because 802.11ax supports uplink and downlink multi-user mode, it can also be called MU-OFDMA), which allocates sub-carriers to different users and adds a multiple access method in the OFDM system to achieve multi-user multiplexing channel resources. So far, it has been adopted by many wireless technologies, such as 3GPP LTE. In addition, the 802.11ax standard also imitates LTE, calling the smallest subchannel a "Resource Unit (RU)", and each RU contains at least 26 subcarriers. Users are distinguished based on time-frequency resource blocks RU. We first divide the resources of the entire channel into small fixed-size time-frequency resource blocks RU. In this mode, the user's data is carried on each RU, so from the perspective of total time-frequency resources, there may be multiple users sending simultaneously on each time slice (as shown in the figure below).
 

Figure 1-2 OFDMA working mode   
 
Compared with OFDM, OFDMA generally has three advantages:

More detailed channel resource allocation.
Especially when the channel status of some nodes is not very well, the transmission power can be allocated according to the channel quality to allocate the channel time-frequency resources more delicately. The following figure shows that the channel quality of different sub-carriers varies greatly in the frequency domain. 802.11ax can select the optimal RU resource for data transmission according to the channel quality.

 

Figure 1-3 Channel quality on different sub-carrier frequency domains
 
Provide better QOS

Because 802.11ac and previous standards occupy the entire channel to transmit data, if a QOS packet needs to be sent, it must wait for the previous sender to release the entire channel, so there will be a long delay. In OFDMA mode, since one sender only occupies part of the resources of the entire channel, data of multiple users can be sent at one time, so the delay of QOS node access can be reduced.

More concurrent users and higher user bandwidth

OFDMA divides the entire channel resource into multiple sub-carriers (also called sub-channels). The sub-carriers are divided into groups according to different RU types. Each user can occupy one or more groups of RUs to meet different bandwidth requirements. business. The smallest RU size in 802.11ax is 2MHz, and the smallest sub-carrier bandwidth is 78.125KHz, so the smallest RU type is 26 sub-carrier RU. By analogy, there are 52 sub-carrier RU, 106 sub-carrier RU, 242 sub-carrier RU, 484 sub-carrier RU and 996 sub-carrier RU. The table below shows the maximum number of RUs under different channel 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 N/A 1-SU/MU-
MIMO
2 4
996-subcarrier RU N/A N/A 1-SU/MU-
MIMO
2
2x996-subcarrier RU N/A N/A N/A 1-SU/MU-MIMO

Table 7 Number of RUs under different bandwidths

Figure 1-4 Schematic diagram of the position of RU in 20MHz

 
The larger number of RUs, the higher the efficiency of multi-user processing when sending small packets, and the higher the throughput. The following figure shows the simulation benefits:

Figure 1-5 Simulation of multi-user throughput in OFDMA and OFDM modes

2. DL/UL MU-MIMO technology

MU-MIMO uses the spatial diversity of the channel to send independent data streams on the same bandwidth. Unlike OFDMA, all users use the full bandwidth, which brings multiplexing gains. The number of antennas of the terminal is limited by the size. Generally speaking, there are only one or two spatial streams (antennas), which are less than the spatial streams (antennas) of the AP. Therefore, the introduction of MU-MIMO into the AP enables simultaneous data transmission between the AP and multiple terminals at the same time, which greatly improves the transmission technology.
 

Figure 2-1 SU-MIMO and MU-MIMO throughput difference
 
DL MU-MIMO technology
 
MU-MIMO has been introduced in 802.11ac, but only supports DL 4x4 MU-MIMO (downlink). In 802.11ax, the number of MU-MIMO is further increased, which can support DL 8x8 MU-MIMO. With the help of DL OFDMA technology (downlink), MU-MIMO transmission can be carried out at the same time and different RUs can be allocated for multi-user multiple access transmission, which increases the amount of concurrent access to the system, but also balances the throughput.
 

Figure 2-2 8x8 MU-MIMO AP downlink multi-user mode scheduling sequence
 

UL MU-MIMO technology

UL MU-MIMO (uplink) is an important feature introduced in 802.11ax. The concept of UL MU-MIMO is similar to that of UL SU-MIMO in that it uses the same channel resources to simultaneously transmit data on multiple spatial streams through the multi-antenna technology of transmitter and receiver. The only difference is that the multiple data streams of UL MU-MIMO come from multiple users. Both 802.11ac and the previous 802.11 standards are UL SU-MIMO, that is, it can only accept data sent by one user, and the efficiency of multi-user concurrent scenarios is low. After 802.11ax supports UL MU-MIMO, with UL OFDMA technology (uplink), MU-MIMO transmission and different RUs can be allocated for multi-user multiple access transmission at the same time, which improves the efficiency of multi-user concurrent scenarios and greatly reduces application delay.

 

Figure 2-3 Uplink scheduling sequence in multi-user mode
 
Although the 802.11ax standard allows OFDMA to be used with MU-MIMO, do not confuse OFDMA and MU-MIMO. OFDMA supports multi-users to improve concurrency efficiency by subdividing channels (sub-channels), and MU-MIMO supports multi-users to improve throughput by using different spatial streams. The following table is the comparison between OFDMA and MU-MIMO:
 
OFDMA MU-MIMO
Improve efficiency Increase capacity
Reduce latency Higher rate per user
Best for low bandwidth applications Best for high bandwidth applications
Most suitable for small packet transmission Most suitable for large packet transmission

Table 8 Comparison of OFDMA and MU-MIMO

3. Higher order modulation technology (1024-QAM)

The main goal of the 802.11ax standard is to increase system capacity, reduce latency, and improve efficiency in multi-user high-density scenarios, but better efficiency and faster speed are not mutually exclusive. 802.11ac uses 256-QAM quadrature amplitude modulation, each symbol transmits 8bit data (2^8=256), 802.11ax will use 1024-QAM quadrature amplitude modulation, and each symbol bit transmits 10bit data (2^10= 1024), the increase from 8 to 10 is 25%, that is, compared to 802.11ac, the data throughput of a single spatial stream of 802.11ax is increased by 25%.

Figure 3-1 Constellation comparison between 256-QAM and 1024-QAM
 
It should be noted that the successful use of 1024-QAM modulation in 802.11ax depends on the channel conditions. A closer constellation point distance requires a stronger EVM (error vector magnitude, used to quantify the performance of the radio receiver or transmitter in terms of modulation accuracy) And accept the sensitivity function, and the channel quality requirement is higher than other modulation types.
 

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