III.EMPIRICAL STUDY We conduct empirical studies to investigate the RF-signal features of the spinning antenna,by using an RFID reader,a ntenna circularly polarized antenna,a linearly polarized antenna and LInear polarization a passive UHF RFID tag.The experiment setup is shown in Fig.2.For the global coordinate system (GCS),the origin O 1a2 B is set to the center of the antenna,the Z-axis is vertically 100cm straight up,the Y-axis is along the spin axis,and the GCS Polarization angle Spin axis is a right-hand system.Specifically,for the linearly polarized antenna,the polarization angle,denoted as o,is defined as the Fig.2.Empirical study setup. incline angle between the X-axis and the linear polarization phase values,and more sensitive orientation information based plane.The polarization direction is defined as the direction on the distinctive RSSI variation.Thus,we prefer the linearly of intersection between the linear polarization plane and the polarized antenna in our work. antenna plane.For simplicity,we use the polarization angle to B. RSSI Variation Pattern of Linearly Polarized Antenna indicate the spin angle of the antenna during the spinning,i.e., Observation 2.For the linearly polarized antenna,the mis- when the linear polarization plane of the antenna is vertical matching direction,corresponding to the minimum RSSI value, as the blue plane in Fig.2.the polarization angle is 900,and is more distinctive for the estimation of the tag orientation the spin angle equals 90.As a dipole,the tag is regarded as than the matching direction,corresponding to the maximum a line and depicted with the 3D orientation of the line. RSSI value.We move the tag from position A (0,200,0)to A.Signal Features between Different Antennas position B(-100,200,0)with the same orientation,and spin Observation 1.During the spinning process,in comparison the linearly polarized antenna to interrogate the tag,the results to the circularly polarized antenna,the phase variation of are plotted in Fig.3(c).By comparing Fig.3(c)with Fig.3(b). the linearly polarized antenna is more stable,and the RSSI it is found that the two RSSI variation patterns of the tag at variation of the linearly polarized antenna is more distinctive. different positions share the similar shape,consisting of two As shown in Fig.2,we put a tag at position A(0,200,0), arches,but they have quite different absolute values.The peak which is parallel to the XZ-plane with the included angle of the arch refers to the perfectly matching polarization direc- against the X-axis (or Z-axis)of 60(or 30),then spin tion with the tag orientation,called the matching direction,and the circular/linearly polarized antenna around the Y-axis to the valley of the arch refers to the mismatching polarization interrogate the tag,separately.We compare the RF-signal direction with the tag orientation,called the mismatching variation of the tag from the circularly polarized antenna with direction.The mismatching direction in Fig.3(b)is 153, that from the linearly polarized antenna,and plot the results which is almost orthogonal to the tag orientation when the in Fig.3(a)and Fig.3(b).It is found that the phase value tag is at position A.But the mismatching direction changes to from the circularly polarized antenna is linearly changing with 166 in Fig.3(c).It indicates that even if the tag orientation is the antenna spinning,while that from the linearly polarized unchanged,the mismatching direction will change according antenna almost keeps unchanged.For the circularly polarized to the position of the tag.Moreover,as shown in Fig.3(c),the antenna,the polarization direction rotates electronically,which RSSI values are similar around the matching direction,while can be regarded as the summation of multiple RF-signals with they are more distinctive around the mismatching direction. different polarization directions and initial phase values.The Hence,we use the mismatching direction to estimate the tag matches the RF-signal with the same polarization direction tag orientation instead of the matching direction,which is to harvest the maximum energy,thus,the phase value of the relatively difficult to deduce. tag changes during the spinning.For the linearly polarized C.Phase Variation Pattern of Linearly Polarized Antenna antenna,it has only one polarization direction with one initial Observation 3.For the linearly polarized antenna,the phase value,so the phase value of the tag is independent phase value keeps stable when the polarization direction of the of the spinning angle and keeps stable.Besides,the RSSI antenna matches the tag orientation perfectly;and the phase values from the two types of antennas are both changing value fluctuates when the polarization direction mismatches periodically with the spinning antenna,but the RSSI variation the tag orientation due to the multi-path effect.We place range from the linearly polarized antenna is much larger,i.e., the tag at position A vertically or horizontally,i.e.,the tag 23dB larger than that from the circularly polarized antenna. orientation is along the Z-axis or the X-axis,and spin the For the circularly polarized antenna,the RSSI reduction is linearly polarized antenna to interrogate the tags with different due to the different antenna gains along different directions. orientations,respectively.We conduct the experiments in an For the linearly polarized antenna,the RSSI reduction is due open lobby,where the antenna is at the height of 150cm and to the mismatch of the linear polarization direction with the 500cm away from the surrounding walls,so the horizontal tag orientation,which leads to large energy loss and causes multi-path effect is relatively smaller than the vertical one more distinctive RSSI variation.Therefore,compared to the As shown in Fig.3(d),the phase value of the vertical tag circularly polarized antenna,the linearly polarized antenna can almost keeps unchanged,while that of the horizontal tag provide more reliable position information based on the stable changes to a certain extent,especially when the tag orientation 3III. EMPIRICAL STUDY We conduct empirical studies to investigate the RF-signal features of the spinning antenna, by using an RFID reader, a circularly polarized antenna, a linearly polarized antenna and a passive UHF RFID tag. The experiment setup is shown in Fig. 2. For the global coordinate system (GCS), the origin O is set to the center of the antenna, the Z-axis is vertically straight up, the Y -axis is along the spin axis, and the GCS is a right-hand system. Specifically, for the linearly polarized antenna, the polarization angle, denoted as φ, is defined as the incline angle between the X-axis and the linear polarization plane. The polarization direction is defined as the direction of intersection between the linear polarization plane and the antenna plane. For simplicity, we use the polarization angle to indicate the spin angle of the antenna during the spinning, i.e., when the linear polarization plane of the antenna is vertical as the blue plane in Fig. 2, the polarization angle is 90◦ , and the spin angle equals 90◦ . As a dipole, the tag is regarded as a line and depicted with the 3D orientation of the line. A. Signal Features between Different Antennas Observation 1. During the spinning process, in comparison to the circularly polarized antenna, the phase variation of the linearly polarized antenna is more stable, and the RSSI variation of the linearly polarized antenna is more distinctive. As shown in Fig. 2, we put a tag at position A (0, 200, 0), which is parallel to the XZ-plane with the included angle against the X-axis (or Z-axis) of 60◦ (or 30◦ ), then spin the circular/linearly polarized antenna around the Y -axis to interrogate the tag, separately. We compare the RF-signal variation of the tag from the circularly polarized antenna with that from the linearly polarized antenna, and plot the results in Fig. 3(a) and Fig. 3(b). It is found that the phase value from the circularly polarized antenna is linearly changing with the antenna spinning, while that from the linearly polarized antenna almost keeps unchanged. For the circularly polarized antenna, the polarization direction rotates electronically, which can be regarded as the summation of multiple RF-signals with different polarization directions and initial phase values. The tag matches the RF-signal with the same polarization direction to harvest the maximum energy, thus, the phase value of the tag changes during the spinning. For the linearly polarized antenna, it has only one polarization direction with one initial phase value, so the phase value of the tag is independent of the spinning angle and keeps stable. Besides, the RSSI values from the two types of antennas are both changing periodically with the spinning antenna, but the RSSI variation range from the linearly polarized antenna is much larger, i.e., 23dB larger than that from the circularly polarized antenna. For the circularly polarized antenna, the RSSI reduction is due to the different antenna gains along different directions. For the linearly polarized antenna, the RSSI reduction is due to the mismatch of the linear polarization direction with the tag orientation, which leads to large energy loss and causes more distinctive RSSI variation. Therefore, compared to the circularly polarized antenna, the linearly polarized antenna can provide more reliable position information based on the stable z � � � Linear polarization Antenna � � Tag � � Polarization angle Spin axis Fig. 2. Empirical study setup. phase values, and more sensitive orientation information based on the distinctive RSSI variation. Thus, we prefer the linearly polarized antenna in our work. B. RSSI Variation Pattern of Linearly Polarized Antenna Observation 2. For the linearly polarized antenna, the mis￾matching direction, corresponding to the minimum RSSI value, is more distinctive for the estimation of the tag orientation than the matching direction, corresponding to the maximum RSSI value. We move the tag from position A (0, 200, 0) to position B (−100, 200, 0) with the same orientation, and spin the linearly polarized antenna to interrogate the tag, the results are plotted in Fig. 3(c). By comparing Fig. 3(c) with Fig. 3(b), it is found that the two RSSI variation patterns of the tag at different positions share the similar shape, consisting of two arches, but they have quite different absolute values. The peak of the arch refers to the perfectly matching polarization direc￾tion with the tag orientation, called the matching direction, and the valley of the arch refers to the mismatching polarization direction with the tag orientation, called the mismatching direction. The mismatching direction in Fig. 3(b) is 153◦ , which is almost orthogonal to the tag orientation when the tag is at position A. But the mismatching direction changes to 166◦ in Fig. 3(c). It indicates that even if the tag orientation is unchanged, the mismatching direction will change according to the position of the tag. Moreover, as shown in Fig. 3(c), the RSSI values are similar around the matching direction, while they are more distinctive around the mismatching direction. Hence, we use the mismatching direction to estimate the tag orientation instead of the matching direction, which is relatively difficult to deduce. C. Phase Variation Pattern of Linearly Polarized Antenna Observation 3. For the linearly polarized antenna, the phase value keeps stable when the polarization direction of the antenna matches the tag orientation perfectly; and the phase value fluctuates when the polarization direction mismatches the tag orientation due to the multi-path effect. We place the tag at position A vertically or horizontally, i.e., the tag orientation is along the Z-axis or the X-axis, and spin the linearly polarized antenna to interrogate the tags with different orientations, respectively. We conduct the experiments in an open lobby, where the antenna is at the height of 150cm and 500cm away from the surrounding walls, so the horizontal multi-path effect is relatively smaller than the vertical one. As shown in Fig. 3(d), the phase value of the vertical tag almost keeps unchanged, while that of the horizontal tag changes to a certain extent, especially when the tag orientation 3