[ath9k-devel] Measurements with Spectral Scan in ath9k

["Gabriel Tolón" <gtolon@inti.gob.ar>]

Hi,

I also want to thank you all for this code. It's a very useful feature!

I've been playing with it to understand how it works and to compare some 
measurements made with a wdr3500 router, and with an RF power meter.

The test consisted of a wifi card transmitting in 802.11n in different 
channels, using MCS7, with an output power of 0dbm for the whole 20 MHz 
channel (measured with the power meter), attenuated with different 
levels, and conducted to one of the antenna connectors of the WDR3500. 
Also a generator was used to transmit a non wifi signal, a carrier 
modulated with FM.

Now, if I understood correctly how ath9k and the code for spectral scan 
work, when at the antenna input there's a signal with for example -60 
dbm in a 20 MHz channel, the RSSI + Noise reported for that channel 
should equal that value. On the other hand, the values in dbm for each 
bin on that channel should be in average 17.48 dbm (10*log(56)) lower, 
because the total power is divided by the 56 bins, right?

Well, for example in 2) I'd expect an RSSI + N equal to -60 dbm, and the 
bins more or less in the line of -77 dbm, but the actual RSSI + N is 
much higher, it's -32 dbm. That difference is not the same in all cases, 
but the RSSI always seems to be higher than expected.

Also, for each channel, when increasing the singal 20 dB, the resulting 
RSSI + N varies much more in same cases.

I know I shouldn't expect a high precision, and I really think al this 
is very useful even if not being an absolute reference, to make relative 
comparisons over time, or over frequency. But I'd like to be sure that 
I'm not misinterpreting the way these measurements are done, and that 
I'm measuring OK. So if someone have done something similar or 
understands better the theory involved I'd appreciate any feedback.

Thanks!



These are the results:

1) Channel 2, 80 dB attenuation:

http://picpaste.com/ch2_att80-xGCxbMXP.png


2) Channel 2, 60 dB att:

http://picpaste.com/ch2_att60-bGozXimM.png


3) Channel 2, 40 dB att:

http://picpaste.com/ch2_att40-bckD2dUU.png


4) Channel 6, 80 dB att:

http://picpaste.com/ch6_att80-upzGPCK8.png


5)Channel 6, 60 dB att:

http://picpaste.com/ch6_att60-CYmnx77n.png


6) Channel 6, 40 dB att:

http://picpaste.com/ch6_att40-MKE99zA0.png


7) Channel 10, 80 dB att:

http://picpaste.com/ch10_att80-Fe1ODcmZ.png


8) Channel 10, 60 dB att:

http://picpaste.com/ch10_att60-HbvPIUuw.png


9) Channel 10, 40 dB att:

http://picpaste.com/ch10_att40-93rJirGQ.png


Using a carrier modulated with FM, with 8.5 MHz width (17 MHz total):


10) Channel 2, 60 dB att:

http://picpaste.com/ch2_att60_FM-urbCkYMf.png


11) Channel 6, 60 dB att:

http://picpaste.com/ch6_att60_FM-5D9aTcJo.png


12) Channel 10, 60 dB att:

http://picpaste.com/ch10_att60_FM-nZGpzKEi.png



El 10/06/13 12:15, Gui Iribarren escribi?:
> On 05/08/2013 07:50 PM, Oleksij Rempel wrote:
>> Am 08.05.2013 18:15, schrieb Claudio:
>>> Hi all,
>>>     first of all thank you for the code you have developed.
>
> Indeed, it opens the door to many interesting possibilities!
> Thanks a lot Simon and Mathias for letting the genie out of the bottle,
> and Adrian, Felix, Zefir for helping its way out :P
>
> I finally got to play with Simon's code and it works better than i expected
> http://blog.altermundi.net/article/playing-with-ath9k-spectral-scan/
>
>>> echo chanscan > /sys/kernel/debug/ieee80211/phy0/ath9k/spectral_scan_ctl
>>> iw dev wlan0 scan
>>> cat /sys/kernel/debug/ieee80211/phy0/ath9k/spectral_scan0 > /tmp/fft_resusts
>>> ./fft_eval /tmp/fft_results
>>>
>>> On average I get 200-400 samples but sometimes I got even 7k samples, which
>>> is very good.
>
> In my case i get a fairly constant 112 samples per "iw wlan0 scan", but
> as always, YMMV.
>
> maybe something is triggering periodic scans on your hardware? (i.e. a
> STA wifi-iface in OpenWrt does this every 2 or 3 seconds until it
> associates to an AP). In that case, just idling between "cat
> spectral_scan"s will increase the number of samples.
> Try running "iw event" in parallel to see what's happening.
>
> Cheers and have spectral fun!!
>
> Gui
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