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"
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