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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