Mitigation of CSI Temporal Phase Rotation with B2B Calibration Method for Fine-Grained Motion Detection Analysis on Commodity Wi-Fi Devices

Abstract

Limitations of optical devices for motion sensing such as small coverage, sensitivity to obstacles, and privacy exposure result in the need for improvement. As motion sensing based on radio frequency signals is not constrained by the limitation above, channel state information (CSI) from Wi-Fi devices could be used to improve sensing performance under the above circumstances. Unfortunately, CSI phase cannot be practically obtained due to the temporal phase rotation generated from Wi-Fi chips. Therefore, it would be rather complicated to realize motion analysis, especially the direction of motion. To mitigate the issue, this paper proposes a CSI calibration method that employs a back-to-back channel between Wi-Fi transceivers for phase rotation removal while preserving the original CSI phase. Through experiment, calibrated CSI showed a high similarity to the channel without phase rotation measured using a Vector Network Analyzer (VNA). Another experiment was conducted to observe Doppler frequency due to simple hand gestures using the Wavelet transform. A visual analysis revealed that the Doppler frequency of calibrated CSI could correctly capture the motion pattern. To the best of the authors’ knowledge, this is the first calibration method that maintains the original CSI and is applicable for in-depth motion analysis.

Modelling, Analysis, and Simulation of the Micro-Doppler Effect in Wideband Indoor Channels with Confirmation Through Pendulum Experiments

“…TV phase distortions exist due to carrier frequency offset [21][22][23], sampling frequency offset [24][25][26], and packet boundary delay [27,28]. These TV phase distortions were eliminated by using a B2B connection between the transmitter station and the receiver station as described in [35]. Since there was only one RF transmission port in the Wi-Fi T x , an RF power splitter ZFSC-2-10G+ from with two output ports was used.…”

“…These attempts do not help to reveal the true Doppler power characteristics of the preprocessed CSI data. In [35] the phase distortions of CSI data have been eliminated by using a back-to-back (B2B) connection between the transmitter and receiver stations. This approach allows for revealing the true Doppler power characteristics of the measured CSI data.…”

Analysis of Non-Intrusive Hand Trajectory Tracking by Utilizing Micro-Doppler Signature Obtained From Wi-Fi Channel State Information

“…CSI has been measured in various wireless communication systems including Wi-Fi to assess the spatial and temporal conditions of the wireless channel. Multiple-input multipleoutput (MIMO) transmission allows the Wi-Fi system to utilize multiple channels thus effectively improving the quality of data communication through the use of beamforming, spatial multiplexing, and diversity techniques [23], [43]. In shadowed region this study, the usage of CSI is further explored for motion detection and tracking through the analysis of Doppler frequency.…”

“…Except for the second experiment, the measurement campaigns were additionally performed to verify the applicability of the simulation result. The hardware configuration was similar to the measurement setup in [23]. Basically, Linux 802.11n CSI Tool [51] was utilized to obtain CSI from each Wi-Fi packet propagating between two commercial laptops at 5.31 GHz center frequency and 40-MHz bandwidth.…”

“…Unfortunately, the temporal pattern of CSI which induces by Doppler frequency depends on the position of the Wi-Fi transmitter (Tx) and receiver (Rx) relative to the user location according to bistatic radar [20]- [22]. The effect of the location-dependent pattern has been visualized in [23] that the same hand gesture could produce different temporal Doppler pattern if the gestures were performed at different positions. As a result, the usability of hand motion sensing may not be able to achieve based on gesture recognition without further in-depth analysis.…”