[ath9k-devel] linearity of ath9k CSI phase

Dear Joe Ayers,
Thank you for response.

I don't have a sort of RF anechoic chamber. So, I've captured CSI in a
quite realistic and practical environemnt with possible interference and
multipath components. There are other WLAN APs, Bluetooth devices. Also,
there exist desks, chairs and walls as reflectors.

Yet, I don't think it does matter to capture CSI and estimate true phase.
My attempts are based on a couple of papers which have done estimating true
phase and AoA of a signal with uniform linear antenna array (ULA) in a
realistic and practical living environment [1, 2, 3]. Which means, those
papers claim that they can identify directpath component and multipath
components with MUSIC algorithm [4].

One suspicious thing is, I think distance between antennas of ULA is
misconfigured. I placed them 6.5 cm apart from each other. With
calculation, a half of wavelength at 2.4 GHz band is less than 6.25 cm.
Does it matter a lot?

Regards,
Jeon.

[1]: K. Qian, C. Wu, Z. Yang, Y. Liu, and Z. Zhou, ?PADS: Passive detection
of moving targets with dynamic speed using PHY layer information,? in 2014
20th IEEE International Conference on Parallel and Distributed Systems
(ICPADS), 2014, pp. 1?8.
[2]: J. Xiong and K. Jamieson, ?ArrayTrack: A Fine-Grained Indoor Location
System,? in Presented as part of the 10th USENIX Symposium on Networked
Systems Design and Implementation (NSDI 13), Lombard, IL, 2013, pp. 71?84.
[3]: M. Kotaru, K. Joshi, D. Bharadia, and S. Katti, ?SpotFi: Decimeter
Level Localization Using WiFi,? SIGCOMM Comput. Commun. Rev., vol. 45, no.
4, pp. 269?282, Aug. 2015.
[4]: R. Schmidt, ?Multiple emitter location and signal parameter
estimation,? IEEE Transactions on Antennas and Propagation, vol. 34, no. 3,
pp. 276?280, Mar. 1986.

2016-06-14 3:28 GMT+09:00 Joe Ayers <[email protected]>:

> Jeon,
>
> "only constant offset across subcarrier seems to be effective".    Could
> this be because there's not just one signal being received anymore, rather
> with microwaves, particularly with lots of nearby reflection surfaces,
> there's now ~10 signals bouncing in to the receive antenna at 10
> AoA's--some with 2x the distance-delay traveled--and resonance/nulls
> occurring?  How perfect is your test environment?
>
> Joe AE6XE
>
> On Mon, Jun 13, 2016 at 10:00 AM, Jeon <[email protected]> wrote:
>
>> 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
>>
>> From the result, in short, removing only constant offset across
>> subcarrier seems to be effective. But I'm not sure.
>> And sometimes, some phase measurement show large dispalcement along
>> y-axis even they are captured within very short duration.
>>
>> Hence the question is,
>> Is ath9k reports CSI with those unwanted linear phase offset removed?
>> If it is not, should I look into Atheros CSI tool? As I look into it, it
>> just captures CSI from the kernel and does not modify it.
>> Or, Is CSI of Atheros different form that of Intel? I don't think so...
>>
>> The final goal of extracting true phase from CSI of ath9k is to determine
>> angle of arrival (AoA) of signal.
>>
>> Regards,
>> Jeon.
>>
>> [1]: http://pdcc.ntu.edu.sg/wands/Atheros/ "Atheros CSI Extraction Tool"
>> [2] K. Qian, C. Wu, Z. Yang, Y. Liu, and Z. Zhou, ?PADS: Passive
>> detection of moving targets with dynamic speed using PHY layer
>> information,? in 2014 20th IEEE International Conference on Parallel and
>> Distributed Systems (ICPADS), 2014, pp. 1?8.
>> [3] M. Kotaru, K. Joshi, D. Bharadia, and S. Katti, ?SpotFi: Decimeter
>> Level Localization Using WiFi,? SIGCOMM Comput. Commun. Rev., vol. 45, no.
>> 4, pp. 269?282, Aug. 2015.
>> [4] http://dhalperi.github.io/linux-80211n-csitool/ "Linux 802.11n CSI
>> Tool"
>>
>> _______________________________________________
>> ath9k-devel mailing list
>> ath9k-devel at lists.ath9k.org
>> https://lists.ath9k.org/mailman/listinfo/ath9k-devel
>>
>>
>
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