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Couldn't better receive sensitivity of an AP's radio possibly impact performance?

Say you have an AP and Wi-Fi client communicating at a relatively strong -30dBm RSSI. And the environment is filled with other 802.11 traffic, but their signals are weak (approx. -80dBm). If the AP is sensitive enough to receive down to -90dBm, then the router will spend unnecessary time processing all that background traffic, and thus decreasing the potential throughput of the client-to-AP connection, right?

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It’s not so much a matter of spending time processing unwanted signal, but that unwanted traffic is “noise” that reduces your signal to noise ratio. Signal to noise ratio (SNR) is more important than absolute received power, and really determines the maximum transmission rate that you can reliably decode signal.

The unwanted traffic (i.e. noise) causes errors in the decoding of your desired signal, causing retransmissions. So your throughput drops off, not because your receiver is “too busy” decoding the unwanted noise (you can only decode one signal at a time), but because the noise causes decoding errors.

In theory, a lower sensitivity will reduce noise, but at the cost of not being able to decode weaker signals. But even if your receiver isn’t activated by the noise, the noise can still cause distortion and errors in the received signal, especially when the signal is marginal.

Since 802.11 uses collision avoidance, at a certain noise threshold, the AP or station won’t transmit because it considers the channel “in use.” It will wait until the channel is idle before transmitting. So this also reduces throughput, much like not being able to say something with a group of friends because one person won’t stop talking.

Also remember that the receiver’s sensitivity is measured at the center of the channel, and drops off as you move higher or lower. So the receiver is less sensitive to interference as you move away from the center of the channel.

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  • The way I understand it, you can place throughput degradation into two categories: physical interference and protocol interference. So, you're suggesting that physical interference (the impact of unwanted 802.11 signals on the SNR of the wanted signal) is more significant than protocol interference (the denial of the channel due to contention/collision avoidance)? If the AP and STA have already established a strong connection, would it be wise or unwise for the AP to decrease it's receive sensitivity to prevent protocol interference, resulting in higher throughput potential? – elin05 Mar 8 '14 at 4:40
  • I'm not saying one is more prevalent than the other -- it would depend on the circumstances. I would use the terms "noise" and "channel contention", though. – Ron Trunk Mar 8 '14 at 5:22
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If you want to 'game the system' and maximize your link performance (to the detriment of others) then you should maximize your transmit power and deafen the receiver (attenuate the RX inputs) so that the -80dBm stuff is not being detected. Or, start off with a deaf receiver. This assumes you have a massive carrier to interference advantage to begin with.

A bit like a shouting conversation between to hard-of hearing people in a restaurant, they don't hear anyone except each other because they are shouting that compensates their deafness. Everyone else shuts up and leaves.

This way your AP/client will effectively cripple CSMA/CD protocol and transmit over the top of everyone else, and because you have sufficient CIR then the other interference won't badly affect you. You will likely force them to silence their TX more too.

Most APS have combined RX and TX ports so it isn't usually possible to attenuate the RX path like this. Perhaps for this very reason.

If everyone did it nothing would work in the presence of a few other APs.

(but I wish I could - I can 'see' 170+ APs in front room and a different 180 APs out back)

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