A WiFi Multi-Link Collaborative Human Tracking Method for Smart Home
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摘要: 在智能家居场景中,WiFi网络将多个物联网(IoT)设备相连,为实现室内被动人体跟踪提供了新路径。然而,多链路跟踪需要考虑信号融合、设备自定位等问题。为此,该文提出了一种适用于商用WiFi的多链路协同跟踪方法。针对商用网卡的测量信号噪声,这种方法首先进行信号预处理,从高维的原始信道状态信息(CSI)数据中提取多普勒频移(DFS)。随后,对多链路DFS进行融合,以实现协同跟踪。该方法在没有WiFi设备位置先验知识的条件下也能实现设备自定位,为跟踪算法提供输入。最后,该文搭建了一个多发单收网络结构的原型验证系统,在未知设备位置的情况下实现了小于50 cm的人体跟踪精度,相比传统方法提升了约70%。在复杂场景的测试中表明,该系统能达到亚米级的跟踪精度,对设备位置误差具有鲁棒性,并显示出一定实时运行潜力。Abstract:
Objective WiFi-based indoor passive human tracking has become increasingly attractive for smart homes due to the widespread interconnection of Internet of Things (IoT) devices via existing WiFi infrastructures. Some approaches estimate Angle of Arrival (AoA) or Time of Flight (ToF) of the target reflection path to achieve tracking, yet which are fundamentally limited by the difficulty of distinguishing weak target reflection component from dense multipath due to small antenna array and narrow bandwidth under the existing mature protocols. Consequently, Doppler Frequency Shift (DFS)-based approaches utilizing multiple links have proven more practical. Dead Reckoning (DR) is commonly applied in such methods. However, it is challenging to dynamically select links for signal fusion. Furthermore, the performance of DR-based methods is significantly degraded when device locations are inaccurate. Additionally, Existing methods usually adopt a single-transmitter-multi-receiver structure, which complicates data aggregation and restricts multi-antenna utilization. To address the challenges of multi-link signal fusion and device-location uncertainty, Particle Filter (PF) is used in this paper. Furthermore, a multi-transmitter-single-receiver structure is adopted, in which multiple STAtions (STAs) transmit uplink packets to a single Access Point (AP). This structure simplifies data aggregation while exploiting the multi-antenna advantage at the AP. Methods In the IEEE 802.11 standard, the wireless channel is estimated at the receiver using pilots embedded in WiFi packets, and Channel State Information (CSI) is continuously reported. Because human motion perturbs multipath propagation and causes time-varying changes in CSI, CSI is utilized as the primary sensing signal since it implicitly encodes the information of target’s movement. In practice, raw CSI is strongly corrupted by amplitude and phase impairments. Therefore, noise reduction is first conducted before Doppler extraction. Subsequently, multi-link DFS are fused via PF, in which the target state is sequentially propagated and updated using likelihoods derived from Doppler measurements. This formulation naturally facilitates dynamic link selection and probabilistic fusion, as unreliable links are down-weighted through their likelihood contributions rather than being deterministically discarded. Moreover, the inaccuracy of device locations can be absorbed into measurement noise, contributing to the robustness against device-location uncertainty. In the absence of prior knowledge of device locations, self-localization is enabled first by jointly estimating the AoA and ToF of the Line of Sight (LoS) path between the AP and each STA. Preliminary self-localization tests of devices ( Fig. 5 ) demonstrate a median STA positioning error of approximately 0.56 m (Table 1 ), which provides a relatively reliable initialization for the tracking algorithm.Results and Discussions A prototype is implemented for evaluation under a multi-transmitter-single-receiver architecture mounted with commodity Intel AX200/AX201 WiFi cards. CSI is collected in a 20 MHz bandwidth channel at a 5.24 GHz center frequency using PicoScenes, with 802.11ac packets transmitted at 100 to 200 Hz. Each CSI sample contains measurements of 57 subcarriers. In parallel, FTM measurement is performed at 2.4 GHz in channel 11 using iw and hostapd. In the prototype, each STA transmitter is equipped with a single antenna, while the AP receiver is equipped with two antennas. Experiments are conducted with one AP and three to four STAs ( Fig. 7 ). WiTraj and PITrack using DR are set as baselines. When device self-localization is required, the proposed method achieves a median tracking error of 0.47 m, compared to 1.78 m for WiTraj and 1.77 m for PITrack (Fig. 9 ). This demonstrates an accuracy improvement of approximately 70% over traditional DR-based methods. Error injection experiments involving random positional disturbances reveal that, unlike traditional DR-based methods which are highly sensitive to location mismatch, the proposed approach exhibits robustness to device-location uncertainty (Fig. 10 ). Additionally, computational complexity analyses indicate that while execution time increases with the number of particles (Table 3 ), tracking performance stabilizes beyond a certain threshold (Fig. 11 ). Thus, real-time operation is attainable without sacrificing tracking quality by selecting an optimal particle count. Finally, evaluations in complex environments confirm that the proposed method consistently maintains accurate tracking performance, which is better than the baselines (Fig. 12 ).Conclusions A Particle-Filter-based approach is proposed for multi-link collaborative passive human tracking using commodity WiFi. By probabilistically fusing multiple links, robust tracking is achieved even when device locations contain errors. When device locations are unavailable, self-localization is supported through combining CSI-based estimation and FTM measurements, which supplies the necessary geometric priors for tracking. A prototype is built on a multi-transmitter-single-receiver network structure, which facilitates data aggregation and exploits AP-side multiple-antenna advantage. Experimental results indicate that (1) a sub-meter level tracking accuracy can be achieved when there exist device-location errors, which is much higher than the traditional method; (2) the proposed method has certain robustness to device-location uncertainty; and (3) the computational cost can meet real-time requirements when the particle count is properly chosen. However, the current study focuses on single-person tracking only. Future work should extend the framework to multi-person scenarios and evaluate performance under more diverse and realistic environmental conditions. -
Key words:
- WiFi sensing /
- Multi-link /
- Passive human tracking /
- Doppler Frequency Shift (DFS)
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表 1 设备自定位测试结果
位置编号 1 2 3 4 5 6 7 8 9 10 11 12 13 14 AoA估计误差 (°) 6.2 8.2 5.0 11.6 13.4 0.0 3.0 10.4 2.0 16.0 22.6 5.4 1.8 5.4 FTM测距误差 (m) 0.07 0.00 0.35 0.17 0.11 0.21 0.08 0.33 0.54 0.42 0.29 0.82 0.68 0.59 STA定位误差 (m) 0.33 0.36 0.42 0.65 0.56 0.21 0.14 0.56 0.54 0.63 1.00 0.84 0.68 0.62 1 基于粒子滤波的多链路跟踪
输入:每条链路$ n $的DFS序列$ \left\{\overrightarrow{f_{\text{D}}^{n}}\right\} $,每个STA$ n $的位置$ \left(x_{\text{STA}}^{n},y_{\text{STA}}^{n}\right) $,链路数$ N $,粒子数$ {N}_{\text{S}} $,初始状态分布$ \boldsymbol{P} $,过程噪声协方差矩阵
$ \boldsymbol{Q} $,测量噪声方差$ {\sigma }^{2} $,时间间隔$ \Delta T $输出:目标的估计状态$ {\left\{{\hat{\boldsymbol{x}}}[t]\right\}}_{t=1,2,\cdots ,T} $ 1 for $ i $= 1 to$ {N}_{\text{S}} $ do 2 从初始状态分布中采样第$ i $个粒子:$ {\boldsymbol{x}}_{i}[1]\sim \boldsymbol{P} $;设置第$ i $个粒子的初始权重:$ {w}_{i}[1]=1/{N}_{\text{S}} $ 3 end for 4 获取粒子初始状态:$ {\hat{\boldsymbol{x}}}[1]=\displaystyle\sum \limits_{i=1}^{{N}_{\text{S}}}{w}_{i}[1]{\boldsymbol{x}}_{i}[1] $ 5 for $ t $= 2 to$ T $ do 6 for $ i $= 1 to$ {N}_{\text{S}} $ do 7 通过式(15)传播粒子$ {\boldsymbol{x}}_{i}[t] $ 8 for $ n $= 1 to$ N $ do 9 通过式估计第$ i $个粒子的DFS$ \hat{f}_{\text{D}i}^{n}[t] $;通过式计算第$ i $个粒子的似然度$ {p}_{i} $;计算权重$ {w}_{i}[t]={w}_{i}[t]\times {p}_{i} $ 10 end for 11 end for 12 权重归一化:$ {w}_{i}[t]={w}_{i}[t]/\displaystyle\sum \limits_{i=1}^{{N}_{\text{S}}}{w}_{i}[t] $;估计目标状态:$ {\hat{\boldsymbol{x}}}[t]=\displaystyle\sum \limits_{i=1}^{{N}_{\text{S}}}{w}_{i}[t]{\boldsymbol{x}}_{i}[t] $ 13 重采样$ {N}_{\text{S}} $个粒子 14 for $ i $= 1 to$ {N}_{\text{S}} $ do 15 重置权重:$ {w}_{i}[t]=1/{N}_{\text{S}} $ 16 end for 17 end for 18 return $ {\left\{{\hat{\boldsymbol{x}}}[t]\right\}}_{t=1,2,\cdots ,T} $ 表 2 基于粒子滤波的多链路跟踪算法参数设置
$ {N}_{\text{S}} $ $ \boldsymbol{P} $ $ \boldsymbol{Q} $ $ {\sigma }^{2} $ $ \Delta T $ 1000 $ \mathcal{N}\left(\left[\begin{array}{c}{x}_{\text{tar}}[1]\\{y}_{\text{tar}}[1]\\0\\0\end{array}\right],\text{diag}\left[\begin{array}{c}{0}^{2}\\{0}^{2}\\{0.01}^{2}\\{0.01}^{2}\end{array}\right]\right) $ $ \mathcal{N}\left(\left[\begin{array}{c}0\\0\\0\\0\end{array}\right],\text{diag}\left[\begin{array}{c}{0.05}^{2}\\{0.05}^{2}\\{0.25}^{2}\\{0.25}^{2}\end{array}\right]\right) $ 0.42 0.1 s 表 3 不同参数下的算法复杂度
参数 N 3 4 NS 500 1000 2000 5000 500 1000 2000 5000 运行时间 (s) 平均值 0.0138 0.0275 0.0551 0.1465 0.0137 0.0270 0.0553 0.1521 最大值 0.0454 0.0808 0.1061 0.2952 0.0440 0.0654 0.1110 0.5602 -
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