[ath9k-devel] ath9k: spectral scan -vs- channel state information

Adrian Chadd <adrian <at> freebsd.org> writes:

> 
> On 12 April 2016 at 15:05, Dan <aidan <at> plezica.com> wrote:
> > Hi
> >
> > I am having some trouble understanding the difference between
> > channel state information (often requested on the mailing list)
> >  and what is currently available via spectral scan.
> >
> >
> > My understanding is as follows;
> > - spectral scan (once triggered via the debugfs) logs
> > channel state estimation for each of the 56 ofdm subcarriers
> >  (HT20 case). In the spectral code (for ath9k), the channel state
> > info is the I and Q values for the subcarriers.
> > For ath9k, the bulk of the code is in ath_process_fft
> 
> Right. And it's sent over as a PHY error frame.
> 
> > Bits I dont understand :
> > Can channel state information be collected on a more general
> > basis ? --- for example for each phy packet received.
> 
> No. There's not enough bandwidth available in the PHY->MAC connection
> to also throw over that much data per packet. So, you get to trigger
> it once, and then the next packet gets the CSI data dumped into a
> memory region in the chip somewhere. Then you read it out.
> 
> > Also, the calibration (ar9003_calib) seems to gather I and Q
> > data also... but its not clear when this is run/collected and used.
> 
> > Any help appreciated....
> 
> That's part of ADC calibration. The idea is to calibrate differences
> in ADCs on the I and Q side of things.
> 
> -adrian
> 

Thanks. A few further questions...
For spectral scan, the I/Q values are 'estimations' based on the
 IFFT/FFT maths magic in the hardware - and not data that comes 
from transmitters (as might be the case in beam forming) 
- right ?

In the data structure for the FFT, I can't see (I may well be blind) 
where the 'phase' values are 
stored (Q).... it seems to mention only 'magnitude' 
which I am assuming is the magnitude of 
the signal (I)
thanks