Clear Channel Assessment

3.5.2.1 Technological Background

Spectrum analysis works only for partial non-Wi-Fi interference. To help analyze Wi-Fi interference and uncontrollable non-Wi-Fi interference, 802.11 introduces a CCA mechanism to monitor channel status (busy or clear). Channel contention occurs between Wi-Fi devices only when channels are clear. With the CCA mechanism, signal transmission in the presence of interference can be avoided to ensure WLAN performance.

3.5.2.2 Working Principles

WLAN data transmission follows a “listen-to-send” principle. Before sending data, a Wi-Fi device does not prepare to send data until detecting that the channel is clear. The technology used is referred to as CCA, which detects whether a channel is clear at the physical layer. Figure 3.41 illustrates the CCA definition of the IEEE. Basically, CCA uses two thresholds to determine whether a channel is clear: signal detect threshold and energy detect threshold.

Signal detect threshold: On a 20MFIz channel, if the strength of a preamble-contained OFDM signal is greater than or equal to -82 dBm, carrier sensing considers that the channel has a 90% probability of being busy for 4 ps.

Energy detect threshold: On a 20 MHz channel, if signal strength is greater than or equal to -62 dBm (signal detect threshold plus 20 dBm), carrier sensing considers that the channel is busy.

Based on IEEE definitions, when the channel bandwidth is 20 MHz, the signal detect threshold is equal to the receiver sensitivity of the smallest modulation and coding scheme (MCS) index (which is -82 dBm), and the energy detect threshold is the sum of the signal detect threshold plus 20 dBm, which is therefore -62 dBm. When the channel bandwidth is 40 MHz, the signal detect threshold is -82 dBm for the primary 20 MHz channel and -72 dBm for the secondary channel. In the case of the channel bandwidth being 80 MHz, the

signal detect threshold is —79 dBm for the primary 40 MHz channel and -72 dBm for the secondary channel.

To further improve CCA, IEEE 802.1 lax introduces the BSS color technology. As illustrated in Figure 3.42, with BSS color recognizing the Intra-BSS (MYBSS) and Inter-BSS (OBSS) frames, it is possible to raise the CCA signal detect threshold for the Inter-BSS frames while maintaining a low CCA signal detect threshold for the Intra- BSS frames. This way, when the signal strength from a neighboring BSS exceeds the CCA signal detect threshold used in earlier standards, with the transmit power lowered for the packets to be sent, the channel can be still considered clear and available for new data transmission. This will minimize the impact of co-channel interference on WLAN performance.

2. Huawei’s dynamic CCA mechanism

Packets are sent only when channels are clear. The CCA mechanism monitors channel status (clear or busy) to reduce the likelihood of collision occurring due to unknown channel status. The mechanism is applicable where both non-Wi-Fi interference and Wi-Fi interference are present. It improves the accuracy of channel status detection based on the signal detect threshold, reducing air interface collision and improving transmission efficiency. However, with the same default signal detect threshold, the CCA mechanism cannot deliver the same performance in all scenarios.

Huawei’s dynamic CCA mechanism dynamically adjusts the signal detect threshold based on scenario conditions, maximizing its network performance.

• Lower the CCA signal detect threshold for reduced probability of collision

As illustrated in Figure 3.43, АР 1 and AP 2 are co-channel APs, use the default CCA signal detect threshold (-82 dBm), and are unaware of each other. When АР 1 sends packets to STA 1, AP 2 considers that the channel is clear based on the CCA mechanism and then sends packets to STA 2. As a result, packets from AP 1 cannot be parsed on STA 1.

With Huawei’s dynamic CCA mechanism, the channel collision as a result of using the default CCA signal detect threshold can be recognized. In case of a high channel collision rate, the CCA signal detect threshold can be adjusted downward to reduce the probability of collision. As illustrated in Figure 3.44, after the CCA signal detect threshold is adjusted to -85 dBm, AP 1 and AP 2 become mutually detectable. Therefore, when AP 1 sends packets to STA 1, AP 2 considers that the channel is busy based on the CCA mechanism and will not send packets to STA 2 until the channel becomes clear. This reduces the probability of channel collision.

FIGURE 3.44 Scenario 1—lowered CCA signal detect threshold.

• Raise the CCA signal detect threshold for increased concurrent data transmission on co-channel APs

As illustrated in Figure 3.45, АР 1 and AP 2 are co-channel APs, use the default CCA protocol threshold (-82 dBm), and are

FIGURE 3.46 Scenario 2—raised CCA signal detect threshold.

aware of each other. However, STA 1 and AP 2 are unaware of each other, so are STA 2 and AP 1. When АР 1 sends packets to STA 1, AP 2 considers that the channel is busy based on the CCA mechanism, and therefore sends packets to STA 2 only after the channel becomes clear.

In this case, with STA 2 unable to sense the signals from AP 1, STA 2 can correctly parse the packets from AP 2. As illustrated in Figure 3.46, with the dynamic CCA mechanism, the scenario is identified and the CCA signal detect threshold is then raised to -78 dBm to make AP 1 and AP 2 mutually unaware. As a result, AP2-to-STA2 packet transmission and APl-to-STAl packet transmission will be free of mutual impact. Therefore, raising the CCA signal detect threshold in such scenarios help increase concurrent data transmission among co-channel APs, improving overall WLAN performance.

3.5.2.3 Applications of CCA

Upon detecting that channels are busy, Wi-Fi devices wait. When the channels are clear, they contend for the channels. The CCA mechanism reduces the probability of collision between interfering and desired signals and is therefore applicable to scenarios where interference is present.

Huawei’s dynamic CCA mechanism adjusts the CCA signal detect threshold for APs based on scenario conditions, ensuring better performance when Wi-Fi co-channel interference is present.