Hi Benoit, Yes. Having a patent is nice and probably significant too. However, if using a preamp with a low noise figure results in more signals or stronger (more error free) signals, then it IS better than the chip itself !! It is VERY surprising that no chips I am aware of provide directly for support of an external low noise preamp AND on a seperate coax a REAL amplifier. In the USA Amateur radio operators can use 1000 Watts (under certain conditions) !! Now two high powered, low noise Acess Points with a clear line of sight would give some real range! warm regards, Wiz On Wed, 28 Apr 2010, Benoit PAPILLAULT wrote: > RHS Linux User a écrit : > > 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. > > > In fact, that's what I thought first, but according to Atheros patent, > it can. It seems pretty logic in fact : if you have no real signal at > the input, you are measuring ambient noise. A spectrum analyzer is able > to do it as well. > > Sensitivity is the ability to decode 802.11 modulation out of the signal > received (which includes any kind of noise). Using the formulas below, > I measure the minimum signal levels of 802.11 packets by moving away > from an AP and I indeed found -95dBm which is the typical sensitivity > levels of an Atheros chip. > > 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 ! > > > Sure, in fact, I /think/ the thermal noise I'm talking is generated > inside the first RX amplifier (it's probably generated in every RX > amplifier, but the first is the biggest since it's amplified more than > the others). So, if you put a low noise preamp, you are feeding the chip > with more signal at the input but the chip has still the same amount of > noise. Am I correct? > > Have fun, > > > > Wiz > > > Regards, > Benoit > > > > 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 > >> > >> > > > > _______________________________________________ > > 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
