Hi All, <preaching> The chip *FOR SURE* *CANNOT* measure the thermal noise level!! It isn't that sensitive. That said under some conditions it CAN measure the local interference level which IS useful. I am *VERY MUCH* in favor of making real time level measurements of various parts of real packets easy to use! Troubleshooting becomes so much easier :). </preaching> Great ideas !! FWIW - I have on occasion used a low noise preamp to feed the chip. Many more signals are detectable which "proves" the chip by itself *IS NOT* that sensitive. Try it yourself ! Have fun, Wiz On Tue, 27 Apr 2010, Benoit PAPILLAULT wrote: > Hello, > > In order to move forward with noise & signal reporting, I'd like to > share my current understanding of the way ath9k HW is working before > sending patches (unfortunately, I did the work before the introduction > of ar9003... so I need to redo the work). > > The ultimate purpose of this work is to be able to measure signal levels > (and noise if possible) as accurately as a spectrum analyzer or power meter. > > First, signal level reporting. It is reported in a per packet basis in > RX descriptors. There are 7 fields: > AR_RxRSSIAnt00 0x000000ff rs_rssi_ctl0 > AR_RxRSSIAnt01 0x0000ff00 rs_rssi_ctl1 > AR_RxRSSIAnt02 0x00ff0000 rs_rssi_ctl2 > AR_RxRSSIAnt10 0x000000ff rs_rssi_ext0 > AR_RxRSSIAnt11 0x0000ff00 rs_rssi_ext1 > AR_RxRSSIAnt12 0x00ff0000 rs_rssi_ext2 > AR_RxRSSICombined 0xff000000 rs_rssi > > Each value is for a 20 MHz wide channel, on the 3 RX chains. "ctl" is > for the primary channel and "ext" is for the secondary channel (using > the 802.11n words). The latter rs_rssi is the sum of the 6 previous > value. However, since each value is dB, the sum is not an arithmetic > sum. Each field is a signed value and the value -128 means that no > measurement has been done (no RX chain, RX chain disabled, no secondary > channel, ...). It seems that in some cases, the combined value is just > plain wrong. Here are few examples: > > RSSI: ctl=(10,7,-128) ext=(-128,-128,-128) => 12 (11.76) correct > > RSSI: ctl=(38,29,-128) ext=(69,-84,-101) => -22 incorrect!!! > > > Next, noise floor calibration. From what I understand, signal levels is > measured using the AGC + RX amplifiers gain (RF, IF and BB). However, > the various gains are not really accurate, only the relative gain are > accurate. This means that reading a signal value of -100dBm might not > exactly means -100dBm. There is a delta between real signal and measured > value. In order to know this value, we need a calibration process with a > known signal. > > One know signal is thermal noise. Thermal noise is generated in any > resistor and can be computed using the well know value N = kTB. For a 20 > MHz bandwidth, this gives -101dBm. If the HW tries to measure signal > strength when the network is supposed to be idle (during SIFS) and with > RX/TX switch disabled (?), then it will in fact measure the thermal > noise at the RX input. > > So, we have : > > Real noise (-101dBm) = Measured noise + delta > > There are type of registers to control noise floor calibration : > > - control register at 0x9860 (AR_PHY_AGC_CONTROL) > > This register allows 3 differents operations : > > 1. start noise floor measurement > > write AR_PHY_MAXCCA_PWR (AR_PHY_CCA & 0x000001ff) : this is apparently > a max value > for noise floor > REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_ENABLE_NF); > REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NO_UPDATE_NF); > REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NF); > > When channel has been changed however, the noise floor needs to be > updated immediately, so AR_PHY_AGC_CONTROL_NO_UPDATE_NF should be > cleared in this particular case. Otherwise, the chip is no longer > receiving (problem since CCA is defined with noise floor as reference). > > 2. read noise floor measurement result > > check REG_READ(ah, AR_PHY_AGC_CONTROL) & AR_PHY_AGC_CONTROL_NF > if 0 (noise floor calibration is finished), read AR_PHY_MINCCA_PWR : > nf = MS(REG_READ(ah, AR_PHY_CCA), AR_PHY_MINCCA_PWR = 0x0ff80000) > > 3. write noise floor reference > > write AR_PHY_MAXCCA_PWR (the value has not the same meaning as > operation 1!) > REG_CLR_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_ENABLE_NF); > REG_CLR_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NO_UPDATE_NF); > REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NF); > > - data register at 0x9864 (AR_PHY_CCA, + more location for other RX chains) > > The fields are different for AR9280+ chipsets, but the mechanism is > the same. > > AR_PHY_MAXCCA_PWR 0x000001ff (half dBm unit!) > AR_PHY_CCA_THRESH62 0x0007f000 > AR_PHY_MINCCA_PWR 0x0ff80000 > > Now, we have : > > Real signal = Measured signal + delta > = RSSI + Noise floor + delta > = RSSI + (-101 dBm) > > Real noise is not thermal noise. There are a lot of definition for noise > since noise is NOT signal. Of course, noise includes thermal noise. > Since the noise measured by the chip is variable, I think we could do : > > - Noise floor = minimum (Noise floor measures) > - Noise = moving average (Noise floor measures) + delta > with delta = (-101 dBm) - Noise floor > > I'd like to get comments before sending patches. Since ath5k and ath9k > are quite close, I'm pretty sure a similar (if not same) process is used > on ath5k. > > Regards, > Benoit > > _______________________________________________ > ath9k-devel mailing list > ath9k-devel@xxxxxxxxxxxxxxx > https://lists.ath9k.org/mailman/listinfo/ath9k-devel > -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
