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