[ath9k-devel] linearity of ath9k CSI phase

Dear ath9k developers,

I am currently working with Atheros CSI Extraction Tool [1] to get a true
phase of each subcarrier.

- Background

[2], [3] and many other papers claim that phase information from extracted
CSI contains two components: true phase and unwanted phase offset due to
subcarrier and time delay.
i.e., measured_phase = true_phase + time_delay * subcarrier_index +
phase_offset_due_to_txrx_mismatch
This equation can be visualized as below:

http://i.imgur.com/rk9Hh1M.png

(Please note that this figure is based on CSI tool for Intel 5300 NIC.)

It contains unwanted linear phase offset and constant phase offset. Since
the true phase is relatively small, it seems that phase is monotonically
increasing or decreasing in macro view due to the unwanted phase offsets.
We cannot see a tiny true phase currently.

To remove phase offset due to subcarrier, the mentioned papers are
attempting to remove it with linear fitting ax + b,
where a = slope of the figure, b = average of measured phase, and x =
subcarrier index.

After removing unwanted phase offset components, the true phase is
estimated.
This estimated true phase seems steady and consistent across a time
duration shorter than < 100 - 1000 ms:

http://i.imgur.com/AO89vYV.png

Note that Y-axis scale is reduece from [-50, 10] to [5, -3]

- My question

I want to extract and manipulate CSI phase WITH ATH9K NIC.

After extracting CSI from my ath9k NIC (AR9580 @ 2.4 GHz) with Atheros CSI
extraction tool,
I've tried various fitting methods to eliminate unwanted components and
stacked results from nearly 100 packets:

http://i.imgur.com/5r9eYwO.png