RF-ECG:Heart Rate Variability Assessment Based on COTS RFID Tag Array.85:5 Water RFID tag bag array RFID antenna Enlarged RFID tag array Fig.2.Empirical study setup. dedicated sensors are fairly expensive and battery-hungry,so it is not suitable for large-scale usage and long-term deployment.HB-phone [22]estimates the human heart rate based on the vibration sensors mounted on the bed,which provides the heartbeat monitoring during sleeping.However,it only provides the average heart rate during a time window without the fine-grained IBI information.Recently,researchers try to measure the heart rate by using the smartphone cameras [19,24,31],which require the user to place her fingertip on the camera.Such vision-based approaches heavily rely on the user's pose as well as the illumination condition; moreover,as the signals are collected from the weak pulse,the accuracy is relatively lower than the approaches of obtaining the signals directly from the heartbeat.In contrast,our solution measures the heartbeat using the signal reflected from the heart,which could be more accurate than the pulse-based measurement.In addition,our solution leverages the battery-free RFID tags for heartbeat detection,which is more lightweight,scalable and not limited by the battery life. RF-based heartbeat detection:Recent research has shown that RF-signals are sensitive to the changes of the multi-path environments [9,14,23,39,41],thus both the heartbeat and breathing can be detected according to the variation of the RF-signals without requiring the user to hold or wear any device.Radar-based approaches, such as FMCW [10,42],doppler radar [17,29],are accurate at measuring such tiny environmental changes However,they usually require dedicated hardware and incur high cost for daily heartbeat monitoring.Nguyen et al [30]try to estimate the respiration rate and heart rate based on a radio transceiver and a radar navigator.But it requires specific motion devices for navigation,and it only provides the coarse-grained heart rate estimation. Wi-Fi based approaches focus on estimating the vital signs using commercial off-the-shelf(COTS)Wi-Fi devices. Specifically,they mainly leverage the Channel State Information(CSI)from both the time and frequency domain to estimate the breathing rate and heart rate [26,28,36].However,both the radar and Wi-Fi based techniques cannot label the subject,due to their device free characteristic.Therefore,it is difficult to distinguish and monitor multiple users simultaneously,especially when users share similar breathing or heartbeat patterns.To address this problem,Adib et al.proposed a radar technique to monitor the vital signs of multiple people simultaneously, by separating reflectors into different buckets depending on the distance between these people and the device [10].Hence,it requires the human subjects to be separated with a considerable distance for efficient distinction In contrast to the previous work,in this paper,we propose a novel approach for heartbeat sensing via RFID tag array,which can be regarded as an extremely lightweight sensor.Moreover,its nature of identification can be used to effectively distinguish multiple human subjects,even if these human subjects are very close to each other. 3 UNDERSTANDING HEART RATE VARIABILITY 3.1 Measurement of Periodic Signal via RFID Tag Array In order to systematically study how to use an RFID tag array to passively sense a periodic signal,e.g.,the heartbeat signal,we first use a controlled experiment to investigate the reflection influence of the periodic signal on the tag array,which is reflected from a signal source without attaching an RFID tag.Specifically,we manually Proc.ACM Interact.Mob.Wearable Ubiquitous Technol,Vol.2,No.2,Article 85.Publication date:June 2018.RF-ECG: Heart Rate Variability Assessment Based on COTS RFID Tag Array • 85:5 Enlarged RFID tag array RFID antenna RFID tag array Water bag Fig. 2. Empirical study setup. dedicated sensors are fairly expensive and battery-hungry, so it is not suitable for large-scale usage and long-term deployment. HB-phone [22] estimates the human heart rate based on the vibration sensors mounted on the bed, which provides the heartbeat monitoring during sleeping. However, it only provides the average heart rate during a time window without the fine-grained IBI information. Recently, researchers try to measure the heart rate by using the smartphone cameras [19, 24, 31], which require the user to place her fingertip on the camera. Such vision-based approaches heavily rely on the user’s pose as well as the illumination condition; moreover, as the signals are collected from the weak pulse, the accuracy is relatively lower than the approaches of obtaining the signals directly from the heartbeat. In contrast, our solution measures the heartbeat using the signal reflected from the heart, which could be more accurate than the pulse-based measurement. In addition, our solution leverages the battery-free RFID tags for heartbeat detection, which is more lightweight, scalable and not limited by the battery life. RF-based heartbeat detection: Recent research has shown that RF-signals are sensitive to the changes of the multi-path environments [9, 14, 23, 39, 41], thus both the heartbeat and breathing can be detected according to the variation of the RF-signals without requiring the user to hold or wear any device. Radar-based approaches, such as FMCW [10, 42], doppler radar [17, 29], are accurate at measuring such tiny environmental changes. However, they usually require dedicated hardware and incur high cost for daily heartbeat monitoring. Nguyen et al. [30] try to estimate the respiration rate and heart rate based on a radio transceiver and a radar navigator. But it requires specific motion devices for navigation, and it only provides the coarse-grained heart rate estimation. Wi-Fi based approaches focus on estimating the vital signs using commercial off-the-shelf (COTS) Wi-Fi devices. Specifically, they mainly leverage the Channel State Information (CSI) from both the time and frequency domain to estimate the breathing rate and heart rate [26, 28, 36]. However, both the radar and Wi-Fi based techniques cannot label the subject, due to their device free characteristic. Therefore, it is difficult to distinguish and monitor multiple users simultaneously, especially when users share similar breathing or heartbeat patterns. To address this problem, Adib et al. proposed a radar technique to monitor the vital signs of multiple people simultaneously, by separating reflectors into different buckets depending on the distance between these people and the device [10]. Hence, it requires the human subjects to be separated with a considerable distance for efficient distinction. In contrast to the previous work, in this paper, we propose a novel approach for heartbeat sensing via RFID tag array, which can be regarded as an extremely lightweight sensor. Moreover, its nature of identification can be used to effectively distinguish multiple human subjects, even if these human subjects are very close to each other. 3 UNDERSTANDING HEART RATE VARIABILITY 3.1 Measurement of Periodic Signal via RFID Tag Array In order to systematically study how to use an RFID tag array to passively sense a periodic signal, e.g., the heartbeat signal, we first use a controlled experiment to investigate the reflection influence of the periodic signal on the tag array, which is reflected from a signal source without attaching an RFID tag. Specifically, we manually Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 2, No. 2, Article 85. Publication date: June 2018