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Wi-Fi 6 Core Technology introduction
Part Two
4. Space Division Multiplexing Technology (SR) & BSS Coloring coloring mechanism
The transmission principle of Wi-Fi radio frequency is that at any specified time, only one user is allowed to transmit data on a channel. If the Wi-Fi AP and the client detect other 802.11 radio transmissions on the same channel, they will automatically avoid conflicts and postpone the transmission, so each user must take turns. Therefore, the channel is a very precious resource in the wireless network. Especially in a high-density scenario, the reasonable division and utilization of the channel will have a greater impact on the capacity and stability of the entire wireless network. 802.11ax can operate in the 2.4GHz or 5GHz frequency band (unlike 802.11ac, it can only operate in the 5GHz frequency band), and high-density deployment may also encounter the problem of too few available channels (especially the 2.4GHz frequency band). If the multiplexing capability of the channel can be improved, it will increase the throughput capacity of the system.
802.11ac and previous standards usually use the mechanism of dynamically adjusting the CCA threshold to optimize the interference between co-frequency channels. By identifying the strength of co-frequency interference, dynamically adjusting the CCA threshold, ignoring weak co-frequency interference signals to achieve simultaneous transmission on the same frequency, improving the system throughput capacity.
Figure 4-1 802.11 default CCA threshold
For example, in Figure 12, STA1 on AP1 is transmitting data. At this time, AP2 also wants to send data to STA2. According to the Wi-Fi radio transmission principle, it needs to detect to whether the channel is available. The CCA threshold defaults to -82dBm, and it is found that the channel is already occupied by STA1, AP2 will postpone sending because it cannot transmit in parallel. In fact, all co-channel clients associated with AP2 will postpone sending. Introduce a dynamic CCA threshold adjustment mechanism. When AP2 detects that the same frequency channel is occupied, the CCA threshold listening range can be adjusted according to the interference strength (for example, from -82dBm to -72dBm) to avoid the impact of interference. Realize concurrent transmission at the same frequency.
Figure 4-2 Dynamic CCA threshold adjustment
802.11ax introduces a new same-frequency transmission identification mechanism, called BSS Coloring coloring mechanism, adding the BSS color field to the PHY header to "color" the data from different BSSs and assign a color to each channel. This color identifies a group of basic service sets (BSS) that should not be interfered, and the receiving end can identify co-frequency transmission interference signals early and stop receiving, avoiding wasting transceiver time. If the colors are the same, it is considered to be an interference signal in the same BSS, and the transmission will be delayed; if the colors are different, it is considered that there is no interference between both, and two Wi-Fi devices can transmit in parallel on the same channel and frequency. In the network designed in this way, those channels with the same color are far away from each other. At this time, we use the dynamic CCA mechanism to set this signal to be insensitive. In fact, they are unlikely to interfere with each other.
Figure 4-3 Comparison of mechanism without BSS Color and mechanism with BSS Color
5. Extended Range (ER)
Since the 802.11ax standard uses the Long OFDM symbol transmission mechanism, the duration of each data transmission is increased from the original 3.2us to 12.8us. A longer transmission time can reduce the terminal packet loss rate; In addition, 802.11ax can only use 2MHz bandwidth for narrowband transmission, effectively reducing frequency band noise interference, improving terminal acceptance sensitivity, and increasing coverage distance.
Figure 5-1 Long OFDM symbol and narrowband transmission increase the coverage distance
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