[ath9k-devel] More on signal and noise

[Peizhao Hu <peizhao.research@gmail.com>]

Does this CMOS assumption apply to ath5k as well?

I thought ath5k has noise recalibration which tune the noise floor 
accordingly (which seems to be true from our experiments). In madwifi 
the noise floor is fixed at -95 or -96, but the ath5k introduces the ani 
and noise recalibration.

But the rs_rssi is SNR??

regards;

Peizhao


On 03/05/11 21:10, Alex Hacker wrote:
> On Mon, May 02, 2011 at 02:46:06PM -0700, Eduard GV wrote:
>> Hi all,
>>
>> Just three questions. I need per-packet SNR information and my first
>> guess was to inspect "last_signal" from debugfs. Values range from -30
>> to -60. last_signal file should contain signal (dBm) of last received
>> frame (from sta_info.h), right? That explains values obtained. But...
>>
>> 1) This value is computed as signal=ATH_DEFAULT_NOISE_FLOOR +
>> rx_stats->rs_rssi, which is confusing me. It would be explained if
>> rs_rssi is actually SNR (not RSSI) measured in dB. Am I wrong?
>>
>> 2) Why is NOISE_FLOOR fixed to -95 (dBm?). Noise varies randomly, e.g.
>> noise reported by iw survey dump vary from -91 to -101 dBm.
>>
>> 3) By the way, what do rs_rssi_ctlX and rs_rssi_extX (-1<  X<  3) measure?
>>
> 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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