Detailed explanation of WIFI 6, CSMA/CA, CCA, and BSS COLOR

Dec 23,2024
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With the advent of the Internet of Things and big data era, the demand for faster and more stable wireless networks is increasing day by day. To meet this demand, WiFi 6, as a representative of the latest generation wireless network technology, has introduced a series of advanced mechanisms, including CSMA/CA, CCA, and BSS Coloring. This article provides a clear explanation of the three strategic principles and mechanisms from the perspectives of communication physical layer algorithms and MAC protocols, leading you to a deeper understanding of the key concepts and principles behind WiFi 6.

1. CSMA/CA: CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

Before WiFi 6, the WiFi system used CSMA/CA mechanism to avoid channel interference. The workflow of CSMA/CA is as follows:

(1) Check if the channel is idle. The conclusion is drawn by comparing the signal power on the channel with the set power threshold, and the following section will explain in detail how to determine channel idle (CCA mechanism). If available, prepare to transmit data; otherwise, continue waiting.

(2) If the channel is idle, the transmitting device sends an RTS (request to send) request to the target device, and the target device replies with a CTS (clear to send). The sending device transmits data, and the target device replies with an ACK signal. See the figure below.

2. CCA (Clear Channel Assessment)

Firstly, we need to clarify a concept called channel floor noise, which is similar to the floor noise of RF receiving antennas. The commonly used calculation method is (-174+logarithmic signal bandwidth+noise figure, in dBm), which is the noise component used to calculate receiver sensitivity. Usually, CCA has two thresholds: Signal Detection (SD) and Energy Detection (ED).

Signal Detection Threshold (CCA-SD): Used to detect whether there is a preamble of an 802.11 signal in the channel. As the preamble is a periodic time-domain signal and is shared by all 802.11 physical packets, it can be detected using relevant algorithms. As long as the signal-to-noise ratio SNR is greater than 4dB, it means that the ratio of preamble power to ground noise power is greater than 4dB.

Energy detection threshold (CCA-ED): usually 20dB above the signal detection threshold (CCA-SD), CCA-ED=CCA-SD+20dB.

To determine whether the channel is busy, the received signal power is usually compared with the signal detection threshold and energy detection threshold for judgment, which varies among different device vendors.

3. BSS coloring

3.1 BSS color labeling

The BSS coloring strategy means that different APs have different identity tags, which are 6bits BSS color tags in the PHY. This tag consists of 6 bits in HE-SIG-A1 at the beginning of the PHY PPDU. The reason for placing it in this position is that the receiver can directly determine without unpacking the entire PPDU. The BSS color label is shown in the following figure. The position of HE-SIG-A in the PPDU packet is based on the 802.11 spec.

HE-SIG-A is divided into two parts, HE-SIG-A1 and HE-SIG-A2, each with 26 bits. HE-SIG-A1 comes first. The BSS color information is in HE-SIG-A1, B8-B13, as shown in the following figure, referring to the 802.11 spec.

3.2 Utilizing BSS color labeling to enhance spatial reuse and increase system capacity

The different numerical symbols in the following figure represent the channel distribution maps used by different WiFi devices.

If there is no BSS coloring, taking the 36 channel as an example, the use of 36 channel devices on the surrounding green and blue lines will interfere with the use of 36 channel devices in the upper left corner, causing the devices in the upper left corner to always believe that the channel is occupied and not transmitting. After using the BSS coloring mechanism, BSS colors of different colors, such as green lines connecting devices using 36 channels, will not cause interference, only two devices using 36 channels connected by red lines will interfere with each other. This is related to the variation of CCA evaluation channel idle method under BSS color.

Under the BSS color mechanism, the receiving end receives data packets, calculates the received signal power, and demodulates the BSS color field in HE-SIG-A1 to determine whether it is consistent with the associated AP. If it is consistent, the signal is considered to come from MYBSS, and the channel interference threshold standard is lower. For example, using the CCA signal detection threshold (CCA-SD) standard mentioned earlier, the received signal power is compared with CCA-SD to determine whether the channel is idle. If the received signal power is higher than the signal detection threshold, the channel is considered busy. If the BSS color field is not consistent with the associated AP, it is considered that the signal comes from OBSS (Overlapping Basic Service Sets). The threshold standard for determining whether the channel is idle is higher than the CCA signal detection threshold, and CCA-ED threshold can be used. This threshold can be adaptively adjusted, so that it is not easy to think that there is channel interference and not dare to send packets. Thereby improving spatial reuse.

On the left side of the following figure (referring to the Cisco diagram) is the method for determining channel interference for WiFi 4 and 5, without BSS mechanism. The CCA-SD threshold is used to determine channel idle, making it easy to determine channel busy. The figure on the right shows that WiFi 6 uses BSS color with two thresholds. If the BSS color field of HE-SIG-A is consistent with the associated AP, CCA-SD threshold is used to determine whether the channel is idle. Otherwise, CCA-ED threshold is used to determine whether the channel is idle.

The introduction of WiFi 6 has provided wireless networks with higher speeds, lower latency, and better capacity, while also offering a series of innovative technological mechanisms. By gaining a deeper understanding of key concepts and principles such as CSMA/CA, CCA, and BSS Coloring in WiFi 6, we can better grasp the performance and stability of wireless networks and make more valuable contributions to future network development. With the continuous innovation and evolution of technology, we can expect WiFi 6 to be widely used in fields such as the Internet of Things and cloud computing, bringing better experiences and convenience to our digital lives.