[ath9k-devel] More on signal and noise

[Eduard GV <eduardgv@gmail.com>]

Understood, big thank you.

However, the noise floor shouldn't take only thermal noise into
account. Man-made noise could raise the noise floor more than 6dB in
the congested 2.4GHz band (in the 5GHz band it should be lower).

By the way, I join the popular demand for having access to CSI data!

>
> I'd spent some time trying to understand how these chips do the RSSI and noise
> measurements and attempt to shortly explain my vision of this process.
>
> Actually these chips unable to measure absolute signal level in dBm. This is
> because of amplifiers in radio are implemented in CMOS technology. Real gain of
> such gain stages are unpredictable and varies with temperature. Instead this
> CMOS technology gives a simple way to realize stable gain step independrnt
> from the temperature. So that Atheros chips can give as a valid SNR which is
> incorrectly called RSSI in descriptor status fields. The value of noise
> reported by "iw survery" is meaningless. This value obtained from a maximum
> gain set by free running AGC within short period of time and then substracted
> by baseband DSP from gain locked on packet's preamble. This process is
> described in much details in Atheros' patent US 7,245,893 B1. Very interesting
> document, should I say. I'm also impressed with 55 claims at the end.
>
> Now how the absolute RSSI is ?calculated in ath9k. Instead of using meaningless
> noisefloor it adds predefined value of -95 dBm to each SNR measured in
> baseband. I will try to guess how this value are calculated. The basic equation
> for calculating noise power at the antenna input is: Pn = k*T*F*B. Where: k -
> Boltzmann constant, T - input noise temperature, F - noise factor of the
> receiver and B - the bandwidth.
> The temperature variation is less then 1dB within working range 250..330K, so
> can be ignored. If we assume T = 300K, F = ~2 for LNAs used in Atheros
> reference boards, we got the following values: 166 fW = -98dBm in 20MHz
> bandwith and 331 fW = -95 dBm in 40 MHz bandwith.
>
> The value -95 programmed in ath9k is valid reference noise level for 40MHz, but
> for 20MHz it should be lowered by 3dB. This difference in measured RSSI can be
> easily shown in monitor mode observing signal level from 20MHz station. When
> monitor node is switched between HT20 and HT40 the RSSI will change by 3dB.
>
>
>
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