4000 that.as the distance between the tag and the reader increases from 50 cm to 150 cm,the RSSI decreases rapidly;when the distance further increases,the RSSI then decreases slowly. Moreover,in regard to a certain distance,the RSSI from the 1000 tag always reaches the maximum value when the antenna is 0事n市男0 directly facing towards the tag.As we further increase the 150-105005010150 offset degree in rotation,the RSSI gradually decreases.This is Depth (cm) The horizontal coordinate:x (cm) because the antenna outputs the maximum transmitting power (a)The depth histogram of multiple (b)The depth value of objects in in the central area of the beam,and thus the RSSI of the objects different horizontal lines Figure 3.Experiment results of depth value backscattered RF-signals reaches the maximum value when the tag is in the center.As the tag's position is deviated from that,for each horizontal line,the depth values of the object the center of the antenna beam,the RSSI of the backscattered keep nearly constant,with rather small deviation;for different RF-signals thus decreases.We call the position of achieving horizontal lines,these depth values have obvious variations. the peak value in RSSI the perpendicular point,since the Due to the limitation of the Kinect's view,the Kinect has perpendicular bisector of the RFID antenna crosses this point. smaller view angle in closer distance.This observation implies that,the depth value collected from the depth cameras depicts the vertical distance rather than the absolute distance between the objects and the depth camera. Depth Feature Extraction To extract the depth of specified objects from the depth his- togram of multiple objects,we set a threshold t to detect the -30-20-i0o9n020 peaks in regard to the number of pixels.We thus iterate from Figure 4.The variation of RSSI via rotating the RFID antenna the minimum depth to the maximum depth in the histogram. Although the RSSI can only be used to measure the vertical if the number of pixels for a certain depth is larger than t,we distance between the tag and the antenna in a coarse granu identify it as a peak p(di,n;)with the depth d;and the number larity,nevertheless,with different offset degrees from the tag of pixels ni.In order to address the multiple-peaks problem to the center of antenna beam,the RSSI changes in a convex of irregularly-shaped objects,we set another threshold Ad.If curve with the peak value at the perpendicular point.We can the differences of these peaks'depth values are smaller than further leverage this property to differentiate the positions of Ad,we then combine them as one peak.Both the value of various objects in the horizontal aspect. t and Ad are selected based on the empirical value from a Extract the Phase Value from RF-Signals number of experimental studies (t=200 and Ad=10cm in our Background implementation).Then,each peak actually represents a speci- fied object.For each peak,we respectively find the leftmost Phase is a basic attribute of a signal along with amplitude and frequency.The phase value of an RF signal describes the depth d and the rightmost depth d,with the number of pixels degree that the received signal offsets from the sent signal, n>0.We then compute the average depth for the specified ranging from 0 to 360 degrees.Let d be the distance between object as follows:d(d).The average depth the RFID antenna and the tag,the signal traverses a round- is calculated in a weighted average approach according to the trip with a distance of 2d in each backscatter communication. number of pixels for each depth around the peak. Therefore,the phase value 0 output by the RFID reader can be expressed as [25,4]: Extract the Received Signal Strength from RF-signals The received signal strength(RSSI)measures the power of 2π =（元x2+四 mod2π. (1) received radio signal,which is inversely proportional to the distance between the tag and the reader.However,according to where A is the wave length.Besides the RF phase rotation the previous study [34],the RSSI is impacted by various issues over distance,the reader's transmitter,the tag's reflection char- like multi-path effect,and path loss,etc.This indicates that acteristic,and the reader's receiver will also introduce some the RSSI does not always have a monotonic relationship with additional phase rotation,denoted as er,Og and erAG respec- the distance.Therefore,with the RSSI from a specified tag, tively.We use u =er+eg+erac to denote this diversity the RFID system can roughly estimate the distance between term in Eq.(1).Since u is rather stable according to the previ- the reader and the tag. ous results [321.and it is only related to the physical properties Experiment Observations of the specified tag-antenna pair,we can record u for different It is found that,inside the RFID antenna's effective scanning tags in advance.Then,according to each tag's response,we can calibrate the phase value by offsetting the diversity term. range,the RSSI from the tag is also impacted by its position Thus,the phase value can be used as an accurate and stable offset from the center of the antenna beam.In order to validate metric to measure distance. the above judgment,we separate the RFID reader and the tag with a distance d,and then we evaluate the average RSSI value Estimate the Vertical Distance from Phase Value by gradually rotating the antenna from an offset degree of According to the definition in Eq.(1),the phase value is a -40 to +40.Figure 4 shows the experiment results.We find periodical function of the distance.Hence,given a specifiedDepth (cm) 100 150 200 250 Number of pixels 0 1000 2000 3000 4000 Depth value:x(cm) 140 145 150 155 160 Number of pixels 0 100 200 300 400 500 Noise A B C (a) The depth histogram of multiple objects The horizontal coordinate: x (cm) -150 -100 -50 0 50 100 150 Depth(cm) 0 50 100 150 200 250 300 (b) The depth value of objects in different horizontal lines Figure 3. Experiment results of depth value that, for each horizontal line, the depth values of the object keep nearly constant, with rather small deviation; for different horizontal lines, these depth values have obvious variations. Due to the limitation of the Kinect’s view, the Kinect has smaller view angle in closer distance. This observation implies that, the depth value collected from the depth cameras depicts the vertical distance rather than the absolute distance between the objects and the depth camera. Depth Feature Extraction To extract the depth of specified objects from the depth his￾togram of multiple objects, we set a threshold t to detect the peaks in regard to the number of pixels. We thus iterate from the minimum depth to the maximum depth in the histogram, if the number of pixels for a certain depth is larger than t, we identify it as a peak p(di ,ni) with the depth di and the number of pixels ni . In order to address the multiple-peaks problem of irregularly-shaped objects, we set another threshold ∆d. If the differences of these peaks’ depth values are smaller than ∆d, we then combine them as one peak. Both the value of t and ∆d are selected based on the empirical value from a number of experimental studies (t=200 and ∆d=10cm in our implementation). Then, each peak actually represents a speci- fied object. For each peak, we respectively find the leftmost depth dl and the rightmost depth dr with the number of pixels nr > 0. We then compute the average depth for the specified object as follows: d = ∑ r i=l (di × ni ∑ r i=l ni ). The average depth is calculated in a weighted average approach according to the number of pixels for each depth around the peak. Extract the Received Signal Strength from RF-signals The received signal strength (RSSI) measures the power of received radio signal, which is inversely proportional to the distance between the tag and the reader. However, according to the previous study [34], the RSSI is impacted by various issues like multi-path effect, and path loss, etc. This indicates that the RSSI does not always have a monotonic relationship with the distance. Therefore, with the RSSI from a specified tag, the RFID system can roughly estimate the distance between the reader and the tag. Experiment Observations It is found that, inside the RFID antenna’s effective scanning range, the RSSI from the tag is also impacted by its position offset from the center of the antenna beam. In order to validate the above judgment, we separate the RFID reader and the tag with a distance d, and then we evaluate the average RSSI value by gradually rotating the antenna from an offset degree of −40◦ to +40◦ . Figure 4 shows the experiment results. We find that, as the distance between the tag and the reader increases from 50 cm to 150 cm, the RSSI decreases rapidly; when the distance further increases, the RSSI then decreases slowly. Moreover, in regard to a certain distance, the RSSI from the tag always reaches the maximum value when the antenna is directly facing towards the tag. As we further increase the offset degree in rotation, the RSSI gradually decreases. This is because the antenna outputs the maximum transmitting power in the central area of the beam, and thus the RSSI of the backscattered RF-signals reaches the maximum value when the tag is in the center. As the tag’s position is deviated from the center of the antenna beam, the RSSI of the backscattered RF-signals thus decreases. We call the position of achieving the peak value in RSSI the perpendicular point, since the perpendicular bisector of the RFID antenna crosses this point. −40 −30 −20 −10 0 10 20 30 40 −70 −65 −60 −55 −50 −45 −40 −35 Rotation angle RSSI value(dBm) Distance=50cm Distance=100cm Distance=150cm Distance=200cm Figure 4. The variation of RSSI via rotating the RFID antenna Although the RSSI can only be used to measure the vertical distance between the tag and the antenna in a coarse granu￾larity, nevertheless, with different offset degrees from the tag to the center of antenna beam, the RSSI changes in a convex curve with the peak value at the perpendicular point. We can further leverage this property to differentiate the positions of various objects in the horizontal aspect. Extract the Phase Value from RF-Signals Background Phase is a basic attribute of a signal along with amplitude and frequency. The phase value of an RF signal describes the degree that the received signal offsets from the sent signal, ranging from 0 to 360 degrees. Let d be the distance between the RFID antenna and the tag, the signal traverses a round￾trip with a distance of 2d in each backscatter communication. Therefore, the phase value θ output by the RFID reader can be expressed as [25, 4]: θ = (2π λ ×2d + µ) mod 2π, (1) where λ is the wave length. Besides the RF phase rotation over distance, the reader’s transmitter, the tag’s reflection char￾acteristic, and the reader’s receiver will also introduce some additional phase rotation, denoted as θT , θR and θTAG respec￾tively. We use µ = θT + θR + θTAG to denote this diversity term in Eq. (1). Since µ is rather stable according to the previ￾ous results [32], and it is only related to the physical properties of the specified tag-antenna pair, we can record µ for different tags in advance. Then, according to each tag’s response, we can calibrate the phase value by offsetting the diversity term. Thus, the phase value can be used as an accurate and stable metric to measure distance. Estimate the Vertical Distance from Phase Value According to the definition in Eq. (1), the phase value is a periodical function of the distance. Hence, given a specified