Re: [ath9k-devel] ath9k: noise floor calibration process

[Benoit PAPILLAULT <benoit.papillault@free.fr>]

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
>>
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>>
>>     
>
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