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