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en:users:drivers:ath9k:spectral_scan

ath9k spectral scan

Atheros 802.11n chipsets include a built-in spectral analysis feature. AR92xx and AR93xx have the ability to report FFT data from the baseband under software controlled conditions, including:

  • absolute magnitude (|i|+|q|, abs() for I/Q phase of the wireless signal) for each FFT bin (56 for subcarriers in HT20 mode and 128 in HT40 mode)
  • an index indicating the strongest FFT bin
  • the maximum signal magnitude for each sample

These information can be used to create an open source spectrum analyzer or interference classifier. To try it out, you can use the following commands to acquire samples for all channels which can be scanned:

echo chanscan > /sys/kernel/debug/ieee80211/phy0/ath9k/spectral_scan_ctl
iw dev wlan0 scan
cat /sys/kernel/debug/ieee80211/phy0/ath9k/spectral_scan0 > samples
echo disable > /sys/kernel/debug/ieee80211/phy0/ath9k/spectral_scan_ctl

Spectral scan configuration parameters can be read and changed /sys/kernel/debug/ieee80211/phy0/ath9k/:

  • spectral_count: number of scan results requested. Note that the “never disable” mode is implemented through the “background” mode in specral_scan_ctl, see below.
  • spectral_short_repeat: controls the short_repeat parameter, see “Spectral scan parameters” below
  • spectral_fft_period: controls the fft_period parameter, see “Spectral scan parameters” below
  • spectral_period: controls the period parameter, see “Spectral scan parameters” below
  • spectral_scan_ctl: Contains the current mode. There are the following modes available:
    • disable: spectral scan is disabled
    • background: spectral scans samples are returned endlessly from the currently configured channel. It is running while the hardware is not busy with sending/receiving. Must be turned on by writing “trigger” into spectral_scan_ctl.
    • manual: as many spectral scan samples as configured in spectral_count are returned from the current channel after writing “trigger” into spectral_scan_ctl.
    • chanscan: as many spectral scan samples as configured in spectral_count are returned for each channel when performing a scan.
  • spectral_scan0: the relayfs file which returns the spectral scan samples. The samples are returned as TLV binary data, see drivers/net/wireless/ath/ath9k/spectral.h and/or drivers/net/wireless/ath/spectral_common.h for the format

Spectral scan parameters

  • fft_period: when active and triggered, PHY passes FFT frames to MAC every (fft_period+1)*4uS
  • period: when active, time period between successive spectral scan entry points (period*256*Tclk). Tclk = 44MHz for HT20 operation, 88MHz for HT40 operation
  • count: number of scan results requested. There are special meanings in some chip revisions:
    • AR92xx: highest bit set (>=128) is interpreted as “never disable”
    • AR9300 and newer: 0 is interpreted as “never disable”
  • short_repeat: controls whether the chip is in spectral scan mode for 4 usec (enabled) or 204 usec (disabled)

Frame format

HT20:

      0       1       2       3       4       5       6       7       8
      +---------------------------------------------------------------+
 0    |       [7:0]: bin -28 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
 1    |       [7:0]: bin -27 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
 2-54 |                                                               |
      +---------------------------------------------------------------+
 55   |       [7:0]: bin 27 magnitude (|i| + |q|) >> max_exp          |
      +---------------------------------------------------------------+
 56   |       [7:0]: all_bins {max_magnite[1:0], bitmap_weight[5:0]}  |
      +---------------------------------------------------------------+
 57   |               [7:0]: all_bins {max_magnite[9:2]}              |
      +---------------------------------------------------------------+
 58   |       [7:0]: all_bins {max_index[5:0], max_magnite[11:10]}    |
      +---------------------------------------------------------------+
 59   |                       [3:0] max_exp                           |
      +---------------------------------------------------------------+

HT40:

      0       1       2       3       4       5       6       7       8
      +---------------------------------------------------------------+
0     |       [7:0]: bin -64 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
1     |       [7:0]: bin -63 magnitude (|i| + |q|) >> max_exp         |
      +---------------------------------------------------------------+
2-125 |                                                               |
      +---------------------------------------------------------------+
127   |       [7:0]: bin 63 magnitude (|i| + |q|) >> max_exp          |
      +---------------------------------------------------------------+
128   |    [7:0]: lower_bins {max_magnite[1:0], bitmap_weight[5:0]}   |
      +---------------------------------------------------------------+
129   |               [7:0]: lower_bins {max_magnite[9:2]}            |
      +---------------------------------------------------------------+
130   |       [7:0]: lower_bins {max_index[5:0], max_magnite[11:10]}  |
      +---------------------------------------------------------------+
131   |    [7:0]: upper_bins {max_magnite[1:0], bitmap_weight[5:0]}   |
      +---------------------------------------------------------------+
132   |               [7:0]: upper_bins {max_magnite[9:2]}            |
      +---------------------------------------------------------------+
133   |       [7:0]: upper_bins {max_index[5:0], max_magnite[11:10]}  |
      +---------------------------------------------------------------+
134   |                       [3:0] max_exp                           |
      +---------------------------------------------------------------+

Received power computation

Assuming the noise floor is equal to -96dbm(*) and the magnitude of each sample in a 20MHz bin equals the RSSI, the received signal strength of each FFT bin on HT20 channel can be computed as follow:

power(i) = nf + RSSI + 10*log(b(i)^2) - bin_sum

where:

  • RSSI is computed on control chain 0
  • b(i) is the magnitude in each bin, unscaled by max_exp
  • bin_sum = 10*log(sum[i=1..56](b(i)^2))

For 40MHz channel, previous formula should be used for 64 bins of control and extension channels, keeping in mind for HT40+ mode the extension channel is above the primary one (lower=ctl, upper=ext) and for HT40- the extension channel is below the primary one (lower=ext, upper=ctl).

(*) nf can differ from -96dbm due to noise and spikes leading to a reduced reported RSSI.

Userspace programs

FFT samples gathered from Atheros NICs could be drawn using userspace programs:

(based on Adrian Chadd's documentation https://wiki.freebsd.org/dev/ath_hal%284%29/SpectralScan)

en/users/drivers/ath9k/spectral_scan.txt · Last modified: 2017/01/09 19:47 by Adam Hunt