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

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 11 '17 at 17:34

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

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