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. _______________________________________________ ath9k-devel mailing list ath9k-devel@lists.ath9k.org https://lists.ath9k.org/mailman/listinfo/ath9k-devel
