I have been learning about the WLAN technology and I couldn't understand the logic behind the choice of the Clear Channel Assessment thresholds protocol used in CSMA/CA:

In 802.11ac for WLAN, a device attempting to transmit over a 20 MHz subchannel should make sure the channel is idle. The threshold at which this device decides whether it should transmit or not is -82 dbm. If a device wants to transmit over primary 40 MHz channel the threshold at which it decides if it can or it can't transmit is -79 dbm (+3db wrt to -82 dbm because we doubled the bandwidth).

1) Why is that a 3db difference in threshold exists, there must be logical explanation for such?

I am just looking for some logical interpretation of why these concepts are made? Thank you

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    Aug 11, 2017 at 17:34

2 Answers 2


This is a matter of fairness between 40MHz and 20MHz devices. Assuming transmitter splits power equally between the primary and secondary 20MHz channels, then a 20MHz device will only hear half of the transmitter's power output. This means under equal circumstances, the 40MHz transmitter will have to be closer to the 20MHz receiver than a 20MHz transmitter to cause the same level of interference.


the reason for 3db is that 40MHz noise floor is 3 dB higher

  • Yes. The transmission power is same for channel widths 20 MHz and 40 MHz. Doubling the bandwidth will double the noise power. Double is 3 dB.
    – JPX
    Mar 14, 2022 at 8:45

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