Review of Noncontact Medical and Health Monitoring Technologies Based on FMCW Radar
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摘要: 非接触式的医疗健康监测系统解决了用户依从性问题,避免了佩戴电极、传感设备进行监测带来的不舒适感,更有助于将健康监测融入日常生活。非接触式监测手段具有持续地监测用户健康状况的潜力,能够在突发急性医疗事件出现时及时示警,且能够满足新生儿、烧伤患者、传染病患者等特殊人群的监测需求。调频连续波(FMCW)雷达能够同时捕获雷达视场内目标的距离、速度信息,可用于非接触式地监测用户的心率、呼吸率等生理体征及跌倒等行为动作,且从技术上易于单片集成,成本可控,因此在医疗健康监测领域有着重要的应用价值。该文首先阐述了将FMCW雷达应用于非接触式医疗健康监测技术的理论基础,然后系统性地归纳了该领域中的典型前沿应用,最后总结了基于FMCW雷达的医疗健康应用这一领域的研究现状及局限性,并对其应用前景与潜在的研究方向进行了展望。Abstract: A contactless health monitoring system can contribute to health assessment in daily life by reducing appliance usage and avoiding discomfort from wearing electrodes or sensors. Such contactless approaches have the potential to continuously monitor the health status of users, alert patients and health personnel in time when acute medical emergencies occur, and meet the monitoring demands of special populations, such as newborns, burn patients, and patients with infectious diseases. The Frequency-Modulated Continuous-Wave (FMCW) radar can measure the range and velocity of sensing targets and be widely applied in heart and respiration rate monitoring and fall detection. Moreover, advances in FMCW radar have enabled low-cost radar-on-chip and antenna-on-chip systems. Thus, FMCW radar has vital application value in the medical and health monitoring fields. In this study, first, we introduce the basic knowledge of the application of FMCW radar in contactless health monitoring. Then, we systematically review the advanced applications and latest papers in this field. Finally, we summarize the present situations and limitations and provide a brief outlook for the application prospects and potential future research in the field.
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Key words:
- FMCW radar /
- Contactless monitoring /
- Vital sign /
- Sleep monitoring /
- Falling detection
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1. 引言
基于导航卫星的双基地SAR(Bistatic Synthetic Aperture Radar based on Global Navigation Satellite System, GNSS-BSAR)是空-地双基地SAR中一种典型的应用[1],使用在轨的导航卫星作为发射源,地面部署接收机(地基、车载、机载)构成双基地SAR系统[2]。由于导航星座的日趋完善,其全球覆盖性以及重轨特性所带来的优势是其他照射源暂时所不能替代的,其中以地基接收机为主的导航卫星干涉合成孔径雷达(Interference Synthetic Aperture Radar based on the Global Navigation Satellite System, GNSS-InSAR)在场景形变监测领域有着广阔的应用前景[3],成为了近年来研究热点。
在GNSS-BSAR系统成像方面,已有研究者分别使用不同的导航卫星星座进行了成像验证,包括了北斗[4,5]、GPS[6]、格洛纳斯[7]、伽利略[8]。除此以外文献[9]还提出了多角度融合方法以增强图像信噪比。在形变监测方面,来自伯明翰大学的学者们[10]使用直达波天线,配合长约50 m的线缆构建了理想点目标,并使用格洛纳斯作为发射源,首次实现了精度约为1 cm的1维形变反演结果。该实验初步验证了GNSS-InSAR应用于形变监测的可行性。为了进一步验证场景形变监测的可能,2017年文献[11]通过对接收机进行高精度移位来模拟场景建筑形变,成功反演出了形变,精度约为1 cm。在3维形变方面,2018年北京理工大学的技术团队[12]通过人为构建转发器,进行了精度可控的强点目标形变模拟,使用我国的北斗IGSO卫星,成功实现了精度优于5 mm的3维形变反演,这些验证性实验充分表明了GNSS-InSAR应用于场景形变检测的可能。
若要实现GNSS-InSAR场景的3维形变反演,需要同时至少3颗卫星从不同角度照射场景。由于GNSS-InSAR系统的拓扑高度非对称性以及导航信号的窄带特性[13],加上导航卫星的重轨并非是严格意义上的重轨,除了不可避免的空间基线外,重轨时间也并非严格一致,因此在实际数据采集中,需要对系统构型以及数据采集时间进行严格的优化设计。文献[14]提出了一种联合优化方法,解决了面向大场景下的多星多角度构型优化问题,配合多个接收机实现综合分辨性能优异的大场景成像。文献[15]提出了空间去相干的理论描述框架,表明了空间去相干在GNSS-InSAR中的必要性,但未对数据采集时间进行说明。从当前实际情况出发,不精确的数据采集时间可能会造成存储资源浪费,空间去相干导致的数据截取进一步降低了数据有效性。具体如图1所示:
针对上述问题,本文提出了一种GNSS-InSAR场景连续数据采集优化方法,通过结合当前数据的卫星轨迹和两行星历数据文件(STK Two-Line Element sets, TLE)预测轨迹,基于相干系数轨迹对齐,获取卫星重轨时间间隔,得到最优的数据采集策略,从源头上降低数据的空间去相干性,提升所采集数据有效性,节约存储资源。在第2部分对GNSS-InSAR场景数据采集优化方法进行了详细介绍。第3部分针对提出的方法进行了实验设计,开展了实测数据采集,并针对采集的数据进行了初步分析。第4部分对全文进行总结。
2. GNSS-InSAR场景数据采集优化模型
2.1 GNSS-InSAR相干系数理论模型
对于GNSS-InSAR图像而言,经过保相成像处理后,场景中任意一点
(x0,y0) 的像素信息分别对应分辨单元内所有散射体回波的相参叠加,可建模为s(x0,y0)=∬f(x,y;t)exp[−j2πλr(x,y;P)]⋅W(x−x0,y−y0;P)dxdy+n(x0,y0;t) (1) 其中,
f(x,y;t) 为时间t 下的地表散射系数,P 为对应的合成孔径中心位置矢量,W(x,y;P) 表示系统的点扩散函数(PSF),n(x,y;t) 为图像的加性噪声。对于SAR图像的同名点像素,其相干系数可表示为[16]ρ=∬sm(x0,y0)s∗s(x0,y0)dxdy√∬sm(x0,y0)s∗m(x0,y0)dxdy∬ss(x0,y0)s∗s(x0,y0)dxdy (2) 其中,下标m表示主图像,
s 表示辅图像。根据柯西不等式可以判断:0≤ρ≤1 ,当ρ=0 时表示同名点完全不相干,当ρ=1 时,同名点完全相干。将点目标像素模型式(1)带入到式(2)并化简得到
ρ=∬sav(x,y)exp[−j2πλ(r(x,y;Pm)−r(x,y;Ps))]|W(x−x0,y−y0;Pm)|2dxdy√(∬sm(x,y)exp[−j2πλr(x,y;Pm)]|W(x−x0,y−y0;Pm)|2dxdy+nm)×(∬ss(x,y)exp[+j2πλr(x,y;Ps)]|W(x−x0,y−y0;Pm)|2dxdy+ns) (3) 式(3)的推导使用了如下近似:
(1) 由于导航卫星的高轨道特性,使得
W(x−x0, y−y0;Pm)≈W(x−x0,y−y0;Ps) 成立;(2) 相邻两天的噪声相干系数为0,即
n(x0,y0;tm)×n(x0,y0;ts)=0 (4) (3) 相邻两天的目标散射系数为
sav(x,y) ,即sav(x,y)≈f(x,y;tm)f∗(x,y;ts) (5) 对式(3)中的相干系数
ρ 进一步分解得到ρ=ρth×ρti×ρsp (6) 其中,热噪声相干系数
ρth 与时间相干系数ρti 分别由系统与实际目标决定。对于PS点[17]而言,地表散射系数相对稳定,不随时间变化,同时为了便于后续分析,假定散射系数为1得到空间相干系数
ρsp 的简化式为ρsp=∬exp[−j2πλ(r(x,y;Pm)−r(x,y;Ps))]|W(x−x0,y−y0;Pm)|2dxdy√(∬|W(x−x0,y−y0;Pm)|2dxdy)(∬|W(x−x0,y−y0;Pm)|2dxdy) (7) 从式(7)推导结果可以知道,空间基线主要是影响
r(x,y;P) 从而导致空间去相干。2.2 数据采集优化模型
导航卫星的重轨时间并非稳定不变,因此需要对数据采集时间进行有效预测,从源头上降低空间去相干,提高数据有效性。
假定主图像数据采集时间为
tm ,该采集时间可以通过文献[14]中的广义优化模型进行求解,辅图像数据采集时间为ts=tm+Δt ,Δt 为时间间隔,那么最优化数据采集模型可通过式(7)推导而来Δt=argmax{∬|˜W(x,y;Pm)|2exp[−j2πλ(r(x,y;Pm)−r(x,y;P(tm+Δt)))]dxdy} (8) 其中,
˜W(x,y;Pm) 为tm 下等效归一化PSF,P(tm+ Δt) 为Δt 时间偏置下得合成孔径中心位置矢量。第1天数据采集需要进行实验设计以确定最优数据采集时刻,往后的重轨天数据采集可以根据数据采集优化模型,同时结合星历文件进行预测。整体的预测流程如图2所示,
n 为任意一天采集的数据,k 为重轨天数间隔。实际卫星位置对应的实际时间设为
tn ,经过模型优化得到的时间偏差为Δt ,那么第n+k 天对应的实际数据采集时间可表示为tn+k=tn+Δt (9) 3. 实验验证
3.1 GNSS-InSAR场景3维形变反演实验设计
对于固定场景的形变监测,首次数据采集的时候需要严格设计系统构型,使分辨率达到最优化。本次实验接收机部署在北京理工大学信息科学试验楼楼顶西北角,实施监测场景位于西偏北30°。使用理论分辨率计算公式[18]对该场景进行分辨率设计。仿真参数具体参见表1。
表 1 数据采集试验仿真参数Table 1. Data acquisition test simulation parameters参数 值 照射源 北斗 IGSO1~5 PRF 1000 Hz 带宽 10.23 MHz 合成孔径时间 600 s TLE文件更新日期 2019年4月29日 预定数据采集日期 2019年4月30日 以分辨单元面积作为判定依据,得到预定采集日期当天全时段下各个卫星在预定场景下所能得到的分辨单元面积如图3所示。
为了实现3维形变反演,需要同一时间下有3颗卫星对场景进行照射。图3中10点前后与17点前后满足当前场景上空有3颗IGSO卫星可见的条件。更进一步,为了使分辨单元面积达到最优,可以得到具体的数据采集时间。具体如图4红框标注,分别是9点30分前后与17点30分前后。
为了配合实验,在场景布置转发器,整体的系统构型如图5所示。
3.2 重轨数据采集时间优化
以2019年4月30日采集的实测数据作为第
n 天数据,对于北斗的IGSO而言,重轨时间约为1天,即m =1,同时下载当天最新的TLE文件。以IGSO1为例,结合图2进行详细说明:(1) 使用实测数据的直达波进行卫星位置解算,同时根据TLE文件推算当天和相邻天的卫星轨迹。经过相干系数轨迹匹配之后,得到的轨迹如图6所示。
(2) 以匹配得到的TLE卫星轨迹作为参考,对重轨天的TLE卫星轨迹进行数据采集优化模型求解,系统的PSF与优化模型仿真结果分别如图7与图8所示。
对图8的结果分析可知,第1个峰值点为其本身,由于空间基线为0,相干系数为1。第2个峰值点相干系数为0.999644,满足除了第1个峰值点外相邻天相干系数最大值条件,因此第2个峰值点就是最佳重轨时的空间相干系数。此时经过模型优化得到的时间间隔为:
Δt=86163s=23h56min3s ,结合第1天的实测数据轨迹对应的时间t1=9h26min0s ,第2天准确的数据采集时间为:t2=9h22min3s 。3.3 结果分析
为了说明优化结果的正确性,在实验场景中放置转发器模拟理想点目标(图5),同时按照优化后的时间进行5月1日数据采集。实际采集时间为
9h21min53s ,总采集时间约650 s。相邻两天的空间相干系数轨迹匹配结果如图9所示。从图9中峰值点位置来看,重轨数据采集优化模型得到的结果和实际结果相吻合。为了进一步说明,图10给出了IGSO1卫星实测数据成像结果。
对相邻两天的图像相干系数进行求解,得到图11所示结果。在同一坐标系下,仿真目标位于[–147, 20, 0],空间相干系数为0.999644;转发器位于[–147, 20, 0],相干系数为0.9996;两者的相干系数基本保持一致。
图9与图10的结果表明经过数据采集优化模型后得到的时间间隔与实际卫星轨迹的重轨时间相互吻合,在保证600 s预期合成孔径时间下,可以最大限度减少数据采集时间,节约存储资源。同时避免后期由于数据对齐带来的数据有效性降低问题。
4. 结论
在GNSS-InSAR场景1维/3维形变反演应用中,针对由于导航卫星重轨时间的非严格一致性与有效数据截取带来的数据冗余,数据有效性低等问题,本文提出了一种面向GNSS-InSAR场景数据采集的优化模型,采用实测数据与TLE文件相结合,根据当天数据采集时间,预测相邻天重轨时间,从而实现精确的数据采集。实测数据验证结果表明了数据采集时间优化模型的正确性。该方法的提出有利于GNSS-InSAR场景1维/3维形变反演实验的开展,在降低原始数据冗余度基础上,保证了有效数据时间长度大于预期合成孔径时间。
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图 1 基于FMCW雷达的接收信号提取微多普勒信号与计算Range-Doppler图的信号处理流程。其中多个Chirp的IF信号经过FFT变换后得到的二维矩阵称为Range Profile
Figure 1. The signal processing flow of extracting micro-Doppler signal and calculating Range-Doppler map based on the received signal of FMCW radar. The two-dimensional matrix of multiple Chirp IF signals after FFT transform is called Range Profile
表 1 基于FMCW雷达的心率、呼吸率监测研究现状总结
Table 1. Summary of heart rate and respiratory rate monitoring based on FMCW radar
作者 信号获取方法 生理参数估计方法 实验设置 监测指标 Adib等人[21] 微多普勒信号提取 频谱分析 被试者保持静止,与雷达相距1 m RR准确率中位数为99.3%,HR准确率中位数为98.5% Mercuri等人[56] 微多普勒信号提取 频谱分析 2名被试者,静止,与雷达距离分别为2.6 m, 5.4 m 98.5%的RR估计误差小于3 次/
min, 95.5%的HR估计误差小于3 次/minWang等人[22] Beamforming,微多普勒信号提取 频谱分析 3名被试者,2名被试者与雷达距离1 m,AOA相差60°,1名被试者与雷达距离为1.5 m,保持静止 RR, HR平均准确率大于92.8% Chen等人[58] 相控阵技术,微多普勒信号提取 频谱分析 2名被试者,与雷达距离相同,约2 m,被试者间距离1 m,静止 97.8%的RR估计误差小于1.5 次/min, 93.6%的HR估计误差小于3 次/min Sun等人[59] EMD,微多普勒信号提取 频谱分析 被试者与雷达间距1.0~2.5 m,静止 HR估计误差RMSE为2.03~5.83 次/min Wang等人[45] VMD,微多普勒信号提取 峰值检测 被试者与雷达间距0.5~2.0 m,AOA为0~60°,静止 IBI RMSE为29.850~68.974 ms Toda等人[60] CNN,微多普勒信号提取 QRS波群检测 被试者静止,距离2.5 m IBI MAE为17.8 ms Ha等人[61] Beamforming,CNN,微多普勒信号提取 Unet[62] 被试者静止,面向雷达,距离25~50 cm 心脏收缩期、舒张期等心脏活动检测准确率90%,召回率为69.8% Zheng等人[63] CFAR,多变量VMD,微多普勒信号提取 频谱分析,峰值检测 被试者驾驶汽车,在不同路况下行驶 RR 误差中位数为 0.06 次/min, HR MAE误差中位数为 0.6 次/
min, IBI误差中位数约50 msChen等人[64] 深度对比学习算法,微多普勒信号提取 频谱分析,峰值检测 被试者存在步行,坐下/站起等大幅度肢体运动 RR HR的MAPE为2% ,3%;不同肢体运动下IBI误差中位数为20~40 ms 表 2 基于FMCW雷达的跌倒检测研究现状总结
Table 2. Summary of research status of falling detection based on FMCW radar
作者 雷达特征信息 算法概述 是否在新用户/
新环境下测试是否需要
采集跌倒样本非跌倒/跌倒
样本比例检测指标 Jokanovic等人[125] Range Profile,Range-Doppler图 Autoencoder+
Logistic回归否 是 43:17 Acc: 96% Tian等人[128] Range-Angle图 级联CNN分类器 是 是 450000:293 F1: 0.929 元志安等人[126] Range-Doppler图 CNN+LSTM 是 是 1:1 Acc: 96.67% Wang等人[127] IF信号 LKCNN 否 是 1:1 Acc: 95.24% Jin等人[46] 点云 VAE+RNN 否 否 4:1 Acc: 98% Li等人[40] Range-Doppler图 LSTM 是 是 5:1 Acc: 96% -
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