How can signalstrength be measured if noise is present? Does interference cause fluctuating signalstrength?

[rutger_jautze]

I have a general question about signalstrength. I will first lay down my thoughts and understandings about this subject and then comes the question (bold).

signal = 802.11 wave which is modulated so it represents and transports data(bits). It applies only to 802.11-traffic in case of wifi. It is processed and filtered raw RF energy.

Interference = signalloss in the air due to negative change of amplitude. Since the amplitude itself represents data, next to phase, this data becomes corrupted. This loss can be caused by:

-FSPL (distance – always there)

-out-of-phase noise-waves (basic noisefloor, ACI, hidden nodes, non-wifi devices using same frequency and nearby frequencies). Note: CCI is not a cause of this kind of interference, because CSMA/CA will manage it, it will only have impact on channel utilisation.

In both cases (FSPL and noise-waves) signalloss in the air will happen before it arrives at receiver. Let’s say the SNR is very low: 5. This is because the desired signal is -67 dBm and the noisefloor is -72 dBm. Since the desired signal is likely to be corrupted by the noise and thus lowered in amplitude, how is it possible the AP can recognize the desired signal? How can the AP measure the signalstrength of desired signal as if there was no signalloss due to out-of-phase noise-waves?

The only explanation I can come up with is that both noise RF energy and desired signal are not constantly sending, there are breaks between each burst of both (Guard interval and IFS for example with 802.11 traffic). So sometimes the receiver is able to see the desired signal like a little sunray through the dark clouds of noise and therefore can measure its strength. I think the desired signal has to deal with arbitrary breaks/pauses from the noise and probably is still influenced before it reaches the receiver and its strength will be smaller than without noise. Also the interference at one moment will be stronger then other moment, which suggests a fluctuation of received signal strength, so in the example at one time it would be -67 dBm, next moment -70 dBm and so on (not sure if -3 dBm is realistic since it halves the signalstrength, but you get the idea). Please confirm if fluctuation can be caused by interference or please give other explanation.

Thanks.

[samantha_lynn]

Signal strength will absolutely fluctuate with interference levels in the environment. FSPL and phase issue are only two of many factors that lead to wireless signal degradation. For an in depth look at what can contribute to interference I would recommend the Radio Frequency chapters in the CWNA guide by Coleman and Westcott.

This guide reviews how to identify and troubleshoot interference in your network using ExtremeCloud IQ: https://thehivecommunity.aerohive.com/s/article/Radio-Frequency-Interference-NG

I'm not sure what you mean when you say the AP measures the desired signal strength? We have a default SNR setting of 15 and that can be adjusted in the configuration, but that isn't something the AP will adjust based on it's own measurements of the environment. Am I understanding your question correctly?