IEEE TRANSACTIONS ON MOBILE COMPUTING,VOL.XX,NO.XX,2020 10 60 cm 0 Touch position(cm) Touch position(cm) Touch position(cm) (a)Swipe with different settings (b)Swipe with different distances (c)Swipe with different angles (d)Swipe with different users Fig.11.RSSI deviation during swipe Antenna Transmitted Error of toucl signal posifon Is de 30 de -45de Distance difference 2 6 ，Reflection Touch position(cm)】 Touch position(cm) Touch position(cm) signal Fig.12.RSSI deviation during Fig.13.Error of touch position Fig.14.Phase deviation during Fig.15.Distance difference during swipe with different scenarios swipe with different angles swipe distances and users.While for different angles,the tag is not deviation as the phase difference between the phase values parallel to the antenna plane,the hand would move away when a tag is or not touched,denoted as Aphase.When from or towards the antenna,thus the power of reflection the tag orientation gets changed,the phase deviation does signals from the hand decreases with the larger distance or not keep steady.Fig.14 plots the phase variation patterns increases with the smaller distance.But thankfully,as shown during the swipe when putting the tag with different angles, in Fig.11(c),such interference is acceptable as long as a tag and these patterns are different from each other.It means can be always activated during the whole swipe.When the that if using the pattern of 0 as the template,we cannot angle gets larger,the tag receives less power from signals, determine the accurate touch position when the angle is 45. it is easy for the tag not to be activated.In our settings,if This phenomenon is likely to be caused by the difference of increasing the angle after 60,the tag cannot continuously the reflection signals from the moving hand.As shown in response to the reader during the swipe.As we deploy one Fig.15,the signal received by the tag contains not only the antenna pair orthogonally to track the rigid motion in Fig.1, transmitted signals from the antenna,but also the reflection the polarization angle between the tag to one of the antenna signals from the surrounding objects,among which the pair is within 45 in the central scanning area,the tag can be hand touching the tag is the main reflector.When the tag is interrogated by at least one antenna,thus we think the RSSI not parallel to the antenna plane,the distance between the deviation is insensitive to the angle. hand and the antenna changes during the swipe process, Although in one scenario,i.e.,a certain tagged object in so the reflection signal from the hand to the tag changes a certain environment,the RSSI deviation is irrelevant to the a bit as well.The larger angle between the tag orientation distance or the user and insensitive to the angle,the static and the antenna plane,the larger distance difference of the multi-path effect in different scenarios differs and influences reflection signal from the moving hand.As the range of the the RSSI variation pattern,as shown in Fig.12.In Scenario 1, distance variation is just several centimeters,the strength of a tag is attached to a paper box in an office room by default, the reflection signal can be viewed unchanged.However,as while we attach the tag to a foam box in Scenario 2 and do mentioned above,the phase is very sensitive to the distance, the experiment in a hall in Scenario 3.It is observed that the so the slight distance difference of the moving hand can ranges of RSSI patterns are different.If using the smoothed lead to the significant phase difference of the reflection pattern of Scenario 1 as template,we cannot always locate signal,which further affects the tag signals collected by the the touch position accurately in other scenarios,as shown antenna.If using the phase,we need to determine the tag in Fig.13.Also,as the RSSI deviation pattern is symmetric, orientation first,then generate the pattern templates for each we would get two candidate touch positions centered of the tag orientation,adding much burden for the training and tag center based on one RSSI deviation value.To tackle this detection.Meanwhile,the amplitude of the phase variance ambiguity,we propose to use two tag antennas for different due to the touch is much smaller than the RSSI variance,so functions,so we can get the unique touch position,details the RSSI deviation is more significant and more robust to are shown in Section 7.Therefore,to ensure the accuracy of the environment interference.Therefore,we select the RSSI the touch position,we need to generate templates for each deviation to detect the touch gesture. scenario,but for the same scenario,one general template is satisfactory for different distances,angles or users. 6.3 Model of RSSI Deviation due to Touch Gesture The power received by the receiver from the transmitter 6.2 Phase Deviation during Touch Gesture can be described by the Friis equation [32].Denote the Compared with the RSSI,the phase is a more sensitive received and transmit power as PR and Pr,the receiver gain parameter of backscattered signals.Similarly,define phase and transmitter gain as GR and Gr,the polarization angleIEEE TRANSACTIONS ON MOBILE COMPUTING, VOL. XX, NO. XX, 2020 10 80cm � � � 100 200 � � 60 � � Antenna � (a) Swipe with different settings 0 2 4 6 8 10 Touch position (cm) -30 -20 -10 0 RSSI (dBm) 60 cm 80 cm 100 cm 120 cm (b) Swipe with different distances 0 2 4 6 8 10 Touch position (cm) -30 -20 -10 0 RSSI (dBm) 0 deg 15 deg 30 deg 45 deg 60 deg (c) Swipe with different angles 0 2 4 6 8 10 Touch position (cm) -30 -20 -10 0 RSSI (dBm) User1 User2 User3 (d) Swipe with different users Fig. 11. RSSI deviation during swipe 0 2 4 6 8 10 Touch position (cm) -30 -20 -10 0 RSSI (dBm) Scenario 1 Scenario 2 Scenario 3 Fig. 12. RSSI deviation during swipe with different scenarios 0 2 4 6 8 10 Touch position (cm) -30 -20 -10 0 RSSI (dBm) Template Test Error of touch position True position Candidate position 1 Candidate position 2 Fig. 13. Error of touch position distances and users. While for different angles, the tag is not parallel to the antenna plane, the hand would move away from or towards the antenna, thus the power of reflection signals from the hand decreases with the larger distance or increases with the smaller distance. But thankfully, as shown in Fig. 11(c), such interference is acceptable as long as a tag can be always activated during the whole swipe. When the angle gets larger, the tag receives less power from signals, it is easy for the tag not to be activated. In our settings, if increasing the angle after 60◦ , the tag cannot continuously response to the reader during the swipe. As we deploy one antenna pair orthogonally to track the rigid motion in Fig. 1, the polarization angle between the tag to one of the antenna pair is within 45◦ in the central scanning area, the tag can be interrogated by at least one antenna, thus we think the RSSI deviation is insensitive to the angle. Although in one scenario, i.e., a certain tagged object in a certain environment, the RSSI deviation is irrelevant to the distance or the user and insensitive to the angle, the static multi-path effect in different scenarios differs and influences the RSSI variation pattern, as shown in Fig. 12. In Scenario 1, a tag is attached to a paper box in an office room by default, while we attach the tag to a foam box in Scenario 2 and do the experiment in a hall in Scenario 3. It is observed that the ranges of RSSI patterns are different. If using the smoothed pattern of Scenario 1 as template, we cannot always locate the touch position accurately in other scenarios, as shown in Fig. 13. Also, as the RSSI deviation pattern is symmetric, we would get two candidate touch positions centered of the tag center based on one RSSI deviation value. To tackle this ambiguity, we propose to use two tag antennas for different functions, so we can get the unique touch position, details are shown in Section 7. Therefore, to ensure the accuracy of the touch position, we need to generate templates for each scenario, but for the same scenario, one general template is satisfactory for different distances, angles or users. 6.2 Phase Deviation during Touch Gesture Compared with the RSSI, the phase is a more sensitive parameter of backscattered signals. Similarly, define phase 0 2 4 6 8 10 Touch position (cm) -5 -4 -3 -2 -1 0 Phase (rad.) 0 deg 15 deg 30 deg 45 deg Fig. 14. Phase deviation during swipe with different angles Antenna Setting1 Left (0cm) Right (9.5cm) Distance difference Transmitted signal Reflection signal Fig. 15. Distance difference during swipe deviation as the phase difference between the phase values when a tag is or not touched, denoted as ∆phase. When the tag orientation gets changed, the phase deviation does not keep steady. Fig. 14 plots the phase variation patterns during the swipe when putting the tag with different angles, and these patterns are different from each other. It means that if using the pattern of 0 ◦ as the template, we cannot determine the accurate touch position when the angle is 45◦ . This phenomenon is likely to be caused by the difference of the reflection signals from the moving hand. As shown in Fig. 15, the signal received by the tag contains not only the transmitted signals from the antenna, but also the reflection signals from the surrounding objects, among which the hand touching the tag is the main reflector. When the tag is not parallel to the antenna plane, the distance between the hand and the antenna changes during the swipe process, so the reflection signal from the hand to the tag changes a bit as well. The larger angle between the tag orientation and the antenna plane, the larger distance difference of the reflection signal from the moving hand. As the range of the distance variation is just several centimeters, the strength of the reflection signal can be viewed unchanged. However, as mentioned above, the phase is very sensitive to the distance, so the slight distance difference of the moving hand can lead to the significant phase difference of the reflection signal, which further affects the tag signals collected by the antenna. If using the phase, we need to determine the tag orientation first, then generate the pattern templates for each tag orientation, adding much burden for the training and detection. Meanwhile, the amplitude of the phase variance due to the touch is much smaller than the RSSI variance, so the RSSI deviation is more significant and more robust to the environment interference. Therefore, we select the RSSI deviation to detect the touch gesture. 6.3 Model of RSSI Deviation due to Touch Gesture The power received by the receiver from the transmitter can be described by the Friis equation [32]. Denote the received and transmit power as PR and PT , the receiver gain and transmitter gain as GR and GT , the polarization angle