-3400 120 0 3600 -3800 81008200 830 -4000 4200 7800 7900800081008200 x30 -4400 hmVI是 102030 40 4500 5000 780079008000810082008300 Grid X Inphase (a)Histogram of signal in I-Q plane (b)Signal in I-Q plane and the state division (C)Extracted R of three tags Fig.9:Extract the physical-layer features from 3-collision signal for the parallelogram among first five states with smaller D.Case Study amplitudes.There are only two possible choices:1)For the Fig.9 is an example of extracting the physical-layer features parallelogram case we can use Ist~4th state to constitute from a 3-collision slot.Firstly,we cluster the samples into a parallelogram.2)For the plane tetrahedron case,we can eight clusters based on the density function in I-Q plane as replace 4th state with 5th and use 1st~3rd and 5th state to shown in Fig.9(a).In the following we denote each state constitute a parallelogram. as the amplitude rank of states in I-Q plane.Secondly,we To decide the correct choice,we leverage the property that determine state So from continuous wave and state S7 from the opposite sides of parallelogram are parallel and equal in the preamble as shown in Fig.9(b).Thirdly,we search for the length.For each choice there are three possible edge pairs. parallelogram based on the first 5 states.The left quadrilateral If we use the rank of state to represent the vertex,the three in Fig.9(b)is the estimated parallelogram from the first 5 pairs are (12,34).(13,24)and (14,23)in choice 1,We use states and the right one is the symmetrical one.Fourthly,we the similarity among the edge pairs to express the likelihood calculate the channel coefficients including the phase profiles of constituting a parallelogram as: as the arrow in the figure.Lastly,we recover the RN16 signal a.b of each tag according to Eq.(7).Here,we use"1"to represent Sim= 1a×阿（园-例 (8)high voltage and"-1"to represent low voltage.We compute the signal length of RN16 as the indicator of BLF based on where (a,b)are the possible edge pair.The first part of Eq(8) cross-correlation values.Fig.9(c)shows the ending part of is the cosine value of the edge pair and the second part is the three separated RN16 signals and points out the slide window reciprocal of the length difference.We choose the edge pair which has the maximum cross-correlation value. with the maximum similarity as the corresponding opposite VI.DETECT THE MOVING TAGS side of the parallelogram.Then we can resolve the vertex A.Motivation and Approaches sequence of the parallelogram based on the similarity. After we extract the phase profiles from the collision signals, Lastly.we measure the channel coefficients based on the we demonstrate how to detect the moving tags by using the geometric construction.The parallelogram will always include phase profiles.Since the phase profile always changes even if either state So or S7 because they are symmetric.Hence,we the tag is moved a small distance,the basic idea is to compare can measure the two channel coefficients in the parallelogram the updated phase profiles with the stationary phase profiles. according to the states So and S7 similar as in the 2-collision Suppose we need to monitor N tags,we obtain the stationary problem.Then the channel coefficient of the 3rd tag can be phase profiles of the N tags in the tag inventory phase.We easily computed based on So,S7 and the first two channel use a vector P =(01,02,...,ON)to represent the phase coefficients,since S7 contains the channel coefficients of three distribution,where is the ith phase profile in distribution tags and So contains none. P.Then in the each cycle of the continuous polling phase,we obtain an updated phase distribution P=(,2,...). C.Measure the Physical-Layer Features from Collision Signal We compare p with P to detect the moving tags in each After extracting the channel coefficients from the collision polling cycle. slot,we can directly compute the phase profile based on Eq. Traditional C1G2 protocol usually costs tens of seconds (3).For backscatter link frequency (BLF),we utilize Eq.(7) to obtain the tag information,which can build the phase to recover the RN16 signal of each tag.It is difficult to decode distribution P'.It is inefficient due to the collision problem these RN16 signals into complete binary sequences due to the and the long EPC-ID signal.Instead,we can extract the phase ambient noise.But according to the special encoded pattern of distribution P'only from the RN16 signal of collision signal preamble and"dummy I"as shown in Fig.5(a),we can decide Hence,we propose a Fast Tag Polling Scheme (FTPS)by the starting and ending point of each RN16 signal using cross- suppressing the transmitting of EPC-ID signal with a new correlation.Here we adopt the same cross-correlation process command OrepSup.The QrepSup command is used to respond as in Section IV-C to decide the signal length.Then we use the RN16 signal of the tags.It not only starts the next slot like the signal length as the indicator of BLF. the Qrep command in C1G2,but also makes the tags,whichGrid X 10 20 30 40 50 Grid Y 10 20 30 40 50 0 20 40 60 80 100 120 (a) Histogram of signal in I-Q plane Inphase 4000 4500 5000 Quadrature -4600 -4400 -4200 -4000 -3800 -3600 -3400 S0 S7 (b) Signal in I-Q plane and the state division Amplitude 7800 7900 8000 8100 8200 8300 -2 0 2 Amplitude 7800 7900 8000 8100 8200 8300 -2 0 2 Sample Counts Amplitude 7800 7900 8000 8100 8200 8300 -2 0 2 (c) Extracted RN16 signals of three tags Fig. 9: Extract the physical-layer features from 3-collision signal for the parallelogram among first five states with smaller amplitudes. There are only two possible choices: 1) For the parallelogram case we can use 1st∼4th state to constitute a parallelogram. 2) For the plane tetrahedron case, we can replace 4th state with 5th and use 1st∼3rd and 5th state to constitute a parallelogram. To decide the correct choice, we leverage the property that the opposite sides of parallelogram are parallel and equal in length. For each choice there are three possible edge pairs. If we use the rank of state to represent the vertex, the three pairs are (ı12, ı34), (ı13, ı24) and (ı14, ı23) in choice 1, We use the similarity among the edge pairs to express the likelihood of constituting a parallelogram as: Sim = # # # # # ⃗a ·⃗b |⃗a| × | ⃗b| × 1 (|⃗a| − | ⃗b|) # # # # # , (8) where (⃗a,⃗b) are the possible edge pair. The first part of Eq(8) is the cosine value of the edge pair and the second part is the reciprocal of the length difference. We choose the edge pair with the maximum similarity as the corresponding opposite side of the parallelogram. Then we can resolve the vertex sequence of the parallelogram based on the similarity. Lastly, we measure the channel coefficients based on the geometric construction. The parallelogram will always include either state S0 or S7 because they are symmetric. Hence, we can measure the two channel coefficients in the parallelogram according to the states S0 and S7 similar as in the 2-collision problem. Then the channel coefficient of the 3rd tag can be easily computed based on S0, S7 and the first two channel coefficients, since S7 contains the channel coefficients of three tags and S0 contains none. C. Measure the Physical-Layer Features from Collision Signal After extracting the channel coefficients from the collision slot, we can directly compute the phase profile based on Eq. (3). For backscatter link frequency (BLF), we utilize Eq. (7) to recover the RN16 signal of each tag. It is difficult to decode these RN16 signals into complete binary sequences due to the ambient noise. But according to the special encoded pattern of preamble and “dummy 1” as shown in Fig. 5(a), we can decide the starting and ending point of each RN16 signal using cross￾correlation. Here we adopt the same cross-correlation process as in Section IV-C to decide the signal length. Then we use the signal length as the indicator of BLF. D. Case Study Fig. 9 is an example of extracting the physical-layer features from a 3-collision slot. Firstly, we cluster the samples into eight clusters based on the density function in I-Q plane as shown in Fig. 9(a). In the following we denote each state as the amplitude rank of states in I-Q plane. Secondly, we determine state S0 from continuous wave and state S7 from the preamble as shown in Fig. 9(b). Thirdly, we search for the parallelogram based on the first 5 states. The left quadrilateral in Fig. 9(b) is the estimated parallelogram from the first 5 states and the right one is the symmetrical one. Fourthly, we calculate the channel coefficients including the phase profiles as the arrow in the figure. Lastly, we recover the RN16 signal of each tag according to Eq.(7). Here, we use “1” to represent high voltage and “-1” to represent low voltage. We compute the signal length of RN16 as the indicator of BLF based on cross-correlation values. Fig. 9(c) shows the ending part of three separated RN16 signals and points out the slide window which has the maximum cross-correlation value. VI. DETECT THE MOVING TAGS A. Motivation and Approaches After we extract the phase profiles from the collision signals, we demonstrate how to detect the moving tags by using the phase profiles. Since the phase profile always changes even if the tag is moved a small distance, the basic idea is to compare the updated phase profiles with the stationary phase profiles. Suppose we need to monitor N tags, we obtain the stationary phase profiles of the N tags in the tag inventory phase. We use a vector P = ⟨θ1, θ2, ··· , θN ⟩ to represent the phase distribution, where θi is the ith phase profile in distribution P. Then in the each cycle of the continuous polling phase, we obtain an updated phase distribution P′ = ⟨θ′ 1, θ′ 2, ··· , θ′ N ⟩. We compare P′ with P to detect the moving tags in each polling cycle. Traditional C1G2 protocol usually costs tens of seconds to obtain the tag information, which can build the phase distribution P′ . It is inefficient due to the collision problem and the long EPC-ID signal. Instead, we can extract the phase distribution P′ only from the RN16 signal of collision signal. Hence, we propose a Fast Tag Polling Scheme (FTPS) by suppressing the transmitting of EPC-ID signal with a new command QrepSup. The QrepSup command is used to respond the RN16 signal of the tags. It not only starts the next slot like the Qrep command in C1G2, but also makes the tags, which