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Robust Device-Free Intrusion Detection Using Physical Layer Information of WiFi Signals

Robust Device-Free Intrusion Detection Using Physical Layer Information of WiFi Signals applied sciences Article Robust Device-Free Intrusion Detection Using Physical Layer Information of WiFi Signals 1 1 , 1 2 3 , 4 Jiguang Lv , Dapeng Man *, Wu Yang , Liangyi Gong , Xiaojiang Du * and Miao Yu Information Security Research Center, Harbin Engineering University, Harbin 150001, China; lvjiguang@hrbeu.edu.cn (J.L.); yangwu@hrbeu.edu.cn (W.Y.) The School of Computer Science and Engineering, Tianjin University of Technology, Tianjin 300072, China; gongliangyi@gmail.com Department of Computer and Information Sciences, Temple University, Philadelphia, PA 19122, USA Institute of Information Engineering, Chinese Academy of Sciences, Beijing 100093, China; yumiao@iie.ac.cn * Correspondence: mandapeng@hrbeu.edu.cn (D.M.); dxj@ieee.org (X.D.); Tel.: +86-451-8258-9638 (D.M.) Received: 26 October 2018; Accepted: 27 December 2018; Published: 5 January 2019 Featured Application: Intrusion Detection and Smart Home. Abstract: WiFi infrastructures are widely deployed in both public and private buildings. They make the connection to the internet more convenient. Recently, researchers find that WiFi signals have the ability to sense the changes in the environment that can detect human motion and even identify human activities and his identity in a device-free manner, and has many potential security applications in a smart home. Previous human detection systems can only detect human motion of regular moving patterns. However, they may have a significant detection performance degradation when used in intrusion detection. In this study, we propose Robust Device-Free Intrusion Detection (RDFID) system leveraging fine-grained Channel State Information (CSI). The noises in the signals are removed by a Principle Component Analysis (PCA) and a low pass filter. We extract a robust feature of frequency domain utilizing Continuous Wavelet Transform (CWT) from all subcarriers. RDFID captures the changes from the whole wireless channel, and a threshold is obtained self-adaptively, which is calibration-free in different environments, and can be deployed in smart home scenarios. We implement RDFID using commodity WiFi devices and evaluate it in three typical office rooms with different moving patterns. The results show that our system can accurately detect intrusion of different moving patterns and different environments without re-calibration. Keywords: intrusion detection; human detection; channel state information; device-free passive 1. Introduction Device-free human detection has attracted a lot of interest in recent years. It can detect human presence in the monitoring area without any sensing-related devices attached to the people [1]. It can be used well in intrusion detection systems, which is a vital security component in a smart home. Aiming at handling the security issues in a smart home, many techniques have been utilized to implement device-free human detection, such as video-based, infrared-based, Radio Frequency Identification (RFID)-based and Ultra-Wide Bandwidth (UWB)-based approaches. Although they have a good detection accuracy, these approaches have limited using conditions and need dedicated devices that hinder their adoption. WiFi-enabled devices become the catalyst of device-free sensing as they have been widely deployed in both public and private buildings. Besides being used for communication, WiFi networks can also be used as sensor networks [2–4]. Many applications have emerged based on WiFi infrastructures, human detection [5], indoor localization [6], and even human identification [7] are some representative applications. Appl. Sci. 2019, 9, 175; doi:10.3390/app9010175 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 175 2 of 17 A typical WiFi-based device-free human detection system usually contains several pairs of transmitters and receivers. A wireless router can act as a transmitter, while a WiFi-enabled device can act as a receiver. As a result, it doesn’t have the problem of key management [8,9] compared with sensor-based approaches. The rational of WiFi-based device-free human detection is that human presence has an impact on signal propagation, which will cause the signal strength fluctuation at the receiver [10]. Previous WiFi-based human detection systems utilize Received Signal Strength Indicator (RSSI) from Media Access Control (MAC) layer for it is easy to obtain. However, RSSI is a coarse-grained measurement. In the typical indoor scenario, RSSI becomes unreliable due to multipath fading. It may increase, decrease, or even remain the same when a person moves in the monitoring area. Recently, many studies explore CSI from physical layer of wireless networks to detect human motion [11–13]. As indicated in [14], CSI is a subcarrier-level measurement that is more fine-grained compared with RSSI. It is more sensitive to environmental changes while keeps quite stable in static scenarios. As a result, CSI succeeds in improving the performance of human detection. However, state-of-the-art human detection techniques still have limitations for intrusion detection systems. Common human detection techniques can only detect a human who is walking with a regular pattern. Nevertheless, an intruder in the building is likely to keep away from the security devices or move very slowly to hide himself from being monitored. Furthermore, most human detection techniques require on-site calibration of both static and dynamic environments. On-site calibration is labor intensive and it needs professional deployment and maintenance that makes a human detection system more complex in practical use. Consequently, human detection techniques will fail in detecting intruders in security systems, and we need to explore effective features to model human motion. To deal with the limitations, in this work, we propose a Robust Device-Free Intrusion Detection (RDFID) system leveraging fine-grained CSI. We investigate the impact of human motion on WiFi signals and demonstrate that different patterns of human motion in different scenarios can be modeled by a unified framework. First, we extract the wavelet variance of CSIs from frequency domain as the feature. It is more sensitive to human motion, and more robust under different moving patterns. In addition, the feature values of static and intrusion can be seen to be generated by different Gaussian Models. As a result, intrusion can be detected using a Gaussian Mixture Model (GMM). As shown in Figure 1, RDFID can detect human motion of different moving patterns. In addition, it can be easily deployed that it can achieve a satisfying performance even using a single pair of transceivers, and needs no re-calibration in different scenarios. We prototyped RDFID in three typical home and office scenarios with commodity WiFi devices composing only one wireless link. We evaluate the system and compare the performance with Fine-grained Real-time passive human motion Detection (FRID), device-free Passive Detection of moving humans with dynamic Speed (PADS) and Fine-grained Indoor Motion Detection (FIMD). The results show that the detection precision of RDFID can achieve over 97% under different moving patterns. Consequently, it makes intrusion detection systems a step closer to practical use. In summary, the contributions of our work are as follows: We propose RDFID, a novel device-free WiFi-based intrusion detection approach, which can detect intruders with different moving patterns at a high accuracy, and needs no re-calibration in different scenarios. It can be deployed in smart home scenarios to ensure security. We extract real-time features from CSIs in frequency domain, which is more sensitive to human motion of various moving patterns. We use the Gaussian Mixture Model (GMM) as the classifier based on the observation that the feature values under different moving patterns and different environments can be seen to be generated by different Gaussian Models. In the rest of this paper, the related works about WiFi-based human detection are reviewed in Section 2. Some preliminaries are introduced in Section 3. Section 4 presents the design details of Appl. Sci. 2019, 9, 175 3 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   3  of  17  our proposed intrusion detection system, while the performance evaluation is provided in Section 5. our proposed intrusion detection system, while the performance evaluation is provided in Section 5.  In Section 6, the potentials and limitations are discussed and we conclude this work in Section 7. In Section 6, the potentials and limitations are discussed and we conclude this work in Section 7.  (b) (a) (c) Figure 1. Different moving patterns in intrusion scenarios. (a) Regular walking; (b) walking while Figure 1. Different moving patterns in intrusion scenarios. (a) Regular walking; (b) walking while  bending down; and (c) creeping. bending down; and (c) creeping.  2. Related Work 2. Related Work  WiFi-based passive human detection is the fundamental technique of various ubiquitous wireless WiFi‐based  passive  human  detection  is  the  fundamental  technique  of  various  ubiquitous  sensing applications, such as indoor localization, human identification and activity recognition. It can wireless  sensing  applications,  such  as  indoor  localization,  human  identification  and  activity  be widely deployed in smart home scenarios to ensure the security. A large quantity of studies about recognition.  It  can  be  widely  deployed  in  smart  home  scenarios  to  ensure  the  security.  A  large  wireless sensing promote the development of wireless sensing. quantity of studies about wireless sensing promote the development of wireless sensing.   Earlier passive human detection systems usually utilize RSSI from the MAC layer of the wireless Earlier  passive  human  detection  systems  usually  utilize  RSSI  from  the  MAC  layer  of  the  network. After Youssef et al. proposed the concept of device-free passive human motion detection, wireless network. After Youssef et al. proposed the concept of device‐free passive human motion  they optimized their approach and made the system work in real environments [10]. Nuzzer leveraged detection,  they  optimized  their  approach  and  made  the  system  work  in  real  environments  [10].  probabilistic techniques, and had the capability to both localize a single entity and estimate the number Nuzzer leveraged probabilistic techniques, and had the capability to both localize a single entity  of people in the area of interest [15]. Since RSSI is a coarse-grained measurement of wireless networks, and  estimate  the  number  of  people  in  the  area  of  interest  [15].  Since  RSSI  is  a  coarse‐grained  many RSSI-based human detection systems deployed multiple pairs of transceivers to achieve a higher measurement of wireless networks, many RSSI‐based human detection systems deployed multiple  accuracy [16]. Another technique of human detection using multiple pairs of transceivers is Radio pairs of transceivers to achieve a higher accuracy [16]. Another technique of human detection using  Tomographic Imaging (RTI) [17]. Researchers also developed various approaches based on RTI, such multiple pairs of transceivers is Radio Tomographic Imaging (RTI) [17]. Researchers also developed  as the kRTI [18] and dRTI [19]. However, RSSI-based human detection systems suffer from severe various  approaches  based  on  RTI,  such  as  the  kRTI  [18]  and  dRTI  [19].  However,  RSSI‐based  multi-path efficiency [20]. As a result, more and more researchers move their attention to the more human detection systems suffer from severe multi‐path efficiency [20]. As a result, more and more  fine-grained measurement, CSI. researchers move their attention to the more fine‐grained measurement, CSI.  To overcome the shortcomings of RSSI-based human detection systems, Fine-grained device-free To  overcome  the  shortcomings  of  RSSI‐based  human  detection  systems,  Fine‐grained  Motion Detection (FIMD) utilized the burst pattern of CSIs during human motion to detection human device‐free  Motion  Detection  (FIMD)  utilized  the  burst  pattern  of  CSIs  during  human  motion  to  presence [21]. Fine-grained Indoor Localization (FILA) explored the frequency diversity of the detection  human  presence  [21].  Fine‐grained  Indoor  Localization  (FILA)  explored  the  frequency  subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems, and constructed a signal diversity of the subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems, and  propagation model [22,23]. As human motion can cause the fluctuation of the signal, Bfp harnessed the constructed a signal propagation model [22,23]. As human motion can cause the fluctuation of the  variance of the amplitude of the CSIs to improve the performance of human detection [11]. PADS took signal,  Bfp  harnessed  the  variance  of  the  amplitude  of  the  CSIs  to  improve  the  performance  of  advantages of the whole information of CSI including both amplitude and phase feature to detect human  detection  [11].  PADS  took  advantages  of  the  whole  information  of  CSI  including  both  human motion with various speeds [24]. It calculates the maximum eigenvalue of covariance matrix of amplitude  and  phase  feature  to  detect  human  motion  with  various  speeds  [24].  It  calculates  the  maximum  eigenvalue  of  covariance  matrix  of  normalized  amplitude  and  phase  information,  respectively, as the feature. Support Vector Machine (SVM) is used as the classifier. FRID explored  Appl. Sci. 2019, 9, 175 4 of 17 normalized amplitude and phase information, respectively, as the feature. Support Vector Machine (SVM) is used as the classifier. FRID explored the phase feature of CSIs and achieved calibration-free human detection without the need of a normal profile [25,26]. Short-term averaged variance ratio (SVR) and long-term averaged variance ratio which are two schemes based on the coefficient of variance of phase are introduced to eliminate the re-calibration cost. Conventional human detection systems demonstrated directional monitoring coverage, and Zimu Zhou et al. utilized CSI features to virtually tune the coverage shape into disk-like [27]. Speed Independent Entity Detection (SIED) extracted a novel feature from the whole wireless channel and transformed human detection into a probabilistic problem to achieve a high detection accuracy [5]. AR-Alarm utilized a self-adaptive learning mechanism to achieve intrusion detection without the need of re-calibration [13]. Besides human detection, wireless signals can be used in indoor localization, activity recognition and even human identification. Abdel-Nasser et al. utilized CSI to provide a localization approach with a high accuracy leveraging only a single pair of transceiver [28]. CSI-MIMO utilized frequency diversity of CSI to construct the fingerprint of different locations and achieved a localization accuracy of 0.95 m [29]. SpotFi computed the Angle of Arrival (AoA) of multipath components of different antennas and improved the localization accuracy to 40 cm [30]. HiDFPL proposed a measurement to represent the sensitivity of the receiver and enhanced the localization accuracy [31]. Xuyu Wang et al. proposed PhaseFi, a fingerprinting system, using phase information of CSIs and incorporated a greedy algorithm to train the weights of a deep network [32]. Rui Zhou et al. proposed an indoor localization system based on CSI and SVM [33]. Density-based Spatial Clustering Of Applications With Noise (DBSCAN) was utilized in the system to reduce the noise in CSIs. CSI based human Activity Recognition and Monitoring (CARM) was proposed based on CSIs of wireless channel that quantified the relationship between the movement speeds of different body parts and activities, and it had the ability to recognize human activities [34]. Activity recognition has a wide range of applications, such as somatosensory games. Wi-Play extracted CSI waveforms from commercial WiFi devices to model some specified activity and achieved an activity recognition system [35]. Wifi-based GEsture Recognition (WiGeR) utilized the fluctuation scheme of CSIs generated by the moving of human hands to recognize gestures [36]. Smokey leveraged WiFi signals and had the ability to recognize smoking activity even in the non-line-of-sight (NLOS) and through-wall environments [37]. Wi-Chase utilized the CSIs from all subcarriers to achieve a higher activity recognition accuracy [38]. It is confirmed that human’s gait is unique among different people, thus it can be used to identify the human’s identity. WifiU was presented to construct the gait profiles of different people utilizing the unique variations in the CSIs [39]. WiWho was presented as a framework of human identification utilizing human’s gait extracted from CSIs [40]. FreeSense combined Principal Component Analysis (PCA), Discrete Wavelet Transform (DWT), and Dynamic Time Warping (DTW) to achieve a nine-user human identification [41]. Wii extracted time and frequency-domain features and used timefrequency analysis to achieve an accurate human identification system [7]. Although there have been quantities of work on human detection, they only perform well when the people move in regular patterns. When an intruder appears, he is more likely to move in an irregular way. As a result, a more robust human detection system is proposed in this paper to meet the challenges of intruder detection. 3. Preliminary CSI is leveraged in this study, and we will give a brief introduction of the background knowledge in this section. The wireless signals propagate through multiple paths from the transmitter to the receiver in a typical indoor scenario. As a result, the received signal is the superposition of the signals from LOS path and several reflection paths. OFDM framework is the basis of 802.11 n wireless networks, in which our system works. In this framework, the wireless channel can be descripted by a Channel Appl. Sci. 2018, 8, x FOR PEER REVIEW   5  of  17   j Appl. Sci. 2019, 9, 175 5 of 17 he () (  )n() ,  (1)  ii i1 th where   ,   , and     denote  the  amplitude,  phase  and  time  delay  of  the  signal  from  i   path,  i i i Impulse Response (CIR) in the time domain. Under the assumption of time-invariant, CIR can be respectively;  N   is the total number of paths;  n()   is complex Gaussian white noise; and  ()   is  expressed as: the Dirac delta function.  jq h(t) = a e d(t t ) + n(t), (1) å i i Nevertheless, precise CIR can be extracted only from dedicated devices rather than commodity  i=1 infrastructures. To overcome this limitation, Channel Frequency Response (CFR) can be extracted  th where a , q , and t denote the amplitude, phase and time delay of the signal from i path, respectively; i i i from frequency domain, which can model the wireless channel. CFR contains amplitudefrequency  N is the total number of paths; n(t) is complex Gaussian white noise; and d(t) is the Dirac response  and  phasefrequency  response.  Under  the  assumption  of  infinite  bandwidth,  CIR  is  delta function. equivalent to CFR, and CFR can be transformed by Fast Fourier Transform (FFT) from CIR: [20]  Nevertheless, precise CIR can be extracted only from dedicated devices rather than commodity H  FFT(( h )) .  (2)  infrastructures. To overcome this limitation, Channel Frequency Response (CFR) can be extracted from frequency domain, which can model the wireless channel. CFR contains amplitudefrequency We can obtain CFRs in the format of CSI:  response and phasefrequency response. Under the assumption of infinite bandwidth, CIR is H  [Hf ( ),Hf ( ), ...,Hf ( )] ,  (3)  equivalent to CFR, and CFR can be transformed12 by Fast Fourier N Transform (FFT) from CIR: [20] where N is the number of subcarriers in the wireless network.  H = FFT(h(t)). (2) The CSI is composed of amplitude and phase of a subcarrier:  j sin(H ) We can obtain CFRs in the format of CSI: Hf()  H(f ) e ,  (4)  kk H = [ H( f ), H( f ), . . . , H( f )], (3) where  f   is the central frequency of the subcarrier, and  H   represents its phase. Thus, a group of  1 2 N CSIs,  H (fk ), (  1,...,K ) , denote K sampled CFRs in subcarrier level.  where N is the number of subcarriers in the wireless network. The CSI is composed of amplitude and phase of a subcarrier: 4. System Design  j sin(\H) H( f ) = k H( f )ke , (4) k k 4.1. System Overview  where f is the central frequency of the subcarrier, and \H represents its phase. Thus, a group of CSIs, The  framework  of  RDFID  is  presented  in  Figure  2.  The  system  has  four  modules:  pre H( f‐processin ), (k = 1,g .;.  .fe , K at)ure , denote   extraction; K sampled   classif CFRs icatio inn;subcarrier   and  post‐ level. processing.  There  are  various  kinds  of  noise  in  the  raw  collected  CSI  data,  and  most  noise  is  removed  in  pre‐processing  module.  We  4. System Design extract  wavelet  variance  as  the  real‐time  feature  from  frequency  domain  in  feature  extraction  module. In the classification module, a portion of data is utilized to train a system to be universal  4.1. System Overview that can be adaptive to different scenarios. In the post‐processing module, the classification result is  The framework of RDFID is presented in Figure 2. The system has four modules: pre-processing; further processed to be closer to reality.  feature extraction; classification; and post-processing. There are various kinds of noise in the raw The  system  can  work  in  typical  indoor  scenarios  with  only  one  pair  of  commodity  WiFi  collected CSI data, and most noise is removed in pre-processing module. We extract wavelet variance devices, which include a wireless router and a laptop. The wireless router is the Transmit Xmt (TX)  as the real-time feature from frequency domain in feature extraction module. In the classification that  supports  Institute  of  Electrical  and  Electronic  Engineers  (IEEE)  802.11n  protocol,  while  the  module, a portion of data is utilized to train a system to be universal that can be adaptive to different laptop is the Receive Xmt (RX) that is equipped with Intel 5300 network interface card (NIC). The  scenarios. In the post-processing module, the classification result is further processed to be closer WiFi  devices  keep  transmitting  data  to  collect  CSIs  in  the  monitoring  area,  and  the  system  to reality. estimated intruder existence according to the extracted feature.  Training CSI Pre‐processing Feature Extraction Post‐processing Classification Figure 2. System Framework. Figure 2. System Framework.  The system can work in typical indoor scenarios with only one pair of commodity WiFi devices, which include a wireless router and a laptop. The wireless router is the Transmit Xmt (TX) that Appl. Sci. 2019, 9, 175 6 of 17 supports Institute of Electrical and Electronic Engineers (IEEE) 802.11n protocol, while the laptop is the Receive Xmt (RX) that is equipped with Intel 5300 network interface card (NIC). The WiFi devices keep transmitting data to collect CSIs in the monitoring area, and the system estimated intruder existence according to the extracted feature. 4.2. Pre-Processing The CSI data is extracted from the respond packets of Internet Control Messages Protocol (ICMP) packets. As a result, the number of the group of CSIs is the same as that of ICMP packets theoretically. However, during data collection period, we find that the number of collected CSI records is larger than that of transmitted ICMP packets we had set in advance. In order to calibrate the frequency of the collected data, we conduct the linear interpolation in the raw data and it has a unified frequency. In 802.11 n wireless networks, there are several subcarriers transmitting signals at the same time under the OFDM framework. The subcarriers are independent theoretically. However, the CSIs of adjacent subcarriers have some relationships. In consequence, PCA is used to extract independent data. The related CSI streams can be combined into several independent principle components. For each ICMP packet, a matrix of 3  30 constructed by CSIs can be extracted from the firmware. It can be further reshaped into a 1  90 vector. For a certain time window, n ICMP packets have been received, and we can obtain an n  90 matrix. During the evaluation of the principle components, we find that in most cases the first principle component can give an 80% contribution rate. As a result, we use the first principle component as the representative data. Unfortunately, there still exist some kinds of noises in the first principle component, and they have negative impact on detection rate. The one that has the most significant impact is high frequency noise induced by environment changes other than human movement. The movement of torso, arms, and legs cause most of signal reflections. The frequency of the movements is lower than 10 Hz according to our observation. As a result, a low pass filter is utilized to filter out the high frequency noise from the collected data with the frequency higher than 10 Hz. 4.3. Feature Extraction A proper feature is critical in classification tasks. Generally, the moving speed of a person is constant in a short period, and some periodicity exists when the person is moving. For instance, when the person walks, two steps construct a period. However, it is a challenging task to analyze the periodicity directly from the waveform of the wireless signals. During our early exploration, we find that besides time-domain features, frequency-domain features can better characterize the waveforms in intrusion detection. As a result, in order to explore a scenario independent feature, we utilize timefrequency analysis on the waveform. Continuous Wavelet Transform (CWT) combined with wavelet variance is a proper tool to analyze the periodicity of the waveform. First, the wavelet coefficient of the first principle component of the CSIs after low-pass filter (cpl) is calculated utilizing CWT in Equation (5): 1 t b W (a, b) = x(t)p y( )dt, (5) where x(t) is the first principle component of the CSIs after low-pass filter (cpl), a and b are scale and time, respectively. y() is the wavelet function, and db6 (Daubechies) wavelet [42] is selected as it provides the best performance after we have tried different wavelet functions. As shown in Figure 3, it can be clearly seen that some periodicity exists in the waveform after we conduct continuous wavelet transform. However, it is necessary to quantitatively calculate the significance of the periodicity to confirm that the periodicity is caused by human behaviors. Appl. Sci. 2019, 9, 175 7 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   7  of  17  Appl. Sci. 2018, 8, x FOR PEER 500 REVIEW   7  of  17  300 100 0 204060 80 100 600 Time (s) FigureFigure 3. Wavelet  3. Wave coef let coeffi ficient cient of of Channel  Channel State State Information Information  (CS (CSI) I) when when  people people  move.move.   During  our  experiment,  we  find  that  the  distribution  of  the  wavelet  variance  is  different  Wavelet variance is widely used in meteorology to calculate the periodicity of precipitation. among whether there is human motion as shown in Figure 4. In consequence, the wavelet variance  It reflects the distribution of the power of the wavelet coefficients of various scales. As a result, it can is a proper feature for intrusion detection.  also describe the significance of the periodicity of human motion. The wavelet variance is calculated as Equation (6): 0 Z +¥ 0 204060 80 100 var(a) = W (a, b) db, (6) Time (s ) Figure 3. Wavelet coefficient of Channel State Information (CSI) when people move.  where W (a, b) is the power of the wavelet coefficient of scale a at time b. During  our  experiment,  we  find  that  the  distribution  of  the  wavelet  variance  is  different  During our experiment, we find that the distribution of the wavelet variance is different among among whether there is human motion as shown in Figure 4. In consequence, the wavelet variance  whether there is human motion as shown in Figure 4. In consequence, the wavelet variance is a proper is a proper feature for intrusion detection.  feature for intrusion detection. Figure 4. The distribution of wavelet variance when there is human motion and static.  4.4. Training and Classification  As the distribution of the wavelet variance when there is human motion is different from that  of static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM,  there  are  two  Gaussian  models,  one  is  static  model  and  the  other  is  human  motion  model.  The  moving data of different volunteers in different moving patterns and the data collected in the static  scenario construct the training data. The GMM only need to be trained once, and it can be used in  different  scenarios  without  being  re‐trained.  As  a  result,  after  a  trained  GMM  is  generated,  the  Figure 4.Figure The distribution  4. The distributi ofon wavelet  of wavelet variance  variance when  whenther  there e is is human human moti motion on andand  static. static.   intrusion detection system is unsupervised. In the training phase, the vectors of wavelet variance of  different scales and the ground truth are utilized to train the GMM. In the classification phase, the  4.4. Training 4.4. and Training Classification  and Classification  inputs are only the vectors of wavelet variance, while the outputs are the detection results whether  As the distribution of the wavelet variance when there is human motion is different from that  there exists human motion.  As the distribution of the wavelet variance when there is human motion is different from that of of static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM,  In  the  end  of  classification,  a  post‐processing  procedure  is  added  to  improve  the  detection  static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM, there are there  are  two  Gaussian  models,  one  is  static  model  and  the  other  is  human  motion  model.  The  accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As  two Gaussian models, one is static model and the other is human motion model. The moving data of moving data of different volunteers in different moving patterns and the data collected in the static  different volunteers in different moving patterns and the data collected in the static scenario construct scenario construct the training data. The GMM only need to be trained once, and it can be used in  the training different data.  sce The nario GMM s  withonly out  being need  re to‐tra be ined. trained   As  a once, result, and after ita can trained be used GMMin  is dif gener fera ent ted,scenarios   the  intrusion detection system is unsupervised. In the training phase, the vectors of wavelet variance of  without being re-trained. As a result, after a trained GMM is generated, the intrusion detection system different scales and the ground truth are utilized to train the GMM. In the classification phase, the  is unsupervised. In the training phase, the vectors of wavelet variance of different scales and the inputs are only the vectors of wavelet variance, while the outputs are the detection results whether  ground truth are utilized to train the GMM. In the classification phase, the inputs are only the vectors there exists human motion.  of wavelet variance, while the outputs are the detection results whether there exists human motion. In  the  end  of  classification,  a  post‐processing  procedure  is  added  to  improve  the  detection  accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As  Appl. Sci. 2019, 9, 175 8 of 17 In the end of classification, a post-processing procedure is added to improve the detection accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As a result, an additional window beyond the detection window is utilized to reduce the detection mistakes. Appl. Sci. 2018, 8, x FOR PEER REVIEW   8  of  17  For example, 0 and 1 represent static and intrusion, respectively. If the detection result is 11011 in this a  result,  an  additional  window  beyond  the  detection  window  is  utilized  to  reduce  the  detection  additional window, we can consider there always exists intrusion in this window. The cost of this mistakes. For example, 0 and 1 represent static and intrusion, respectively. If the detection result is  procedure is the time delay in detection, but the detection accuracy can be higher. 11011 in this additional window, we can consider there always exists intrusion in this window. The  cost of this procedure is the time delay in detection, but the detection accuracy can be higher.  5. Evaluation 5. Evaluation  5.1. Experiment Setup T 5.o1.evaluate  Experimen the t Se detection tup  performance of the system, some real experiments are conducted in three typical rooms from several aspects. The three rooms are a meeting room, a typical living room, and a To evaluate the detection performance of the system, some real experiments are conducted in  large office, and the sizes of the three room are 5 m  4 m, 5 m  4 m and 10 m  6 m, respectively. three typical rooms from several aspects. The three rooms are a meeting room, a typical living room,  The layout of the three rooms and transceiver deployment are shown in Figure 5. There are desks with and  a  large  office,  and  the  sizes  of  the  three  room  are  5  m  ×  4  m,  5  m  ×  4  m  and  10  m  ×  6  m,  glassrespectively. dam-boards  The and  lay chairs out  of in  the the  three office,  rooms while  and a meeting   transcei table ver  de and ployment chairs in are the   shown meeting   in  Fig room, ure  5. which   There are desks with glass dam‐boards and chairs in the office, while a meeting table and chairs in  causes different multipath effects. Especially, in order to present a reasonable evaluation in a smart the  meeting  room,  which  causes  different  multipath  effects.  Especially,  in  order  to  present  a  home scenario, a typical living room was used as a scenario. In the living room a television, there is a reasonable evaluation in a smart home scenario, a typical living room was used as a scenario. In the  television on the wall, a sofa, a piano, a refrigerator, some other furniture, and some doors to other living room a television, there is a television on the wall, a sofa, a piano, a refrigerator, some other  rooms, which will cause much more complex multipath effects. A TP-Link 802.11n wireless router furniture, and some doors to other rooms, which will cause much more complex multipath effects.  with a single antenna is used as the transmitter and a Lenovo laptop equipped with a three-antenna A TP‐Link 802.11n wireless router with a single antenna is used as the transmitter and a Lenovo  Intel WiFi Link 5300 (iwl 5300) NIC running Ubuntu 11.04 OS as the receiver. The firmware of the laptop equipped with a three‐antenna Intel WiFi Link 5300 (iwl 5300) NIC running Ubuntu 11.04  NIC is modified in order to extract CSIs from data packets utilizing the CSI tools. In addition, we OS as the receiver. The firmware of the NIC is modified in order to extract CSIs from data packets  upgrade the antennas of the NIC using three 6dbi gain antennas as shown in Figure 6 in order to utilizing  the  CSI  tools.  In  addition,  we  upgrade  the  antennas  of  the  NIC  using  three  6dbi  gain  increase antennas the signal-noise-ratio.  as shown in Figure 6 in order to increase the signal‐noise‐ratio.  5m 5m TX TX RX RX (a) meeting room (b) living room 10m TX RX (c) large office Figure 5. Experimental scenario. Figure 5. Experimental scenario.  According to CSI tools, the sensing data is the CSIs of the respond packets when the transmitter is According  to  CSI  tools,  the  sensing  data  is  the  CSIs  of  the  respond  packets  when  the  continuously transmitter sending  is continuously ICMP packets  sending to IC the MPr eceiver packets. to W ethe recr  receiver uited.four  We re volunteers cruited fouin r volunteers our experiments  in  our experiments with the basic information shown in Table 1. During data collection period, only a  with the basic information shown in Table 1. During data collection period, only a single person single person moves back and forth in different moving patterns respectively in the room without a  6m 4m 4m Appl. Sci. 2019, 9, 175 9 of 17 moves back and forth in different moving patterns respectively in the room without a fixed path. Appl. Sci. 2018, 8, x FOR PEER REVIEW   9  of  17  The transmission rate in our experiments is configured to 200 Hz. A few cycles of data collection process are conducted for one person, while each cycle contains only one moving pattern and lasts for fixed path. The transmission rate in our experiments is configured to 200 Hz. A few cycles of data  100 s. Data collection lasts for one week, and about 20 min moving data is collected for one person collection process are conducted for one person, while each cycle contains only one moving pattern  moving in one pattern. and lasts for 100 s. Data collection lasts for one week, and about 20 min moving data is collected for  False negative (FN), false positive (FP), and the probability of detection (PD) are used as the one person moving in one pattern.  evaluation metrics in this paper. False negative is the ratio that RDFID fails to detect intrusion, while False negative (FN), false positive (FP), and the probability of detection (PD) are used as the  false positive is the ratio it reports intrusion when nobody is in the room. The probability of detection evaluation  metrics  in  this  paper.  False  negative  is  the  ratio  that  RDFID  fails  to  detect  intrusion,  Appl. Sci. 2018, 8, x FOR PEER REVIEW   9  of  17  is the ratio that it successfully detects the existence of the intruder. The three metrics can be illustrated while false positive is the ratio it reports intrusion when nobody is in the room. The probability of  by Figure 7, wher fixed e P1–P4 path. Thear  tran e the smission elements  rate in our of exper the iconfusion ments is configure matrix d to 20 in0 the Hz. A form  few cycles of per  of centage. data  As shown detection is the ratio that it successfully detects the existence of the intruder. The three metrics can  collection process are conducted for one person, while each cycle contains only one moving pattern  in be Figur   illu est 7rat , P4 edr epr by esents Figure FN 7,  where and P1  P1– repr P4 esents   are  the FP .ele PD mis ents described   of  the  confusion in Equation   ma(7). trix  in  the  form  of  and lasts for 100 s. Data collection lasts for one week, and about 20 min moving data is collected for  percentage.  As  shown  in  Figure  7,  P4  represents  FN  and  P1  represents  FP.  PD  is  described  in  one person moving in one pattern.  False negative (FN), false positive PD (F= P), P3 and / the (P3 pro+babil P4it)y , of detection (PD) are used as the  (7) Equation (7).  evaluation  metrics  in  this  paper.  False  negative  is  the  ratio  that  RDFID  fails  to  detect  intrusion,  while false positive is the ratio it reports intrusion when nobody is in the room. The probability of  detection is the ratio that it successfully detects the existence of the intruder. The three metrics can  be  illustrated  by  Figure  7,  where  P1–P4  are  the  elements  of  the  confusion  matrix  in  the  form  of  percentage.  As  shown  in  Figure  7,  P4  represents  FN  and  P1  represents  FP.  PD  is  described  in  Equation (7).  Figure 6. The modified receiver.  Figure 6. The modified receiver.  Figure 6. The modified receiver. Classified as Classified as intrusion clear intrusion clear P1 P2 P1 P2 P3 P4 Figure 7. Confusion matrix of intrusion detection.  Figure 7. ConfusionP3 matrix of intrusion P4 detection. PD=+ P3 / (P 3 P 4),  (7)  Table 1. Basic information of volunteers. Figure 7. Confusion matrix of intrusion detection.  Volunteers Gender Table 1. BasicHeight  information (cm)  of volunteers.W   eight (kg) Age Volunteers  Gender  Height (cm)  Weight (kg)  Age  1 male 174 63 30 1  male  174  63  30  2 male 175 70 27 PD=+ P3 / (P 3 P 4),  (7)  2  male  175  70  27  3 male 170 62 27 3  male  170  62  27  4  female  163  51  26  4 female 163 51 26 Table 1. Basic information of volunteers.  5.2. Performance Evaluation  5.2. Performance Evaluation Volunteers  Gender  Height (cm)  Weight (kg)  Age  1  male  174  63  30  5.2.1. Intrusion Detection in Different Scenarios 2  male  175  70  27  3  male  170  62  27  In order to confirm that the performance of RDFID is independent of scenarios, we first evaluate 4  female  163  51  26  the system in different rooms. In addition, we compare the system with two other device-free human 5.2. Performance Evaluation  Actual state intrusion clear Actual state intrusion clear Appl. Sci. 2018, 8, x FOR PEER REVIEW   10  of  17  5.2.1. Intrusion Detection in Different Scenarios  Appl. Sci. 2019, 9, 175 10 of 17 In  order  to  confirm  that  the  performance  of  RDFID  is  independent  of  scenarios,  we  first  evaluate  the  system  in  different  rooms.  In  addition,  we  compare  the  system  with  two  other  detection device systems, ‐free hu FRID man detection and PADS.  systems, When FR constr ID and ucting  PADS. the When training  constructi set, we nguse  the the traini combination ng set, we us ofe  the the combination of the data from the three scenarios to form six groups of training set and we name  data from the three scenarios to form six groups of training set and we name them a, b, c, ab, ac, and them a, b, c, ab, ac, and bc, respectively, according to Figure 7, and all training sets contain the three  bc, respectively, according to Figure 7, and all training sets contain the three moving patterns. Datasets moving patterns. Datasets that are opposite to the training sets are used as test sets, which are bc, ac,  that are opposite to the training sets are used as test sets, which are bc, ac, ab, c, b, and a, respectively. ab, c, b, and a, respectively. To ensure the reliability of the evaluation, each training set is equally  To ensure the reliability of the evaluation, each training set is equally divided into five parts, and five divided into five parts, and five experiments are conducted in which the classifier is trained using  experiments are conducted in which the classifier is trained using each part respectively. The result is each  part respectively.  The result is  the  mean of  the  five experiments.  The window  size in  these  the mean of the five experiments. The window size in these experiments is 5 s. The FN and FP rate of experiments is 5 s. The FN and FP rate of the three approaches is shown in Table 2. As indicated in  the three approaches is shown in Table 2. As indicated in the table, the FN rate of RDFID in different the table, the FN rate of RDFID in different scenarios is around 2%, which is the lowest among the  scenarios three is approa around ches. 2%, The which  FN rais te of the PA lowest DS is af among fected more the thr signif eeica appr ntly oaches.  by the se The lection FN of rate  the of training PADS  is set because it uses SVM as its classifier, the support vectors in different scenarios are not the same.  affected more significantly by the selection of the training set because it uses SVM as its classifier, the As a result, the FN rate of PADS is higher. As FRID does not need training data, the estimation of  support vectors in different scenarios are not the same. As a result, the FN rate of PADS is higher. As the parameters has particular influence on the performance of human detection.  FRID does not need training data, the estimation of the parameters has particular influence on the The FP rate of RDFID is lower than the other two approaches. Most of the FP rate is around 2%,  performance of human detection. which  indicates  RDFID  generates  less  false  alarms  when  detecting  intruders.  PADS  uses  phase  information  in  CSIs  that  is  more  sensitive  to  environmental  changes;  therefore,  it  achieves  the  Table 2. False negative/false positive (FN/FP) of human detection in different scenarios (%). highest FP rate among the three approaches.  FN FP Table 2. False negative/false positive (FN/FP) of human detection in different scenarios (%).  Training Set a b c ab ac bc a b c ab ac bc RDFID 2.5  2.3 2.4 2.4FN  2.6 2.5 1.7 2.2 FP 2.1  2.2 2.1 1.9 Training Set  a  b  c  ab  ac  bc  a  b  c  ab  ac  bc  FRID 4.8 5.8 4.4 4.3 5.7 5.2 8.5 8.2 8.7 8.7 8.2 8.6 RDFID  2.5  2.3  2.4  2.4  2.6  2.5  1.7  2.2  2.1  2.2  2.1  1.9  PADS 6.0 6.7 6.2 6.1 6.8 5.8 10.8 10.2 10.5 11.8 11.0 10.6 FRID  4.8  5.8  4.4  4.3  5.7  5.2  8.5  8.2  8.7  8.7  8.2  8.6  PADS  6.0  6.7  6.2  6.1  6.8  5.8  10.8  10.2  10.5  11.8  11.0  10.6  The FP rate of RDFID is lower than the other two approaches. Most of the FP rate is around Figure  8  indicates  the  PD  of  the  approaches  in  different  scenarios.  It  can  be  seen  from  the  2%, which indicates RDFID generates less false alarms when detecting intruders. PADS uses phase figure that RDFID achieves the most stable and lowest probability of detection.  information in CSIs that is more sensitive to environmental changes; therefore, it achieves the highest It  can  be  seen  from  the  results  that  the  detection  performance  of  RDFID  is  independent  of  FP rate among the three approaches. scenarios. The detection model trained in one scenario can be adapted to other scenarios directly in  Figure 8 indicates the PD of the approaches in different scenarios. It can be seen from the figure a relative high detection performance.  that RDFID achieves the most stable and lowest probability of detection. Figure 8. The probability of detection (PD) of human detection in different scenarios.  Figure 8. The probability of detection (PD) of human detection in different scenarios. 5.2.2. Intrusion Detection among Different People  It can be seen from the results that the detection performance of RDFID is independent of scenarios. The detection model trained in one scenario can be adapted to other scenarios directly in a relative high detection performance. 5.2.2. Intrusion Detection among Different People In order to evaluate the independence of the intrusion detection performance among different people, we use the moving data of only one volunteer as the training data, while the moving data of Appl. Sci. 2018, 8, x FOR PEER REVIEW   11  of  17  In order to evaluate the independence of the intrusion detection performance among different  people, we use the moving data of only one volunteer as the training data, while the moving data of  all the four volunteers as the test data. The training data and test data of the first volunteer has no  Appl. Sci. 2019, 9, 175 11 of 17 intersection.  In  addition,  the  performance  of  RDFID  is  compared  to  that  of  PADS.  When  constructing the training set, the moving data of the first volunteer is used as the training set. It  all the four volunteers as the test data. The training data and test data of the first volunteer has no contains the moving data in all three scenarios and three different moving patterns. The evaluation  intersection. In addition, the performance of RDFID is compared to that of PADS. When constructing is conducted five times, and each time the training data is selected randomly from the moving data  the training set, the moving data of the first volunteer is used as the training set. It contains the moving of the first volunteer. The result is the mean of the five times. The window size is 5 s; the FN and FP  data in all three scenarios and three different moving patterns. The evaluation is conducted five times, rate of the two approaches are presented in Table 3. It is indicated in the table that the FN rate of  and each time the training data is selected randomly from the moving data of the first volunteer. The RDFID is relatively stable when detecting different people. However, the FN rate of PADS is more  result is the mean of the five times. The window size is 5 s; the FN and FP rate of the two approaches sensitive to different people. Its FN rate is even lower than that of RDFID when the test data and  are presented in Table 3. It is indicated in the table that the FN rate of RDFID is relatively stable when training  data  is  from  the  same  person.  In  contrast,  the  FN  rate  of  PADS  suffers  significant  detecting different people. However, the FN rate of PADS is more sensitive to different people. Its FN fluctuation when the test data and the training data is from different people. The result shows that  rate is even lower than that of RDFID when the test data and training data is from the same person. In the  FN  rate  of  PADS  is  sensitive  to  training  data  and  test  data,  the  moving  data  from  different  contrast, the FN rate of PADS suffers significant fluctuation when the test data and the training data is people  can  affect  the  detection  performance.  As  a  result,  RDFID  has  a  better  adaptability  to  from different people. The result shows that the FN rate of PADS is sensitive to training data and test different people.  data, the moving data from different people can affect the detection performance. As a result, RDFID has a better adaptability to different people. Table 3. FN/FP of human detection of different people (%).  FN  FP  Table 3. FN/FP of human detection of different people (%). Volunteer  1  2  3  4  1  2  3  4  FN FP RDFID  2.1  2.9  3.3  2.8  1.7  1.8  1.8  2  Volunteer 1 PADS 2  1.9  37.3  9.1  4 7.4  2.4 1 8.8  8.5 2 9.8  3 4 RDFID 2.1 2.9 3.3 2.8 1.7 1.8 1.8 2 The trend of the FP rate of the two approaches is similar to that of the FN rate. The FP rate of  PADS 1.9 7.3 9.1 7.4 2.4 8.8 8.5 9.8 RDFID  is  still  stable  in  the  four  tests  and  maintains  about  2%.  However,  the  FP  rate  of  PADS  achieves a low level only when the test data and training data is from the same person, and raises  The trend of the FP rate of the two approaches is similar to that of the FN rate. The FP rate of significantly using the test data of the other three people.  RDFID is still stable in the four tests and maintains about 2%. However, the FP rate of PADS achieves Figure 9 shows the PD of the two approaches when detection different people. Besides PADS  a low level only when the test data and training data is from the same person, and raises significantly achieves a lower PD when the data of the same volunteer is used in both training set and test set,  using the test data of the other three people. RDFID has a higher PD when using the moving data of the other volunteers as test set.  Figure 9 shows the PD of the two approaches when detection different people. Besides PADS In consequence, RDFID is less sensitive to the training and test data, and can achieve a better  achieves a lower PD when the data of the same volunteer is used in both training set and test set, human detection performance.  RDFID has a higher PD when using the moving data of the other volunteers as test set. Figure 9. PD of human detection of different people. Figure 9. PD of human detection of different people.  In consequence, RDFID is less sensitive to the training and test data, and can achieve a better 5.2.3. Intrusion Detection with Different Window Sizes  human detection performance. Appl. Sci. 2019, 9, 175 12 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   12  of  17  5.2.3. Intrusion Detection with Different Window Sizes As RDFID is  a  window‐based  human detection  approach,  the  detection  performance is also  As RDFID is a window-based human detection approach, the detection performance is also evaluated under different window sizes. To examine the advancement of RDFID, it is compared to  evaluated under different window sizes. To examine the advancement of RDFID, it is compared to two other human detection approaches, FRID and PADS. In the construction phase of the training  two other human detection approaches, FRID and PADS. In the construction phase of the training set, a 30 s data segment is randomly divided from the regular walking data of the first volunteer in  set, a 30 s data segment is randomly divided from the regular walking data of the first volunteer in scenario (a). The test data contains the regular walking data of the other three volunteers, while the  scenario (a). The test data contains the regular walking data of the other three volunteers, while the window size ranges from 1 s to 5 s.  window size ranges from 1 s to 5 s. The  results  are  the  mean  values  of  the  three  people.  The  FN  and  FP  rate  of  the  three  The results are the mean values of the three people. The FN and FP rate of the three approaches approaches under different window sizes are shown in Table 4. It is indicated from the table that  under different window sizes are shown in Table 4. It is indicated from the table that the FN rate of the FN rate of RDFID is as high as 11.7% when the window size is 1 s, but it decreases to 5.2% when  RDFID is as high as 11.7% when the window size is 1 s, but it decreases to 5.2% when the window the window size changes to 2 s. Moreover, the FN rate of RDFID keeps decreasing as the window  size changes to 2 s. Moreover, the FN rate of RDFID keeps decreasing as the window size increases. size increases. It is because the 1‐s window is too narrow for human motion, and people can only  It is because the 1-s window is too narrow for human motion, and people can only walk less than two walk  less  than  two  steps  within  the  window.  As  a  result,  the  periodicity  in  the  extracted  steps within the window. As a result, the periodicity in the extracted frequency-domain features is not frequency‐domain features is not significant enough, which leads to a higher FN rate. Although the  significant enough, which leads to a higher FN rate. Although the FN rate of FRID is lower than that FN rate of FRID is lower than that of RDFID when the window size is 1 s, it decreases slower when  of RDFID when the window size is 1 s, it decreases slower when the window size increases. On the the  window  size  increases.  On  the  other  hand,  as  the  training  and  test  data  is  from  the  same  other hand, as the training and test data is from the same scenario in this experiment, the variation of scenario in this experiment, the variation of the support vector of the features is insignificant; the  the support vector of the features is insignificant; the FN rate of PADS can achieve a low level. FN rate of PADS can achieve a low level.  Table 4. FN/FP of human detection under different window sizes (%). Table 4. FN/FP of human detection under different window sizes (%).  FN FP   FN  FP  Window Size Window Size (s)  1  2  3  4  5  1  2  3  4  5  1 2 3 4 5 1 2 3 4 5 (s) RDFID  11.7  5.2  4.6  3  2.1  2.1  2.3  1.7  1.8  1.7  RDFID 11.7 5.2 4.6 3 2.1 2.1 2.3 1.7 1.8 1.7 FRID  9.8  7.4  5.6  4.9  4.4  7  7.6  6.8  7.2  7.3  FRID 9.8 7.4 5.6 4.9 4.4 7 7.6 6.8 7.2 7.3 PADS  6.3  4.6  4.6  3.8  3.2  3.5  3.8  3.5  4  3.3  PADS 6.3 4.6 4.6 3.8 3.2 3.5 3.8 3.5 4 3.3 The FP rate of the three approaches all undergoes a low fluctuation, which indicates that the  The FP rate of the three approaches all undergoes a low fluctuation, which indicates that the FP FP rate of the three approaches can be less affected by the window size. However, as the extracted  rate of the three approaches can be less affected by the window size. However, as the extracted feature feature in RDFID has a better discernibility between static and dynamic, this approach achieves the  in RDFID has a better discernibility between static and dynamic, this approach achieves the lowest lowest FP rate.  FP rate. Figure 10 shows the PD of the three approaches when the window size is different. It can be  Figure 10 shows the PD of the three approaches when the window size is different. It can be seen seen that PADS achieves a higher PD when the window size is no larger than 3 s, but the PD of  that PADS achieves a higher PD when the window size is no larger than 3 s, but the PD of RDFID RDFID increases fast as the window size gets larger, and gets the highest of the three approaches  increases fast as the window size gets larger, and gets the highest of the three approaches when the when the window size is larger than 3 s.   window size is larger than 3 s. RDFID FRID PADS 12 345 Window Size (s) Figure 10. PD of human detection under different window sizes. Figure 10. PD of human detection under different window sizes.  5.2.4. Intrusion Detection with Different Moving Patterns  PD (%) Appl. Sci. 2018, 8, x FOR PEER REVIEW   13  of  17  Appl. Sci. 2019, 9, 175 13 of 17 The most important problem that RDFID solves is human detection under different moving  patterns.  In  consequence,  to  evaluate  the  ability  of  RDFID  in  this  problem,  the  data  of  different  5.2.4. Intrusion Detection with Different Moving Patterns moving patterns is used in this evaluation. To address the importance of this problem, RDFID is  compared  to FRID,  PADS,  and  FIMD  [21].  A  30 s moving  data  segment  of  the  first  volunteer  in  The most important problem that RDFID solves is human detection under different moving scenario (b) under regular moving pattern is randomly divided as training data, while the data of  patterns. In consequence, to evaluate the ability of RDFID in this problem, the data of different moving the other three volunteers in scenario (b) under three different moving patterns is used as the test  patterns is used in this evaluation. To address the importance of this problem, RDFID is compared data.  The  results  of  the  three  approaches  are  the  mean  values  of  the  three  volunteers,  and  the  to FRID, PADS, and FIMD [21]. A 30 s moving data segment of the first volunteer in scenario (b) window size is 5 s.   under regular moving pattern is randomly divided as training data, while the data of the other three The FN and FP rate of the four approaches under different moving patterns is shown in Table  volunteers in scenario (b) under three different moving patterns is used as the test data. The results of 5. It can be seen from the table that the FN rate of RDFID remains stable under different moving  the three approaches are the mean values of the three volunteers, and the window size is 5 s. patterns.  However,  the  FN  rate  of  the  other  three  approaches  raises  significantly  when  the  The FN and FP rate of the four approaches under different moving patterns is shown in Table 5. volunteers creep on the floor. FRID, PADS, and FIMD are affected more significantly because the  It can be seen from the table that the FN rate of RDFID remains stable under different moving patterns. influence  of  the  human  body  to  the  transmission  of  the  wireless  signal  becomes  weak  when  the  However, the FN rate of the other three approaches raises significantly when the volunteers creep on the volunteers creep on the floor. The FN rate of RDFID has a small fluctuation because the extracted  floor. FRID, PADS, and FIMD are affected more significantly because the influence of the human body to feature is related to the periodicity of human motion. It can detect human at a high accuracy as long  the transmission of the wireless signal becomes weak when the volunteers creep on the floor. The FN as there exists a periodicity of human motion.  rate of RDFID has a small fluctuation because the extracted feature is related to the periodicity of human motion. It can detect human at a high accuracy as long as there exists a periodicity of human motion. Table 5. FN/FP of human detection under different moving patterns (%).  Table 5. FN/FP of human detection under different moving patterns (%).   FN  FP  Moving  Regular  Bending  Regular  Bending  FN FP Creeping  Creeping  Pattern  Walking  Down  Walking  Down  Moving Regular Bending Regular Bending Creeping Creeping RDFID  2.3  2.5  2.6  1.7  1.7  1.5  Pattern Walking Down Walking Down FRID  4.8  5.2  9.8  7.8  6.2  3.6  RDFID 2.3 2.5 2.6 1.7 1.7 1.5 PADS  4.3  4.8  6.4  4.3  4.1  2.8  FRID 4.8 5.2 9.8 7.8 6.2 3.6 FIMD  5.4  5.7  12.5  6.8  6.4  4.8  PADS 4.3 4.8 6.4 4.3 4.1 2.8 FIMD 5.4 5.7 12.5 6.8 6.4 4.8 The FP rate of RDFID is still stable under the three moving patterns, while the change trend of  the FP rate of the other three approaches is the opposite to that of the FN rate. The reason is the  The FP rate of RDFID is still stable under the three moving patterns, while the change trend of the same that the influence of human body to the transmission of the wireless signal becomes less when  FP rate of the other three approaches is the opposite to that of the FN rate. The reason is the same that the person creeps on the floor. The low FP rate of the other three approaches is on the cost of the  the influence of human body to the transmission of the wireless signal becomes less when the person high FN rate. In consequence, RDFID has the ability to detect human of different moving patterns.  creeps on the floor. The low FP rate of the other three approaches is on the cost of the high FN rate. In It  has  the  advancement  of  human  detection  especially  when  the  person  moves  in  an  irregular  consequence, RDFID has the ability to detect human of different moving patterns. It has the advancement pattern.  The  robustness  of  RDFID  is  higher  that  the  detection  performance  is  less  affected  by  of human detection especially when the person moves in an irregular pattern. The robustness of RDFID different moving patterns.  is higher that the detection performance is less affected by different moving patterns. As illustrated in Figure 11, the PD of RDFID is the highest and stable under the three different  As illustrated in Figure 11, the PD of RDFID is the highest and stable under the three different moving patterns benefiting from the frequency–domain feature.   moving patterns benefiting from the frequency–domain feature. Figure 11. PD of human detection under different moving patterns. Figure 11. PD of human detection under different moving patterns.  Appl. Sci. 2019, 9, 175 14 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   14  of  17  5.2.5. Intrusion Detection under Different Moving Speeds 5.2.5. Intrusion Detection under Different Moving Speeds  As a special case, human detection under different moving speeds plays an important role in As a special case, human detection under different moving speeds plays an important role in  intrusion detection systems. The four volunteers are asked to walk in a regular pattern at 1.5 m/s, intrusion detection systems. The four volunteers are asked to walk in a regular pattern at 1.5 m/s,  0.7 m/s, and 0.2 m/s, respectively, in the meeting room. A 30 s data segment is randomly divided from 0.7 m/s, and 0.2 m/s, respectively, in the meeting room. A 30 s data segment is randomly divided  the data of the first volunteer walking at the speed of 0.7 m/s as the training data, while the walking from the data of the first volunteer walking at the speed of 0.7 m/s as the training data, while the  data of the other three volunteers under different speeds is used as the test data. The window size is 5 s, walking  data  of  the  other  three  volunteers  under  different  speeds  is  used  as  the  test  data.  The  and the results are the mean value of the three volunteers. The human detection performance of RDFID window size is 5 s, and the results are the mean value of the three volunteers. The human detection  is compared to PADS and FRID. The FN and FP rate of the three approaches under different moving performance of RDFID is compared to PADS and FRID. The FN and FP rate of the three approaches  speeds is shown in Table 6. As indicated in the table, the trends of the FN rate of the approaches are under different moving speeds is shown in Table 6. As indicated in the table, the trends of the FN  the same that they all increase as the moving speed becomes slower. The influence of human motion to rate of the approaches are the same that they all increase as the moving speed becomes slower. The  the transmission of the wireless signal decreases when the moving speed becomes slower. Especially influence of human motion to the transmission of the wireless signal decreases when the moving  when the person moves far away from the first Fresnel zone, the reflected signal is submerged in the speed becomes slower. Especially when the person moves far away from the first Fresnel zone, the  signal from the LOS path. As a result, it is of great difficulties to extract effect environmental change reflected signal is submerged in the signal from the LOS path. As a result, it is of great difficulties to  information from the received signal. On the other hand, it can be seen that the FN rate of RDFID is extract effect environmental change information from the received signal. On the other hand, it can  lower than the other approaches. be seen that the FN rate of RDFID is lower than the other approaches.  Table 6. FN/FP of human detection under different moving speeds (%). Table 6. FN/FP of human detection under different moving speeds (%).  FN FP   FN  FP  Moving Speed (m/s) 1.5 0.7 0.2 1.5 0.7 0.2 Moving Speed (m/s)  1.5  0.7  0.2  1.5  0.7  0.2  RDFID  1.2  1.8  3.4  1.2  1.2  0.9  RDFID 1.2 1.8 3.4 1.2 1.2 0.9 FRID  2.1  3  4.2  2.1  2.3  1.8  FRID 2.1 3 4.2 2.1 2.3 1.8 PADS  2.2  3.3  5.7  3.3  2.3  1.2  PADS 2.2 3.3 5.7 3.3 2.3 1.2 It  can  be  seen  that  the  FP  rate  of  the  three  approaches  under  different  moving  speeds  is  It can be seen that the FP rate of the three approaches under different moving speeds is relatively relatively stable and keeps at a low level. It indicates that the probability of false alarm of the three  stable and keeps at a low level. It indicates that the probability of false alarm of the three approaches is approaches is low when detecting human motion.   low when detecting human motion. As can be seen from Figure 12, the PD of the three approaches all suffer a decrease when the  As can be seen from Figure 12, the PD of the three approaches all suffer a decrease when the moving speed becomes slower. The performance of human detection can be affected by different  moving speed becomes slower. The performance of human detection can be affected by different moving  speeds,  but  the  overall  detection  performance  can  meet  the  requirement  of  security  in a  moving speeds, but the overall detection performance can meet the requirement of security in a regular regular smart home environment.  smart home environment. RDFID FRID PADS 1.5 0.7 0.2 Moving speed (m/s) Figure 12. PD of human detection under different moving speeds. Figure 12. PD of human detection under different moving speeds.  6. Discussion  We did a set of evaluations in this work and demonstrated the effectiveness of RDFID to detect  human  motion  of  different  moving  patterns  using  WiFi  signals.  However,  there  are  still  some  PD (%) Appl. Sci. 2019, 9, 175 15 of 17 6. Discussion We did a set of evaluations in this work and demonstrated the effectiveness of RDFID to detect human motion of different moving patterns using WiFi signals. However, there are still some limitations in RDFID. In this section, we will give a discussion about the limitations and potentials of RDFID. Although the approach can achieve a high intrusion detection accuracy, it may be influenced by several factors. First, the relative location of the intruder and transceivers can affect the detection accuracy. There exists a relationship between the impact of the intruder to the signal transmission and the distance of the intruder to the transceivers. When the intruder moves far away from the transceivers or the first Fresnel zone, it becomes more difficult to extract effective features from the collected CSI of the ambient wireless signal. As a result, the detection accuracy suffers a degradation when the distance of the intruder to the transceivers. In addition, in real scenarios there may exist more than one intruder. Nevertheless, the movement of multiple intruders will break the periodicity of the received CSI. In consequence, the detection performance will be affected directly. Despite these limitations, WiFi signal-based intrusion detection systems have much potential in a smart home. In our future work, we will explore more effective features that less affected as much by the distance of the intruder to the transceivers and the number of the intruders in the environment to make the approach more robust in smart home applications. 7. Conclusions In this paper, we propose RDFID, a robust device-free passive intrusion detection approach. The moving pattern of the intruder has less influence to the detection performance of RDFID. Furthermore, the detection accuracy can achieve a high level without re-calibration when the scenario has changed. It only need commodity off-the-shelf (COTS) WiFi devices, and extract fine-grained channel state information from the physical layer of the wireless network. The time-frequency analysis technique is utilized to extract the features that are affected less by the environment from the frequency domain. As a result, the performance of RDFID is less affected by the moving pattern of the intruder and the different indoor scenarios. In order to evaluate the effectiveness of RDFID, a set of experiments were conducted from several perspectives. The results demonstrate that RDFID can achieve a high performance of intrusion detection, and can meet the security requirement in a smart home. 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In Proceedings of the IEEE INFOCOM 2016—The 35th Annual IEEE International Conference on Computer Communications, San Francisco, CA, USA, 10–14 April 2016; pp. 1–9. 38. Arshad, S.; Feng, C.; Liu, Y.; Hu, Y.; Yu, R.; Zhou, S.; Li, H. Wi-chase: A wifi based human activity recognition system for sensorless environments. In Proceedings of the 2017 IEEE 18th International Symposium on A World of Wireless, Mobile and Multimedia Networks (WoWMoM), Macau, China, 12–15 June 2017; pp. 1–6. 39. Wang, W.; Liu, A.X.; Shahzad, M. Gait recognition using wifi signals. In Proceedings of the 2016 ACM International Joint Conference on Pervasive and Ubiquitous Computing, Heidelberg, Germany, 12–16 September 2016; pp. 363–373. 40. Zeng, Y.; Pathak, P.H.; Mohapatra, P. Wiwho: Wifi-based person identification in smart spaces. In Proceedings of the 15th International Conference on Information Processing in Sensor Networks, Vienna, Austria, 11–14 April 2016; pp. 1–12. 41. Xin, T.; Guo, B.; Wang, Z.; Li, M.; Yu, Z.; Zhou, X. Freesense: Indoor human identification with wi-fi signals. In Proceedings of the 2016 IEEE Global Communications Conference (GLOBECOM), Washington, DC, USA, 4–8 December 2016; pp. 1–7. 42. Daubechies, I. Ten Lectures on Wavelets; SIAM: Philadelphia, PA, USA, 1992. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Robust Device-Free Intrusion Detection Using Physical Layer Information of WiFi Signals

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applied sciences Article Robust Device-Free Intrusion Detection Using Physical Layer Information of WiFi Signals 1 1 , 1 2 3 , 4 Jiguang Lv , Dapeng Man *, Wu Yang , Liangyi Gong , Xiaojiang Du * and Miao Yu Information Security Research Center, Harbin Engineering University, Harbin 150001, China; lvjiguang@hrbeu.edu.cn (J.L.); yangwu@hrbeu.edu.cn (W.Y.) The School of Computer Science and Engineering, Tianjin University of Technology, Tianjin 300072, China; gongliangyi@gmail.com Department of Computer and Information Sciences, Temple University, Philadelphia, PA 19122, USA Institute of Information Engineering, Chinese Academy of Sciences, Beijing 100093, China; yumiao@iie.ac.cn * Correspondence: mandapeng@hrbeu.edu.cn (D.M.); dxj@ieee.org (X.D.); Tel.: +86-451-8258-9638 (D.M.) Received: 26 October 2018; Accepted: 27 December 2018; Published: 5 January 2019 Featured Application: Intrusion Detection and Smart Home. Abstract: WiFi infrastructures are widely deployed in both public and private buildings. They make the connection to the internet more convenient. Recently, researchers find that WiFi signals have the ability to sense the changes in the environment that can detect human motion and even identify human activities and his identity in a device-free manner, and has many potential security applications in a smart home. Previous human detection systems can only detect human motion of regular moving patterns. However, they may have a significant detection performance degradation when used in intrusion detection. In this study, we propose Robust Device-Free Intrusion Detection (RDFID) system leveraging fine-grained Channel State Information (CSI). The noises in the signals are removed by a Principle Component Analysis (PCA) and a low pass filter. We extract a robust feature of frequency domain utilizing Continuous Wavelet Transform (CWT) from all subcarriers. RDFID captures the changes from the whole wireless channel, and a threshold is obtained self-adaptively, which is calibration-free in different environments, and can be deployed in smart home scenarios. We implement RDFID using commodity WiFi devices and evaluate it in three typical office rooms with different moving patterns. The results show that our system can accurately detect intrusion of different moving patterns and different environments without re-calibration. Keywords: intrusion detection; human detection; channel state information; device-free passive 1. Introduction Device-free human detection has attracted a lot of interest in recent years. It can detect human presence in the monitoring area without any sensing-related devices attached to the people [1]. It can be used well in intrusion detection systems, which is a vital security component in a smart home. Aiming at handling the security issues in a smart home, many techniques have been utilized to implement device-free human detection, such as video-based, infrared-based, Radio Frequency Identification (RFID)-based and Ultra-Wide Bandwidth (UWB)-based approaches. Although they have a good detection accuracy, these approaches have limited using conditions and need dedicated devices that hinder their adoption. WiFi-enabled devices become the catalyst of device-free sensing as they have been widely deployed in both public and private buildings. Besides being used for communication, WiFi networks can also be used as sensor networks [2–4]. Many applications have emerged based on WiFi infrastructures, human detection [5], indoor localization [6], and even human identification [7] are some representative applications. Appl. Sci. 2019, 9, 175; doi:10.3390/app9010175 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 175 2 of 17 A typical WiFi-based device-free human detection system usually contains several pairs of transmitters and receivers. A wireless router can act as a transmitter, while a WiFi-enabled device can act as a receiver. As a result, it doesn’t have the problem of key management [8,9] compared with sensor-based approaches. The rational of WiFi-based device-free human detection is that human presence has an impact on signal propagation, which will cause the signal strength fluctuation at the receiver [10]. Previous WiFi-based human detection systems utilize Received Signal Strength Indicator (RSSI) from Media Access Control (MAC) layer for it is easy to obtain. However, RSSI is a coarse-grained measurement. In the typical indoor scenario, RSSI becomes unreliable due to multipath fading. It may increase, decrease, or even remain the same when a person moves in the monitoring area. Recently, many studies explore CSI from physical layer of wireless networks to detect human motion [11–13]. As indicated in [14], CSI is a subcarrier-level measurement that is more fine-grained compared with RSSI. It is more sensitive to environmental changes while keeps quite stable in static scenarios. As a result, CSI succeeds in improving the performance of human detection. However, state-of-the-art human detection techniques still have limitations for intrusion detection systems. Common human detection techniques can only detect a human who is walking with a regular pattern. Nevertheless, an intruder in the building is likely to keep away from the security devices or move very slowly to hide himself from being monitored. Furthermore, most human detection techniques require on-site calibration of both static and dynamic environments. On-site calibration is labor intensive and it needs professional deployment and maintenance that makes a human detection system more complex in practical use. Consequently, human detection techniques will fail in detecting intruders in security systems, and we need to explore effective features to model human motion. To deal with the limitations, in this work, we propose a Robust Device-Free Intrusion Detection (RDFID) system leveraging fine-grained CSI. We investigate the impact of human motion on WiFi signals and demonstrate that different patterns of human motion in different scenarios can be modeled by a unified framework. First, we extract the wavelet variance of CSIs from frequency domain as the feature. It is more sensitive to human motion, and more robust under different moving patterns. In addition, the feature values of static and intrusion can be seen to be generated by different Gaussian Models. As a result, intrusion can be detected using a Gaussian Mixture Model (GMM). As shown in Figure 1, RDFID can detect human motion of different moving patterns. In addition, it can be easily deployed that it can achieve a satisfying performance even using a single pair of transceivers, and needs no re-calibration in different scenarios. We prototyped RDFID in three typical home and office scenarios with commodity WiFi devices composing only one wireless link. We evaluate the system and compare the performance with Fine-grained Real-time passive human motion Detection (FRID), device-free Passive Detection of moving humans with dynamic Speed (PADS) and Fine-grained Indoor Motion Detection (FIMD). The results show that the detection precision of RDFID can achieve over 97% under different moving patterns. Consequently, it makes intrusion detection systems a step closer to practical use. In summary, the contributions of our work are as follows: We propose RDFID, a novel device-free WiFi-based intrusion detection approach, which can detect intruders with different moving patterns at a high accuracy, and needs no re-calibration in different scenarios. It can be deployed in smart home scenarios to ensure security. We extract real-time features from CSIs in frequency domain, which is more sensitive to human motion of various moving patterns. We use the Gaussian Mixture Model (GMM) as the classifier based on the observation that the feature values under different moving patterns and different environments can be seen to be generated by different Gaussian Models. In the rest of this paper, the related works about WiFi-based human detection are reviewed in Section 2. Some preliminaries are introduced in Section 3. Section 4 presents the design details of Appl. Sci. 2019, 9, 175 3 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   3  of  17  our proposed intrusion detection system, while the performance evaluation is provided in Section 5. our proposed intrusion detection system, while the performance evaluation is provided in Section 5.  In Section 6, the potentials and limitations are discussed and we conclude this work in Section 7. In Section 6, the potentials and limitations are discussed and we conclude this work in Section 7.  (b) (a) (c) Figure 1. Different moving patterns in intrusion scenarios. (a) Regular walking; (b) walking while Figure 1. Different moving patterns in intrusion scenarios. (a) Regular walking; (b) walking while  bending down; and (c) creeping. bending down; and (c) creeping.  2. Related Work 2. Related Work  WiFi-based passive human detection is the fundamental technique of various ubiquitous wireless WiFi‐based  passive  human  detection  is  the  fundamental  technique  of  various  ubiquitous  sensing applications, such as indoor localization, human identification and activity recognition. It can wireless  sensing  applications,  such  as  indoor  localization,  human  identification  and  activity  be widely deployed in smart home scenarios to ensure the security. A large quantity of studies about recognition.  It  can  be  widely  deployed  in  smart  home  scenarios  to  ensure  the  security.  A  large  wireless sensing promote the development of wireless sensing. quantity of studies about wireless sensing promote the development of wireless sensing.   Earlier passive human detection systems usually utilize RSSI from the MAC layer of the wireless Earlier  passive  human  detection  systems  usually  utilize  RSSI  from  the  MAC  layer  of  the  network. After Youssef et al. proposed the concept of device-free passive human motion detection, wireless network. After Youssef et al. proposed the concept of device‐free passive human motion  they optimized their approach and made the system work in real environments [10]. Nuzzer leveraged detection,  they  optimized  their  approach  and  made  the  system  work  in  real  environments  [10].  probabilistic techniques, and had the capability to both localize a single entity and estimate the number Nuzzer leveraged probabilistic techniques, and had the capability to both localize a single entity  of people in the area of interest [15]. Since RSSI is a coarse-grained measurement of wireless networks, and  estimate  the  number  of  people  in  the  area  of  interest  [15].  Since  RSSI  is  a  coarse‐grained  many RSSI-based human detection systems deployed multiple pairs of transceivers to achieve a higher measurement of wireless networks, many RSSI‐based human detection systems deployed multiple  accuracy [16]. Another technique of human detection using multiple pairs of transceivers is Radio pairs of transceivers to achieve a higher accuracy [16]. Another technique of human detection using  Tomographic Imaging (RTI) [17]. Researchers also developed various approaches based on RTI, such multiple pairs of transceivers is Radio Tomographic Imaging (RTI) [17]. Researchers also developed  as the kRTI [18] and dRTI [19]. However, RSSI-based human detection systems suffer from severe various  approaches  based  on  RTI,  such  as  the  kRTI  [18]  and  dRTI  [19].  However,  RSSI‐based  multi-path efficiency [20]. As a result, more and more researchers move their attention to the more human detection systems suffer from severe multi‐path efficiency [20]. As a result, more and more  fine-grained measurement, CSI. researchers move their attention to the more fine‐grained measurement, CSI.  To overcome the shortcomings of RSSI-based human detection systems, Fine-grained device-free To  overcome  the  shortcomings  of  RSSI‐based  human  detection  systems,  Fine‐grained  Motion Detection (FIMD) utilized the burst pattern of CSIs during human motion to detection human device‐free  Motion  Detection  (FIMD)  utilized  the  burst  pattern  of  CSIs  during  human  motion  to  presence [21]. Fine-grained Indoor Localization (FILA) explored the frequency diversity of the detection  human  presence  [21].  Fine‐grained  Indoor  Localization  (FILA)  explored  the  frequency  subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems, and constructed a signal diversity of the subcarriers in Orthogonal Frequency Division Multiplexing (OFDM) systems, and  propagation model [22,23]. As human motion can cause the fluctuation of the signal, Bfp harnessed the constructed a signal propagation model [22,23]. As human motion can cause the fluctuation of the  variance of the amplitude of the CSIs to improve the performance of human detection [11]. PADS took signal,  Bfp  harnessed  the  variance  of  the  amplitude  of  the  CSIs  to  improve  the  performance  of  advantages of the whole information of CSI including both amplitude and phase feature to detect human  detection  [11].  PADS  took  advantages  of  the  whole  information  of  CSI  including  both  human motion with various speeds [24]. It calculates the maximum eigenvalue of covariance matrix of amplitude  and  phase  feature  to  detect  human  motion  with  various  speeds  [24].  It  calculates  the  maximum  eigenvalue  of  covariance  matrix  of  normalized  amplitude  and  phase  information,  respectively, as the feature. Support Vector Machine (SVM) is used as the classifier. FRID explored  Appl. Sci. 2019, 9, 175 4 of 17 normalized amplitude and phase information, respectively, as the feature. Support Vector Machine (SVM) is used as the classifier. FRID explored the phase feature of CSIs and achieved calibration-free human detection without the need of a normal profile [25,26]. Short-term averaged variance ratio (SVR) and long-term averaged variance ratio which are two schemes based on the coefficient of variance of phase are introduced to eliminate the re-calibration cost. Conventional human detection systems demonstrated directional monitoring coverage, and Zimu Zhou et al. utilized CSI features to virtually tune the coverage shape into disk-like [27]. Speed Independent Entity Detection (SIED) extracted a novel feature from the whole wireless channel and transformed human detection into a probabilistic problem to achieve a high detection accuracy [5]. AR-Alarm utilized a self-adaptive learning mechanism to achieve intrusion detection without the need of re-calibration [13]. Besides human detection, wireless signals can be used in indoor localization, activity recognition and even human identification. Abdel-Nasser et al. utilized CSI to provide a localization approach with a high accuracy leveraging only a single pair of transceiver [28]. CSI-MIMO utilized frequency diversity of CSI to construct the fingerprint of different locations and achieved a localization accuracy of 0.95 m [29]. SpotFi computed the Angle of Arrival (AoA) of multipath components of different antennas and improved the localization accuracy to 40 cm [30]. HiDFPL proposed a measurement to represent the sensitivity of the receiver and enhanced the localization accuracy [31]. Xuyu Wang et al. proposed PhaseFi, a fingerprinting system, using phase information of CSIs and incorporated a greedy algorithm to train the weights of a deep network [32]. Rui Zhou et al. proposed an indoor localization system based on CSI and SVM [33]. Density-based Spatial Clustering Of Applications With Noise (DBSCAN) was utilized in the system to reduce the noise in CSIs. CSI based human Activity Recognition and Monitoring (CARM) was proposed based on CSIs of wireless channel that quantified the relationship between the movement speeds of different body parts and activities, and it had the ability to recognize human activities [34]. Activity recognition has a wide range of applications, such as somatosensory games. Wi-Play extracted CSI waveforms from commercial WiFi devices to model some specified activity and achieved an activity recognition system [35]. Wifi-based GEsture Recognition (WiGeR) utilized the fluctuation scheme of CSIs generated by the moving of human hands to recognize gestures [36]. Smokey leveraged WiFi signals and had the ability to recognize smoking activity even in the non-line-of-sight (NLOS) and through-wall environments [37]. Wi-Chase utilized the CSIs from all subcarriers to achieve a higher activity recognition accuracy [38]. It is confirmed that human’s gait is unique among different people, thus it can be used to identify the human’s identity. WifiU was presented to construct the gait profiles of different people utilizing the unique variations in the CSIs [39]. WiWho was presented as a framework of human identification utilizing human’s gait extracted from CSIs [40]. FreeSense combined Principal Component Analysis (PCA), Discrete Wavelet Transform (DWT), and Dynamic Time Warping (DTW) to achieve a nine-user human identification [41]. Wii extracted time and frequency-domain features and used timefrequency analysis to achieve an accurate human identification system [7]. Although there have been quantities of work on human detection, they only perform well when the people move in regular patterns. When an intruder appears, he is more likely to move in an irregular way. As a result, a more robust human detection system is proposed in this paper to meet the challenges of intruder detection. 3. Preliminary CSI is leveraged in this study, and we will give a brief introduction of the background knowledge in this section. The wireless signals propagate through multiple paths from the transmitter to the receiver in a typical indoor scenario. As a result, the received signal is the superposition of the signals from LOS path and several reflection paths. OFDM framework is the basis of 802.11 n wireless networks, in which our system works. In this framework, the wireless channel can be descripted by a Channel Appl. Sci. 2018, 8, x FOR PEER REVIEW   5  of  17   j Appl. Sci. 2019, 9, 175 5 of 17 he () (  )n() ,  (1)  ii i1 th where   ,   , and     denote  the  amplitude,  phase  and  time  delay  of  the  signal  from  i   path,  i i i Impulse Response (CIR) in the time domain. Under the assumption of time-invariant, CIR can be respectively;  N   is the total number of paths;  n()   is complex Gaussian white noise; and  ()   is  expressed as: the Dirac delta function.  jq h(t) = a e d(t t ) + n(t), (1) å i i Nevertheless, precise CIR can be extracted only from dedicated devices rather than commodity  i=1 infrastructures. To overcome this limitation, Channel Frequency Response (CFR) can be extracted  th where a , q , and t denote the amplitude, phase and time delay of the signal from i path, respectively; i i i from frequency domain, which can model the wireless channel. CFR contains amplitudefrequency  N is the total number of paths; n(t) is complex Gaussian white noise; and d(t) is the Dirac response  and  phasefrequency  response.  Under  the  assumption  of  infinite  bandwidth,  CIR  is  delta function. equivalent to CFR, and CFR can be transformed by Fast Fourier Transform (FFT) from CIR: [20]  Nevertheless, precise CIR can be extracted only from dedicated devices rather than commodity H  FFT(( h )) .  (2)  infrastructures. To overcome this limitation, Channel Frequency Response (CFR) can be extracted from frequency domain, which can model the wireless channel. CFR contains amplitudefrequency We can obtain CFRs in the format of CSI:  response and phasefrequency response. Under the assumption of infinite bandwidth, CIR is H  [Hf ( ),Hf ( ), ...,Hf ( )] ,  (3)  equivalent to CFR, and CFR can be transformed12 by Fast Fourier N Transform (FFT) from CIR: [20] where N is the number of subcarriers in the wireless network.  H = FFT(h(t)). (2) The CSI is composed of amplitude and phase of a subcarrier:  j sin(H ) We can obtain CFRs in the format of CSI: Hf()  H(f ) e ,  (4)  kk H = [ H( f ), H( f ), . . . , H( f )], (3) where  f   is the central frequency of the subcarrier, and  H   represents its phase. Thus, a group of  1 2 N CSIs,  H (fk ), (  1,...,K ) , denote K sampled CFRs in subcarrier level.  where N is the number of subcarriers in the wireless network. The CSI is composed of amplitude and phase of a subcarrier: 4. System Design  j sin(\H) H( f ) = k H( f )ke , (4) k k 4.1. System Overview  where f is the central frequency of the subcarrier, and \H represents its phase. Thus, a group of CSIs, The  framework  of  RDFID  is  presented  in  Figure  2.  The  system  has  four  modules:  pre H( f‐processin ), (k = 1,g .;.  .fe , K at)ure , denote   extraction; K sampled   classif CFRs icatio inn;subcarrier   and  post‐ level. processing.  There  are  various  kinds  of  noise  in  the  raw  collected  CSI  data,  and  most  noise  is  removed  in  pre‐processing  module.  We  4. System Design extract  wavelet  variance  as  the  real‐time  feature  from  frequency  domain  in  feature  extraction  module. In the classification module, a portion of data is utilized to train a system to be universal  4.1. System Overview that can be adaptive to different scenarios. In the post‐processing module, the classification result is  The framework of RDFID is presented in Figure 2. The system has four modules: pre-processing; further processed to be closer to reality.  feature extraction; classification; and post-processing. There are various kinds of noise in the raw The  system  can  work  in  typical  indoor  scenarios  with  only  one  pair  of  commodity  WiFi  collected CSI data, and most noise is removed in pre-processing module. We extract wavelet variance devices, which include a wireless router and a laptop. The wireless router is the Transmit Xmt (TX)  as the real-time feature from frequency domain in feature extraction module. In the classification that  supports  Institute  of  Electrical  and  Electronic  Engineers  (IEEE)  802.11n  protocol,  while  the  module, a portion of data is utilized to train a system to be universal that can be adaptive to different laptop is the Receive Xmt (RX) that is equipped with Intel 5300 network interface card (NIC). The  scenarios. In the post-processing module, the classification result is further processed to be closer WiFi  devices  keep  transmitting  data  to  collect  CSIs  in  the  monitoring  area,  and  the  system  to reality. estimated intruder existence according to the extracted feature.  Training CSI Pre‐processing Feature Extraction Post‐processing Classification Figure 2. System Framework. Figure 2. System Framework.  The system can work in typical indoor scenarios with only one pair of commodity WiFi devices, which include a wireless router and a laptop. The wireless router is the Transmit Xmt (TX) that Appl. Sci. 2019, 9, 175 6 of 17 supports Institute of Electrical and Electronic Engineers (IEEE) 802.11n protocol, while the laptop is the Receive Xmt (RX) that is equipped with Intel 5300 network interface card (NIC). The WiFi devices keep transmitting data to collect CSIs in the monitoring area, and the system estimated intruder existence according to the extracted feature. 4.2. Pre-Processing The CSI data is extracted from the respond packets of Internet Control Messages Protocol (ICMP) packets. As a result, the number of the group of CSIs is the same as that of ICMP packets theoretically. However, during data collection period, we find that the number of collected CSI records is larger than that of transmitted ICMP packets we had set in advance. In order to calibrate the frequency of the collected data, we conduct the linear interpolation in the raw data and it has a unified frequency. In 802.11 n wireless networks, there are several subcarriers transmitting signals at the same time under the OFDM framework. The subcarriers are independent theoretically. However, the CSIs of adjacent subcarriers have some relationships. In consequence, PCA is used to extract independent data. The related CSI streams can be combined into several independent principle components. For each ICMP packet, a matrix of 3  30 constructed by CSIs can be extracted from the firmware. It can be further reshaped into a 1  90 vector. For a certain time window, n ICMP packets have been received, and we can obtain an n  90 matrix. During the evaluation of the principle components, we find that in most cases the first principle component can give an 80% contribution rate. As a result, we use the first principle component as the representative data. Unfortunately, there still exist some kinds of noises in the first principle component, and they have negative impact on detection rate. The one that has the most significant impact is high frequency noise induced by environment changes other than human movement. The movement of torso, arms, and legs cause most of signal reflections. The frequency of the movements is lower than 10 Hz according to our observation. As a result, a low pass filter is utilized to filter out the high frequency noise from the collected data with the frequency higher than 10 Hz. 4.3. Feature Extraction A proper feature is critical in classification tasks. Generally, the moving speed of a person is constant in a short period, and some periodicity exists when the person is moving. For instance, when the person walks, two steps construct a period. However, it is a challenging task to analyze the periodicity directly from the waveform of the wireless signals. During our early exploration, we find that besides time-domain features, frequency-domain features can better characterize the waveforms in intrusion detection. As a result, in order to explore a scenario independent feature, we utilize timefrequency analysis on the waveform. Continuous Wavelet Transform (CWT) combined with wavelet variance is a proper tool to analyze the periodicity of the waveform. First, the wavelet coefficient of the first principle component of the CSIs after low-pass filter (cpl) is calculated utilizing CWT in Equation (5): 1 t b W (a, b) = x(t)p y( )dt, (5) where x(t) is the first principle component of the CSIs after low-pass filter (cpl), a and b are scale and time, respectively. y() is the wavelet function, and db6 (Daubechies) wavelet [42] is selected as it provides the best performance after we have tried different wavelet functions. As shown in Figure 3, it can be clearly seen that some periodicity exists in the waveform after we conduct continuous wavelet transform. However, it is necessary to quantitatively calculate the significance of the periodicity to confirm that the periodicity is caused by human behaviors. Appl. Sci. 2019, 9, 175 7 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   7  of  17  Appl. Sci. 2018, 8, x FOR PEER 500 REVIEW   7  of  17  300 100 0 204060 80 100 600 Time (s) FigureFigure 3. Wavelet  3. Wave coef let coeffi ficient cient of of Channel  Channel State State Information Information  (CS (CSI) I) when when  people people  move.move.   During  our  experiment,  we  find  that  the  distribution  of  the  wavelet  variance  is  different  Wavelet variance is widely used in meteorology to calculate the periodicity of precipitation. among whether there is human motion as shown in Figure 4. In consequence, the wavelet variance  It reflects the distribution of the power of the wavelet coefficients of various scales. As a result, it can is a proper feature for intrusion detection.  also describe the significance of the periodicity of human motion. The wavelet variance is calculated as Equation (6): 0 Z +¥ 0 204060 80 100 var(a) = W (a, b) db, (6) Time (s ) Figure 3. Wavelet coefficient of Channel State Information (CSI) when people move.  where W (a, b) is the power of the wavelet coefficient of scale a at time b. During  our  experiment,  we  find  that  the  distribution  of  the  wavelet  variance  is  different  During our experiment, we find that the distribution of the wavelet variance is different among among whether there is human motion as shown in Figure 4. In consequence, the wavelet variance  whether there is human motion as shown in Figure 4. In consequence, the wavelet variance is a proper is a proper feature for intrusion detection.  feature for intrusion detection. Figure 4. The distribution of wavelet variance when there is human motion and static.  4.4. Training and Classification  As the distribution of the wavelet variance when there is human motion is different from that  of static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM,  there  are  two  Gaussian  models,  one  is  static  model  and  the  other  is  human  motion  model.  The  moving data of different volunteers in different moving patterns and the data collected in the static  scenario construct the training data. The GMM only need to be trained once, and it can be used in  different  scenarios  without  being  re‐trained.  As  a  result,  after  a  trained  GMM  is  generated,  the  Figure 4.Figure The distribution  4. The distributi ofon wavelet  of wavelet variance  variance when  whenther  there e is is human human moti motion on andand  static. static.   intrusion detection system is unsupervised. In the training phase, the vectors of wavelet variance of  different scales and the ground truth are utilized to train the GMM. In the classification phase, the  4.4. Training 4.4. and Training Classification  and Classification  inputs are only the vectors of wavelet variance, while the outputs are the detection results whether  As the distribution of the wavelet variance when there is human motion is different from that  there exists human motion.  As the distribution of the wavelet variance when there is human motion is different from that of of static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM,  In  the  end  of  classification,  a  post‐processing  procedure  is  added  to  improve  the  detection  static scenario, the Gaussian Mixture Model (GMM) is an appropriate classifier. In this GMM, there are there  are  two  Gaussian  models,  one  is  static  model  and  the  other  is  human  motion  model.  The  accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As  two Gaussian models, one is static model and the other is human motion model. The moving data of moving data of different volunteers in different moving patterns and the data collected in the static  different volunteers in different moving patterns and the data collected in the static scenario construct scenario construct the training data. The GMM only need to be trained once, and it can be used in  the training different data.  sce The nario GMM s  withonly out  being need  re to‐tra be ined. trained   As  a once, result, and after ita can trained be used GMMin  is dif gener fera ent ted,scenarios   the  intrusion detection system is unsupervised. In the training phase, the vectors of wavelet variance of  without being re-trained. As a result, after a trained GMM is generated, the intrusion detection system different scales and the ground truth are utilized to train the GMM. In the classification phase, the  is unsupervised. In the training phase, the vectors of wavelet variance of different scales and the inputs are only the vectors of wavelet variance, while the outputs are the detection results whether  ground truth are utilized to train the GMM. In the classification phase, the inputs are only the vectors there exists human motion.  of wavelet variance, while the outputs are the detection results whether there exists human motion. In  the  end  of  classification,  a  post‐processing  procedure  is  added  to  improve  the  detection  accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As  Appl. Sci. 2019, 9, 175 8 of 17 In the end of classification, a post-processing procedure is added to improve the detection accuracy. In this procedure, it is assumed that a person cannot appear and disappear suddenly. As a result, an additional window beyond the detection window is utilized to reduce the detection mistakes. Appl. Sci. 2018, 8, x FOR PEER REVIEW   8  of  17  For example, 0 and 1 represent static and intrusion, respectively. If the detection result is 11011 in this a  result,  an  additional  window  beyond  the  detection  window  is  utilized  to  reduce  the  detection  additional window, we can consider there always exists intrusion in this window. The cost of this mistakes. For example, 0 and 1 represent static and intrusion, respectively. If the detection result is  procedure is the time delay in detection, but the detection accuracy can be higher. 11011 in this additional window, we can consider there always exists intrusion in this window. The  cost of this procedure is the time delay in detection, but the detection accuracy can be higher.  5. Evaluation 5. Evaluation  5.1. Experiment Setup T 5.o1.evaluate  Experimen the t Se detection tup  performance of the system, some real experiments are conducted in three typical rooms from several aspects. The three rooms are a meeting room, a typical living room, and a To evaluate the detection performance of the system, some real experiments are conducted in  large office, and the sizes of the three room are 5 m  4 m, 5 m  4 m and 10 m  6 m, respectively. three typical rooms from several aspects. The three rooms are a meeting room, a typical living room,  The layout of the three rooms and transceiver deployment are shown in Figure 5. There are desks with and  a  large  office,  and  the  sizes  of  the  three  room  are  5  m  ×  4  m,  5  m  ×  4  m  and  10  m  ×  6  m,  glassrespectively. dam-boards  The and  lay chairs out  of in  the the  three office,  rooms while  and a meeting   transcei table ver  de and ployment chairs in are the   shown meeting   in  Fig room, ure  5. which   There are desks with glass dam‐boards and chairs in the office, while a meeting table and chairs in  causes different multipath effects. Especially, in order to present a reasonable evaluation in a smart the  meeting  room,  which  causes  different  multipath  effects.  Especially,  in  order  to  present  a  home scenario, a typical living room was used as a scenario. In the living room a television, there is a reasonable evaluation in a smart home scenario, a typical living room was used as a scenario. In the  television on the wall, a sofa, a piano, a refrigerator, some other furniture, and some doors to other living room a television, there is a television on the wall, a sofa, a piano, a refrigerator, some other  rooms, which will cause much more complex multipath effects. A TP-Link 802.11n wireless router furniture, and some doors to other rooms, which will cause much more complex multipath effects.  with a single antenna is used as the transmitter and a Lenovo laptop equipped with a three-antenna A TP‐Link 802.11n wireless router with a single antenna is used as the transmitter and a Lenovo  Intel WiFi Link 5300 (iwl 5300) NIC running Ubuntu 11.04 OS as the receiver. The firmware of the laptop equipped with a three‐antenna Intel WiFi Link 5300 (iwl 5300) NIC running Ubuntu 11.04  NIC is modified in order to extract CSIs from data packets utilizing the CSI tools. In addition, we OS as the receiver. The firmware of the NIC is modified in order to extract CSIs from data packets  upgrade the antennas of the NIC using three 6dbi gain antennas as shown in Figure 6 in order to utilizing  the  CSI  tools.  In  addition,  we  upgrade  the  antennas  of  the  NIC  using  three  6dbi  gain  increase antennas the signal-noise-ratio.  as shown in Figure 6 in order to increase the signal‐noise‐ratio.  5m 5m TX TX RX RX (a) meeting room (b) living room 10m TX RX (c) large office Figure 5. Experimental scenario. Figure 5. Experimental scenario.  According to CSI tools, the sensing data is the CSIs of the respond packets when the transmitter is According  to  CSI  tools,  the  sensing  data  is  the  CSIs  of  the  respond  packets  when  the  continuously transmitter sending  is continuously ICMP packets  sending to IC the MPr eceiver packets. to W ethe recr  receiver uited.four  We re volunteers cruited fouin r volunteers our experiments  in  our experiments with the basic information shown in Table 1. During data collection period, only a  with the basic information shown in Table 1. During data collection period, only a single person single person moves back and forth in different moving patterns respectively in the room without a  6m 4m 4m Appl. Sci. 2019, 9, 175 9 of 17 moves back and forth in different moving patterns respectively in the room without a fixed path. Appl. Sci. 2018, 8, x FOR PEER REVIEW   9  of  17  The transmission rate in our experiments is configured to 200 Hz. A few cycles of data collection process are conducted for one person, while each cycle contains only one moving pattern and lasts for fixed path. The transmission rate in our experiments is configured to 200 Hz. A few cycles of data  100 s. Data collection lasts for one week, and about 20 min moving data is collected for one person collection process are conducted for one person, while each cycle contains only one moving pattern  moving in one pattern. and lasts for 100 s. Data collection lasts for one week, and about 20 min moving data is collected for  False negative (FN), false positive (FP), and the probability of detection (PD) are used as the one person moving in one pattern.  evaluation metrics in this paper. False negative is the ratio that RDFID fails to detect intrusion, while False negative (FN), false positive (FP), and the probability of detection (PD) are used as the  false positive is the ratio it reports intrusion when nobody is in the room. The probability of detection evaluation  metrics  in  this  paper.  False  negative  is  the  ratio  that  RDFID  fails  to  detect  intrusion,  Appl. Sci. 2018, 8, x FOR PEER REVIEW   9  of  17  is the ratio that it successfully detects the existence of the intruder. The three metrics can be illustrated while false positive is the ratio it reports intrusion when nobody is in the room. The probability of  by Figure 7, wher fixed e P1–P4 path. Thear  tran e the smission elements  rate in our of exper the iconfusion ments is configure matrix d to 20 in0 the Hz. A form  few cycles of per  of centage. data  As shown detection is the ratio that it successfully detects the existence of the intruder. The three metrics can  collection process are conducted for one person, while each cycle contains only one moving pattern  in be Figur   illu est 7rat , P4 edr epr by esents Figure FN 7,  where and P1  P1– repr P4 esents   are  the FP .ele PD mis ents described   of  the  confusion in Equation   ma(7). trix  in  the  form  of  and lasts for 100 s. Data collection lasts for one week, and about 20 min moving data is collected for  percentage.  As  shown  in  Figure  7,  P4  represents  FN  and  P1  represents  FP.  PD  is  described  in  one person moving in one pattern.  False negative (FN), false positive PD (F= P), P3 and / the (P3 pro+babil P4it)y , of detection (PD) are used as the  (7) Equation (7).  evaluation  metrics  in  this  paper.  False  negative  is  the  ratio  that  RDFID  fails  to  detect  intrusion,  while false positive is the ratio it reports intrusion when nobody is in the room. The probability of  detection is the ratio that it successfully detects the existence of the intruder. The three metrics can  be  illustrated  by  Figure  7,  where  P1–P4  are  the  elements  of  the  confusion  matrix  in  the  form  of  percentage.  As  shown  in  Figure  7,  P4  represents  FN  and  P1  represents  FP.  PD  is  described  in  Equation (7).  Figure 6. The modified receiver.  Figure 6. The modified receiver.  Figure 6. The modified receiver. Classified as Classified as intrusion clear intrusion clear P1 P2 P1 P2 P3 P4 Figure 7. Confusion matrix of intrusion detection.  Figure 7. ConfusionP3 matrix of intrusion P4 detection. PD=+ P3 / (P 3 P 4),  (7)  Table 1. Basic information of volunteers. Figure 7. Confusion matrix of intrusion detection.  Volunteers Gender Table 1. BasicHeight  information (cm)  of volunteers.W   eight (kg) Age Volunteers  Gender  Height (cm)  Weight (kg)  Age  1 male 174 63 30 1  male  174  63  30  2 male 175 70 27 PD=+ P3 / (P 3 P 4),  (7)  2  male  175  70  27  3 male 170 62 27 3  male  170  62  27  4  female  163  51  26  4 female 163 51 26 Table 1. Basic information of volunteers.  5.2. Performance Evaluation  5.2. Performance Evaluation Volunteers  Gender  Height (cm)  Weight (kg)  Age  1  male  174  63  30  5.2.1. Intrusion Detection in Different Scenarios 2  male  175  70  27  3  male  170  62  27  In order to confirm that the performance of RDFID is independent of scenarios, we first evaluate 4  female  163  51  26  the system in different rooms. In addition, we compare the system with two other device-free human 5.2. Performance Evaluation  Actual state intrusion clear Actual state intrusion clear Appl. Sci. 2018, 8, x FOR PEER REVIEW   10  of  17  5.2.1. Intrusion Detection in Different Scenarios  Appl. Sci. 2019, 9, 175 10 of 17 In  order  to  confirm  that  the  performance  of  RDFID  is  independent  of  scenarios,  we  first  evaluate  the  system  in  different  rooms.  In  addition,  we  compare  the  system  with  two  other  detection device systems, ‐free hu FRID man detection and PADS.  systems, When FR constr ID and ucting  PADS. the When training  constructi set, we nguse  the the traini combination ng set, we us ofe  the the combination of the data from the three scenarios to form six groups of training set and we name  data from the three scenarios to form six groups of training set and we name them a, b, c, ab, ac, and them a, b, c, ab, ac, and bc, respectively, according to Figure 7, and all training sets contain the three  bc, respectively, according to Figure 7, and all training sets contain the three moving patterns. Datasets moving patterns. Datasets that are opposite to the training sets are used as test sets, which are bc, ac,  that are opposite to the training sets are used as test sets, which are bc, ac, ab, c, b, and a, respectively. ab, c, b, and a, respectively. To ensure the reliability of the evaluation, each training set is equally  To ensure the reliability of the evaluation, each training set is equally divided into five parts, and five divided into five parts, and five experiments are conducted in which the classifier is trained using  experiments are conducted in which the classifier is trained using each part respectively. The result is each  part respectively.  The result is  the  mean of  the  five experiments.  The window  size in  these  the mean of the five experiments. The window size in these experiments is 5 s. The FN and FP rate of experiments is 5 s. The FN and FP rate of the three approaches is shown in Table 2. As indicated in  the three approaches is shown in Table 2. As indicated in the table, the FN rate of RDFID in different the table, the FN rate of RDFID in different scenarios is around 2%, which is the lowest among the  scenarios three is approa around ches. 2%, The which  FN rais te of the PA lowest DS is af among fected more the thr signif eeica appr ntly oaches.  by the se The lection FN of rate  the of training PADS  is set because it uses SVM as its classifier, the support vectors in different scenarios are not the same.  affected more significantly by the selection of the training set because it uses SVM as its classifier, the As a result, the FN rate of PADS is higher. As FRID does not need training data, the estimation of  support vectors in different scenarios are not the same. As a result, the FN rate of PADS is higher. As the parameters has particular influence on the performance of human detection.  FRID does not need training data, the estimation of the parameters has particular influence on the The FP rate of RDFID is lower than the other two approaches. Most of the FP rate is around 2%,  performance of human detection. which  indicates  RDFID  generates  less  false  alarms  when  detecting  intruders.  PADS  uses  phase  information  in  CSIs  that  is  more  sensitive  to  environmental  changes;  therefore,  it  achieves  the  Table 2. False negative/false positive (FN/FP) of human detection in different scenarios (%). highest FP rate among the three approaches.  FN FP Table 2. False negative/false positive (FN/FP) of human detection in different scenarios (%).  Training Set a b c ab ac bc a b c ab ac bc RDFID 2.5  2.3 2.4 2.4FN  2.6 2.5 1.7 2.2 FP 2.1  2.2 2.1 1.9 Training Set  a  b  c  ab  ac  bc  a  b  c  ab  ac  bc  FRID 4.8 5.8 4.4 4.3 5.7 5.2 8.5 8.2 8.7 8.7 8.2 8.6 RDFID  2.5  2.3  2.4  2.4  2.6  2.5  1.7  2.2  2.1  2.2  2.1  1.9  PADS 6.0 6.7 6.2 6.1 6.8 5.8 10.8 10.2 10.5 11.8 11.0 10.6 FRID  4.8  5.8  4.4  4.3  5.7  5.2  8.5  8.2  8.7  8.7  8.2  8.6  PADS  6.0  6.7  6.2  6.1  6.8  5.8  10.8  10.2  10.5  11.8  11.0  10.6  The FP rate of RDFID is lower than the other two approaches. Most of the FP rate is around Figure  8  indicates  the  PD  of  the  approaches  in  different  scenarios.  It  can  be  seen  from  the  2%, which indicates RDFID generates less false alarms when detecting intruders. PADS uses phase figure that RDFID achieves the most stable and lowest probability of detection.  information in CSIs that is more sensitive to environmental changes; therefore, it achieves the highest It  can  be  seen  from  the  results  that  the  detection  performance  of  RDFID  is  independent  of  FP rate among the three approaches. scenarios. The detection model trained in one scenario can be adapted to other scenarios directly in  Figure 8 indicates the PD of the approaches in different scenarios. It can be seen from the figure a relative high detection performance.  that RDFID achieves the most stable and lowest probability of detection. Figure 8. The probability of detection (PD) of human detection in different scenarios.  Figure 8. The probability of detection (PD) of human detection in different scenarios. 5.2.2. Intrusion Detection among Different People  It can be seen from the results that the detection performance of RDFID is independent of scenarios. The detection model trained in one scenario can be adapted to other scenarios directly in a relative high detection performance. 5.2.2. Intrusion Detection among Different People In order to evaluate the independence of the intrusion detection performance among different people, we use the moving data of only one volunteer as the training data, while the moving data of Appl. Sci. 2018, 8, x FOR PEER REVIEW   11  of  17  In order to evaluate the independence of the intrusion detection performance among different  people, we use the moving data of only one volunteer as the training data, while the moving data of  all the four volunteers as the test data. The training data and test data of the first volunteer has no  Appl. Sci. 2019, 9, 175 11 of 17 intersection.  In  addition,  the  performance  of  RDFID  is  compared  to  that  of  PADS.  When  constructing the training set, the moving data of the first volunteer is used as the training set. It  all the four volunteers as the test data. The training data and test data of the first volunteer has no contains the moving data in all three scenarios and three different moving patterns. The evaluation  intersection. In addition, the performance of RDFID is compared to that of PADS. When constructing is conducted five times, and each time the training data is selected randomly from the moving data  the training set, the moving data of the first volunteer is used as the training set. It contains the moving of the first volunteer. The result is the mean of the five times. The window size is 5 s; the FN and FP  data in all three scenarios and three different moving patterns. The evaluation is conducted five times, rate of the two approaches are presented in Table 3. It is indicated in the table that the FN rate of  and each time the training data is selected randomly from the moving data of the first volunteer. The RDFID is relatively stable when detecting different people. However, the FN rate of PADS is more  result is the mean of the five times. The window size is 5 s; the FN and FP rate of the two approaches sensitive to different people. Its FN rate is even lower than that of RDFID when the test data and  are presented in Table 3. It is indicated in the table that the FN rate of RDFID is relatively stable when training  data  is  from  the  same  person.  In  contrast,  the  FN  rate  of  PADS  suffers  significant  detecting different people. However, the FN rate of PADS is more sensitive to different people. Its FN fluctuation when the test data and the training data is from different people. The result shows that  rate is even lower than that of RDFID when the test data and training data is from the same person. In the  FN  rate  of  PADS  is  sensitive  to  training  data  and  test  data,  the  moving  data  from  different  contrast, the FN rate of PADS suffers significant fluctuation when the test data and the training data is people  can  affect  the  detection  performance.  As  a  result,  RDFID  has  a  better  adaptability  to  from different people. The result shows that the FN rate of PADS is sensitive to training data and test different people.  data, the moving data from different people can affect the detection performance. As a result, RDFID has a better adaptability to different people. Table 3. FN/FP of human detection of different people (%).  FN  FP  Table 3. FN/FP of human detection of different people (%). Volunteer  1  2  3  4  1  2  3  4  FN FP RDFID  2.1  2.9  3.3  2.8  1.7  1.8  1.8  2  Volunteer 1 PADS 2  1.9  37.3  9.1  4 7.4  2.4 1 8.8  8.5 2 9.8  3 4 RDFID 2.1 2.9 3.3 2.8 1.7 1.8 1.8 2 The trend of the FP rate of the two approaches is similar to that of the FN rate. The FP rate of  PADS 1.9 7.3 9.1 7.4 2.4 8.8 8.5 9.8 RDFID  is  still  stable  in  the  four  tests  and  maintains  about  2%.  However,  the  FP  rate  of  PADS  achieves a low level only when the test data and training data is from the same person, and raises  The trend of the FP rate of the two approaches is similar to that of the FN rate. The FP rate of significantly using the test data of the other three people.  RDFID is still stable in the four tests and maintains about 2%. However, the FP rate of PADS achieves Figure 9 shows the PD of the two approaches when detection different people. Besides PADS  a low level only when the test data and training data is from the same person, and raises significantly achieves a lower PD when the data of the same volunteer is used in both training set and test set,  using the test data of the other three people. RDFID has a higher PD when using the moving data of the other volunteers as test set.  Figure 9 shows the PD of the two approaches when detection different people. Besides PADS In consequence, RDFID is less sensitive to the training and test data, and can achieve a better  achieves a lower PD when the data of the same volunteer is used in both training set and test set, human detection performance.  RDFID has a higher PD when using the moving data of the other volunteers as test set. Figure 9. PD of human detection of different people. Figure 9. PD of human detection of different people.  In consequence, RDFID is less sensitive to the training and test data, and can achieve a better 5.2.3. Intrusion Detection with Different Window Sizes  human detection performance. Appl. Sci. 2019, 9, 175 12 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   12  of  17  5.2.3. Intrusion Detection with Different Window Sizes As RDFID is  a  window‐based  human detection  approach,  the  detection  performance is also  As RDFID is a window-based human detection approach, the detection performance is also evaluated under different window sizes. To examine the advancement of RDFID, it is compared to  evaluated under different window sizes. To examine the advancement of RDFID, it is compared to two other human detection approaches, FRID and PADS. In the construction phase of the training  two other human detection approaches, FRID and PADS. In the construction phase of the training set, a 30 s data segment is randomly divided from the regular walking data of the first volunteer in  set, a 30 s data segment is randomly divided from the regular walking data of the first volunteer in scenario (a). The test data contains the regular walking data of the other three volunteers, while the  scenario (a). The test data contains the regular walking data of the other three volunteers, while the window size ranges from 1 s to 5 s.  window size ranges from 1 s to 5 s. The  results  are  the  mean  values  of  the  three  people.  The  FN  and  FP  rate  of  the  three  The results are the mean values of the three people. The FN and FP rate of the three approaches approaches under different window sizes are shown in Table 4. It is indicated from the table that  under different window sizes are shown in Table 4. It is indicated from the table that the FN rate of the FN rate of RDFID is as high as 11.7% when the window size is 1 s, but it decreases to 5.2% when  RDFID is as high as 11.7% when the window size is 1 s, but it decreases to 5.2% when the window the window size changes to 2 s. Moreover, the FN rate of RDFID keeps decreasing as the window  size changes to 2 s. Moreover, the FN rate of RDFID keeps decreasing as the window size increases. size increases. It is because the 1‐s window is too narrow for human motion, and people can only  It is because the 1-s window is too narrow for human motion, and people can only walk less than two walk  less  than  two  steps  within  the  window.  As  a  result,  the  periodicity  in  the  extracted  steps within the window. As a result, the periodicity in the extracted frequency-domain features is not frequency‐domain features is not significant enough, which leads to a higher FN rate. Although the  significant enough, which leads to a higher FN rate. Although the FN rate of FRID is lower than that FN rate of FRID is lower than that of RDFID when the window size is 1 s, it decreases slower when  of RDFID when the window size is 1 s, it decreases slower when the window size increases. On the the  window  size  increases.  On  the  other  hand,  as  the  training  and  test  data  is  from  the  same  other hand, as the training and test data is from the same scenario in this experiment, the variation of scenario in this experiment, the variation of the support vector of the features is insignificant; the  the support vector of the features is insignificant; the FN rate of PADS can achieve a low level. FN rate of PADS can achieve a low level.  Table 4. FN/FP of human detection under different window sizes (%). Table 4. FN/FP of human detection under different window sizes (%).  FN FP   FN  FP  Window Size Window Size (s)  1  2  3  4  5  1  2  3  4  5  1 2 3 4 5 1 2 3 4 5 (s) RDFID  11.7  5.2  4.6  3  2.1  2.1  2.3  1.7  1.8  1.7  RDFID 11.7 5.2 4.6 3 2.1 2.1 2.3 1.7 1.8 1.7 FRID  9.8  7.4  5.6  4.9  4.4  7  7.6  6.8  7.2  7.3  FRID 9.8 7.4 5.6 4.9 4.4 7 7.6 6.8 7.2 7.3 PADS  6.3  4.6  4.6  3.8  3.2  3.5  3.8  3.5  4  3.3  PADS 6.3 4.6 4.6 3.8 3.2 3.5 3.8 3.5 4 3.3 The FP rate of the three approaches all undergoes a low fluctuation, which indicates that the  The FP rate of the three approaches all undergoes a low fluctuation, which indicates that the FP FP rate of the three approaches can be less affected by the window size. However, as the extracted  rate of the three approaches can be less affected by the window size. However, as the extracted feature feature in RDFID has a better discernibility between static and dynamic, this approach achieves the  in RDFID has a better discernibility between static and dynamic, this approach achieves the lowest lowest FP rate.  FP rate. Figure 10 shows the PD of the three approaches when the window size is different. It can be  Figure 10 shows the PD of the three approaches when the window size is different. It can be seen seen that PADS achieves a higher PD when the window size is no larger than 3 s, but the PD of  that PADS achieves a higher PD when the window size is no larger than 3 s, but the PD of RDFID RDFID increases fast as the window size gets larger, and gets the highest of the three approaches  increases fast as the window size gets larger, and gets the highest of the three approaches when the when the window size is larger than 3 s.   window size is larger than 3 s. RDFID FRID PADS 12 345 Window Size (s) Figure 10. PD of human detection under different window sizes. Figure 10. PD of human detection under different window sizes.  5.2.4. Intrusion Detection with Different Moving Patterns  PD (%) Appl. Sci. 2018, 8, x FOR PEER REVIEW   13  of  17  Appl. Sci. 2019, 9, 175 13 of 17 The most important problem that RDFID solves is human detection under different moving  patterns.  In  consequence,  to  evaluate  the  ability  of  RDFID  in  this  problem,  the  data  of  different  5.2.4. Intrusion Detection with Different Moving Patterns moving patterns is used in this evaluation. To address the importance of this problem, RDFID is  compared  to FRID,  PADS,  and  FIMD  [21].  A  30 s moving  data  segment  of  the  first  volunteer  in  The most important problem that RDFID solves is human detection under different moving scenario (b) under regular moving pattern is randomly divided as training data, while the data of  patterns. In consequence, to evaluate the ability of RDFID in this problem, the data of different moving the other three volunteers in scenario (b) under three different moving patterns is used as the test  patterns is used in this evaluation. To address the importance of this problem, RDFID is compared data.  The  results  of  the  three  approaches  are  the  mean  values  of  the  three  volunteers,  and  the  to FRID, PADS, and FIMD [21]. A 30 s moving data segment of the first volunteer in scenario (b) window size is 5 s.   under regular moving pattern is randomly divided as training data, while the data of the other three The FN and FP rate of the four approaches under different moving patterns is shown in Table  volunteers in scenario (b) under three different moving patterns is used as the test data. The results of 5. It can be seen from the table that the FN rate of RDFID remains stable under different moving  the three approaches are the mean values of the three volunteers, and the window size is 5 s. patterns.  However,  the  FN  rate  of  the  other  three  approaches  raises  significantly  when  the  The FN and FP rate of the four approaches under different moving patterns is shown in Table 5. volunteers creep on the floor. FRID, PADS, and FIMD are affected more significantly because the  It can be seen from the table that the FN rate of RDFID remains stable under different moving patterns. influence  of  the  human  body  to  the  transmission  of  the  wireless  signal  becomes  weak  when  the  However, the FN rate of the other three approaches raises significantly when the volunteers creep on the volunteers creep on the floor. The FN rate of RDFID has a small fluctuation because the extracted  floor. FRID, PADS, and FIMD are affected more significantly because the influence of the human body to feature is related to the periodicity of human motion. It can detect human at a high accuracy as long  the transmission of the wireless signal becomes weak when the volunteers creep on the floor. The FN as there exists a periodicity of human motion.  rate of RDFID has a small fluctuation because the extracted feature is related to the periodicity of human motion. It can detect human at a high accuracy as long as there exists a periodicity of human motion. Table 5. FN/FP of human detection under different moving patterns (%).  Table 5. FN/FP of human detection under different moving patterns (%).   FN  FP  Moving  Regular  Bending  Regular  Bending  FN FP Creeping  Creeping  Pattern  Walking  Down  Walking  Down  Moving Regular Bending Regular Bending Creeping Creeping RDFID  2.3  2.5  2.6  1.7  1.7  1.5  Pattern Walking Down Walking Down FRID  4.8  5.2  9.8  7.8  6.2  3.6  RDFID 2.3 2.5 2.6 1.7 1.7 1.5 PADS  4.3  4.8  6.4  4.3  4.1  2.8  FRID 4.8 5.2 9.8 7.8 6.2 3.6 FIMD  5.4  5.7  12.5  6.8  6.4  4.8  PADS 4.3 4.8 6.4 4.3 4.1 2.8 FIMD 5.4 5.7 12.5 6.8 6.4 4.8 The FP rate of RDFID is still stable under the three moving patterns, while the change trend of  the FP rate of the other three approaches is the opposite to that of the FN rate. The reason is the  The FP rate of RDFID is still stable under the three moving patterns, while the change trend of the same that the influence of human body to the transmission of the wireless signal becomes less when  FP rate of the other three approaches is the opposite to that of the FN rate. The reason is the same that the person creeps on the floor. The low FP rate of the other three approaches is on the cost of the  the influence of human body to the transmission of the wireless signal becomes less when the person high FN rate. In consequence, RDFID has the ability to detect human of different moving patterns.  creeps on the floor. The low FP rate of the other three approaches is on the cost of the high FN rate. In It  has  the  advancement  of  human  detection  especially  when  the  person  moves  in  an  irregular  consequence, RDFID has the ability to detect human of different moving patterns. It has the advancement pattern.  The  robustness  of  RDFID  is  higher  that  the  detection  performance  is  less  affected  by  of human detection especially when the person moves in an irregular pattern. The robustness of RDFID different moving patterns.  is higher that the detection performance is less affected by different moving patterns. As illustrated in Figure 11, the PD of RDFID is the highest and stable under the three different  As illustrated in Figure 11, the PD of RDFID is the highest and stable under the three different moving patterns benefiting from the frequency–domain feature.   moving patterns benefiting from the frequency–domain feature. Figure 11. PD of human detection under different moving patterns. Figure 11. PD of human detection under different moving patterns.  Appl. Sci. 2019, 9, 175 14 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW   14  of  17  5.2.5. Intrusion Detection under Different Moving Speeds 5.2.5. Intrusion Detection under Different Moving Speeds  As a special case, human detection under different moving speeds plays an important role in As a special case, human detection under different moving speeds plays an important role in  intrusion detection systems. The four volunteers are asked to walk in a regular pattern at 1.5 m/s, intrusion detection systems. The four volunteers are asked to walk in a regular pattern at 1.5 m/s,  0.7 m/s, and 0.2 m/s, respectively, in the meeting room. A 30 s data segment is randomly divided from 0.7 m/s, and 0.2 m/s, respectively, in the meeting room. A 30 s data segment is randomly divided  the data of the first volunteer walking at the speed of 0.7 m/s as the training data, while the walking from the data of the first volunteer walking at the speed of 0.7 m/s as the training data, while the  data of the other three volunteers under different speeds is used as the test data. The window size is 5 s, walking  data  of  the  other  three  volunteers  under  different  speeds  is  used  as  the  test  data.  The  and the results are the mean value of the three volunteers. The human detection performance of RDFID window size is 5 s, and the results are the mean value of the three volunteers. The human detection  is compared to PADS and FRID. The FN and FP rate of the three approaches under different moving performance of RDFID is compared to PADS and FRID. The FN and FP rate of the three approaches  speeds is shown in Table 6. As indicated in the table, the trends of the FN rate of the approaches are under different moving speeds is shown in Table 6. As indicated in the table, the trends of the FN  the same that they all increase as the moving speed becomes slower. The influence of human motion to rate of the approaches are the same that they all increase as the moving speed becomes slower. The  the transmission of the wireless signal decreases when the moving speed becomes slower. Especially influence of human motion to the transmission of the wireless signal decreases when the moving  when the person moves far away from the first Fresnel zone, the reflected signal is submerged in the speed becomes slower. Especially when the person moves far away from the first Fresnel zone, the  signal from the LOS path. As a result, it is of great difficulties to extract effect environmental change reflected signal is submerged in the signal from the LOS path. As a result, it is of great difficulties to  information from the received signal. On the other hand, it can be seen that the FN rate of RDFID is extract effect environmental change information from the received signal. On the other hand, it can  lower than the other approaches. be seen that the FN rate of RDFID is lower than the other approaches.  Table 6. FN/FP of human detection under different moving speeds (%). Table 6. FN/FP of human detection under different moving speeds (%).  FN FP   FN  FP  Moving Speed (m/s) 1.5 0.7 0.2 1.5 0.7 0.2 Moving Speed (m/s)  1.5  0.7  0.2  1.5  0.7  0.2  RDFID  1.2  1.8  3.4  1.2  1.2  0.9  RDFID 1.2 1.8 3.4 1.2 1.2 0.9 FRID  2.1  3  4.2  2.1  2.3  1.8  FRID 2.1 3 4.2 2.1 2.3 1.8 PADS  2.2  3.3  5.7  3.3  2.3  1.2  PADS 2.2 3.3 5.7 3.3 2.3 1.2 It  can  be  seen  that  the  FP  rate  of  the  three  approaches  under  different  moving  speeds  is  It can be seen that the FP rate of the three approaches under different moving speeds is relatively relatively stable and keeps at a low level. It indicates that the probability of false alarm of the three  stable and keeps at a low level. It indicates that the probability of false alarm of the three approaches is approaches is low when detecting human motion.   low when detecting human motion. As can be seen from Figure 12, the PD of the three approaches all suffer a decrease when the  As can be seen from Figure 12, the PD of the three approaches all suffer a decrease when the moving speed becomes slower. The performance of human detection can be affected by different  moving speed becomes slower. The performance of human detection can be affected by different moving  speeds,  but  the  overall  detection  performance  can  meet  the  requirement  of  security  in a  moving speeds, but the overall detection performance can meet the requirement of security in a regular regular smart home environment.  smart home environment. RDFID FRID PADS 1.5 0.7 0.2 Moving speed (m/s) Figure 12. PD of human detection under different moving speeds. Figure 12. PD of human detection under different moving speeds.  6. Discussion  We did a set of evaluations in this work and demonstrated the effectiveness of RDFID to detect  human  motion  of  different  moving  patterns  using  WiFi  signals.  However,  there  are  still  some  PD (%) Appl. Sci. 2019, 9, 175 15 of 17 6. Discussion We did a set of evaluations in this work and demonstrated the effectiveness of RDFID to detect human motion of different moving patterns using WiFi signals. However, there are still some limitations in RDFID. In this section, we will give a discussion about the limitations and potentials of RDFID. Although the approach can achieve a high intrusion detection accuracy, it may be influenced by several factors. First, the relative location of the intruder and transceivers can affect the detection accuracy. There exists a relationship between the impact of the intruder to the signal transmission and the distance of the intruder to the transceivers. When the intruder moves far away from the transceivers or the first Fresnel zone, it becomes more difficult to extract effective features from the collected CSI of the ambient wireless signal. As a result, the detection accuracy suffers a degradation when the distance of the intruder to the transceivers. In addition, in real scenarios there may exist more than one intruder. Nevertheless, the movement of multiple intruders will break the periodicity of the received CSI. In consequence, the detection performance will be affected directly. Despite these limitations, WiFi signal-based intrusion detection systems have much potential in a smart home. In our future work, we will explore more effective features that less affected as much by the distance of the intruder to the transceivers and the number of the intruders in the environment to make the approach more robust in smart home applications. 7. Conclusions In this paper, we propose RDFID, a robust device-free passive intrusion detection approach. The moving pattern of the intruder has less influence to the detection performance of RDFID. Furthermore, the detection accuracy can achieve a high level without re-calibration when the scenario has changed. It only need commodity off-the-shelf (COTS) WiFi devices, and extract fine-grained channel state information from the physical layer of the wireless network. The time-frequency analysis technique is utilized to extract the features that are affected less by the environment from the frequency domain. As a result, the performance of RDFID is less affected by the moving pattern of the intruder and the different indoor scenarios. In order to evaluate the effectiveness of RDFID, a set of experiments were conducted from several perspectives. The results demonstrate that RDFID can achieve a high performance of intrusion detection, and can meet the security requirement in a smart home. Author Contributions: Conceptualization, J.L. and D.M.; Funding acquisition, W.Y.; Methodology, J.L.; Project administration, W.Y.; Supervision, D.M.; Validation, L.G. and M.Y.; Writing—original draft, J.L.; Writing—review & editing, W.Y. and X.D. Funding: This research was funded by the National Natural Science Foundation of China, grant number 6177010612 and 61831007, and Natural Science Foundation of Tianjin, grant number No. 18JCQNJC69900. Conflicts of Interest: The authors declare no conflict of interest. References 1. Youssef, M.; Mah, M.; Agrawala, A. Challenges: Device-free passive localization for wireless environments. In Proceedings of the 13th annual ACM international conference on Mobile computing and networking, Montreal, QC, Canada, 9–14 September 2007; pp. 222–229. 2. Du, X.; Chen, H. Security in wireless sensor networks. IEEE Wirel. Commun. 2008, 15, 60–66. 3. Du, X.; Guizani, M.; Xiao, Y.; Chen, H. 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Published: Jan 5, 2019

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