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 analyzeror interference classifier
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:
Riceved power computation:
Userspace programs:
this document is based on https://wiki.freebsd.org/dev/ath_hal%284%29/SpectralScan