CN110850401B - RFID label positioning method based on motion model and synthetic aperture - Google Patents

RFID label positioning method based on motion model and synthetic aperture Download PDF

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CN110850401B
CN110850401B CN201910798139.8A CN201910798139A CN110850401B CN 110850401 B CN110850401 B CN 110850401B CN 201910798139 A CN201910798139 A CN 201910798139A CN 110850401 B CN110850401 B CN 110850401B
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马永涛
刘涵凯
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Tianjin University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/06—Systems determining position data of a target
    • G01S13/46—Indirect determination of position data

Abstract

The invention relates to a method for positioning an RFID label based on a motion model and a synthetic aperture, wherein the initial position of the label, namely the position to be positioned, is p0＝[x0,y0]The speed of the label is v, and the label comprises a label motion model establishing stage, a phase unfolding stage, a phase combining stage, a fixed frequency offset eliminating stage and a steepest descent method positioning stage.

Description

RFID label positioning method based on motion model and synthetic aperture
Technical Field
The invention belongs to the technical field of RFID positioning, and aims to solve the problem of high-precision low-delay positioning of a tag by using tag phase information obtained by a reader antenna moving along a known track.
Background
With the popularization of mobile internet and smart phones, Location Based Service (LBS) has gained wide attention, driving the development of various modern navigation and positioning technologies. The well-known outdoor positioning technology GPS cannot be applied to indoor positioning due to occlusion of buildings and the like. In recent years, many indoor positioning technologies based on wireless networks, such as WiFi positioning, bluetooth positioning, ZigBee positioning, RFID positioning, and the like, have appeared. The rapid development of the internet of things technology enables the RFID technology to be widely applied to positioning, tracking and backtracking of production, logistics, medicines and the like. In the aspect of indoor positioning, compared with other positioning methods, the RFID has the characteristics of low cost, high positioning accuracy, high identification speed, strong anti-interference performance and the like, and has the advantages of non-contact, non-line-of-sight and capability of identifying and tracking multiple targets simultaneously, so that the RFID gradually becomes the first choice for indoor positioning.
The wireless positioning technology of the RFID is mainly divided into two types: one is based on a non-ranging method, usually a determined signal propagation model is not needed, a large number of reference labels are arranged in a positioning area in advance, the reference labels with similar positions are screened out after certain operation, and final positioning coordinates are determined by using the reference labels; the other method is based on distance measurement, the distances between a person and a plurality of wireless point devices are determined according to a signal propagation model, and the position of the person is determined through geometric relation transformation. The method based on the distance measurement mainly comprises the following steps: aoa (angle of arrival) Signal angle of arrival method, toa (time of arrival) Signal time of arrival method, tdoa (time Difference of arrival) Signal time Difference of arrival method, and rssi (received Signal Strength indication) received Signal Strength method. The RSSI positioning method estimates the received label by establishing a propagation model through signal strength and distance, has low power and low cost, does not need additional equipment function support, and is limited in positioning precision because signal energy information is influenced by various factors such as non-line-of-sight, multipath and the like besides distance factors in the propagation process. TOA and TDOA location methods calculate distance primarily by measuring the propagation time of electromagnetic waves, which requires or requires an accurate reference time for clock synchronization of hardware facilities. The AOA positioning method mainly measures the arrival direction of the label signal through an antenna array of a reader, and the method needs the antenna array with special functions, so that the accuracy is limited by the performance of equipment, and the cost is relatively high.
In a scene that the tag and the reader move relatively, the synthetic aperture technology can form a virtual antenna array by using the relative movement between the antenna and the target, and the position of the target is determined by coherent superposition of a plurality of sampling phase values, so that the synthetic aperture technology has good anti-noise and multi-path interference capabilities, and can improve the positioning accuracy. And the method is suitable for unique dynamic application scenes such as handheld equipment, and the like, and does not need to arrange a reference label additionally or need a time-consuming calibration stage. Meanwhile, the cost is relatively low, and positioning can be realized only by commercial equipment. The conventional SAR method uses a mesh matching method whose positioning accuracy is affected by the mesh density and requires a long calculation time. Therefore, the invention relates to an RFID label positioning method based on a motion model and a synthetic aperture, and aims to overcome the defects that the existing SAR grid matching positioning mode is influenced by grid density and has long calculation time.
Disclosure of Invention
The invention relates to an RFID (radio frequency identification) tag positioning method based on a motion model and a synthetic aperture, which aims to overcome the defects that the existing SAR grid matching positioning mode is influenced by grid density and has long calculation time, analyze the motion model of an SAR scene, introduce a convex optimization strategy into the process of target positioning, perform phase expansion and phase combination on acquired phase information, eliminate fixed frequency deviation, and finally perform optimal estimation on a similarity function by using a steepest descent method. And finally, positioning with high precision and extremely low calculation time is realized. The technical scheme is as follows:
An RFID label positioning method based on a motion model and a synthetic aperture is provided, wherein the initial position of a label, namely the position to be positioned, is p0＝[x0,y0]The speed of the label is v, and the positioning steps are as follows:
(1) a motion model stage:
1) the reader performs the interrogation process at N different times, pn＝[xn,yn]Indicating the position of the tag at the nth interrogation, the relative distance between the tag and the stationary reader antenna at the nth interrogation is expressed as:
dn＝||pa-pn||
wherein p isaIs the antenna position, | | | | · | |, is the modulo function;
2) let r denote the length of the antenna perpendicular to the direction of label movement, ξnIs from dnThe length of the antenna after r subtraction is called the residual distance, and the position of the antenna to the perpendicular point of the perpendicular line of the motion trail is called pvL represents p0And pvDistance between, snRepresenting the displacement of the label from the first interrogation to the nth interrogation, and obtaining a motion model:
Figure GDA0003635719280000021
(2) phase unwrapping phase
1) Using the phase offset information to locate the tag, the first step in the data processing is phase unwrapping:
Figure GDA0003635719280000031
wherein the content of the first and second substances,
Figure GDA0003635719280000032
is the measured phase, N1, 2nIs the phase after phase unwrapping, N ═ 1,2]Is a rounding function; sign (·) is a step function; through phase unwrapping, the problem of jump of measured phase data can be solved;
(3) A phase combination stage:
1) one antenna is selected as a central antenna in an antenna array, and of two antennas to be connected, an antenna close to the central antenna is called a primary antenna, and the other antenna is called a secondary antenna, and data collected by the w-th antenna is set as follows:
Figure GDA0003635719280000033
wherein psiwIs a phase sequence obtained by phase spreading the phase measured by the w-th antenna, SwIs the sequence of displacements measured at the w-th antenna, [ phi ]w,nN is a phase obtained by phase-spreading a phase measured by the w-th antenna, and s is a phase obtained by phase-spreading a phase measured by the w-th antennaw,nN is the displacement measured by the w-th antenna, and T is the transposed symbol;
2) if the phases measured by the m-th and m-1-th antennas are to be connected together, S is first updatedm-1The value of (c):
Figure GDA0003635719280000034
wherein d isa,m,m-1＝||pa,m-pa,m-1||，Sm-1' is the updated displacement sequence;
3) the initial phase difference of the two antennas is called delta phim,m-1Constructing a cost function
Figure GDA0003635719280000035
Where Σ · is the summation function, I is the number of measurements in the coincidence range of the two antennas,
Figure GDA0003635719280000036
and
Figure GDA0003635719280000037
indicating displacement data measured by the m-th antenna on both sides of the displacement data measured by the m-1 th antenna in the coincidence range,
Figure GDA0003635719280000041
and
Figure GDA0003635719280000042
then it is its corresponding phase data;
Figure GDA0003635719280000043
the phase data measured by the (m-1) th antenna in the coincidence range is the ith; (.) 2Is a squared symbol;
4) the initial phase difference is calculated by minimizing the cost function J:
Figure GDA0003635719280000044
wherein
Figure GDA0003635719280000045
For the predicted symbol, arg min is the function of solving the minimum value; the initial phase difference is obtained through calculation so as to solve the problem of the initial phase difference of different antennas and further carry out phase combination;
(4) and eliminating the fixed phase offset stage:
1) the unwrapped phase still contains a fuzzy number of cycles and the fixed phase offset caused by r is cancelled out in the following way
φr,n＝φn-φmin
φr,nIn order to eliminate the phase after the phase offset is fixed; phi is aminIs the minimum value in the measured phase data; by all the above processes, we get a curve with the lowest point value of 0, which eliminates the ambiguityThe information and the shape of the curve are kept;
(5) and a steepest descent method positioning stage:
1) by the above data processing, the phase sequence psi is obtainedr＝[φ1,φ2,...,φN]TAnd the shift sequence S ═ S1,s2,...,sN]TFurther, the remaining distance sequence xi ═ xi [ xi ] is obtained1,ξ2,...,ξN]TBy:
Figure GDA0003635719280000046
2) constructing a similarity function according to a Cauchy-Schwarz inequality
Figure GDA0003635719280000047
Wherein<·,·>Is inner product symbol, | ·| non-conducting phosphor2Is a 2-norm symbol;
3) because | | | xi | non-calculation2Is invariant, and the similarity function is simplified to
Figure GDA0003635719280000048
Wherein
Figure GDA0003635719280000051
4) Obtaining an estimated unknown quantity by steepest descent
Figure GDA0003635719280000052
And
Figure GDA0003635719280000053
Figure GDA0003635719280000054
wherein argmax is the sign of the maximum value;
5) Obtaining a label position:
Figure GDA0003635719280000055
wherein x isa,cAnd ya,cRepresenting the x and y coordinates of the center antenna in the antenna array, respectively.
Drawings
Fig. 1 is a motion model.
Fig. 2 is a phase unwrapping.
Fig. 3 is a phase combination.
Fig. 4 is a positioning flowchart.
Detailed Description
The following describes a method for positioning an RFID tag based on the synthetic aperture radar technology with reference to the accompanying drawings.
A tag localization scenario based on synthetic aperture technology is shown in fig. 1. In a tag data acquisition stage, a 4m × 4m scene to be positioned is established, a moving track of a reader antenna is a straight line along an X axis, the moving track is located at a boundary of an area to be positioned by 20cm, the total length is 4m, and measurement data of a tag is acquired at a space interval of 4cm every time the reader moves, so that 100(N equals to 100) readings are acquired by the antenna during the moving period when the working frequency of the reader is 867.5MHz and the corresponding wavelength is 34.6 cm. The distance between the pixel points of the region to be positioned in the pixel database establishing stage can be set to be 5cm, and if higher positioning precision is expected, the pixel points can be set more densely.
The positioning method estimates the position of the label measurement data in real time according to the acquired label measurement data, the algorithm flow is shown in figure 4, and the steps are as follows:
1) Assume that the initial position of the tag (i.e., the position to be located) is p0＝[x0,y0]The velocity of the label is v. The reader performs the interrogation process at N different times. p is a radical ofn＝[xn,yn]Indicating the nth questionThe position of the tag. Thus, the relative distance between the tag and the stationary reader antenna at the nth interrogation can be expressed as:
dn＝||pa-pn||
wherein p isaIs the antenna position and is the modulo function.
2) As shown in fig. 1, r represents the length of the antenna perpendicular to the direction of tag movement. XinIs from dnThe length after r is subtracted, which we refer to as the remaining distance. We call the vertical point location pv. l represents p0And pvThe distance between them. snIndicating the displacement of the tag from the first challenge to the nth challenge. We can then derive a motion model:
Figure GDA0003635719280000061
3) although distance information cannot be obtained directly from phase information due to ambiguity over the number of cycles, we can still use the phase offset information to locate the tag. The first step in data processing is phase unwrapping:
Figure GDA0003635719280000062
wherein the content of the first and second substances,
Figure GDA0003635719280000063
is the measured phase. Phi is an(N ═ 1, 2.., N) is the phase after phase unwrapping, | · | is an absolute value function, [ · N) is the phase after phase unwrapping]Is a rounding function. sign () is a step function. Through phase unwrapping, the problem of jump in measured phase data can be solved.
4) First, one antenna is selected as a center antenna in an antenna array. Of the two antennas to be connected, the antenna near the center antenna is referred to as a primary antenna, and the other is referred to as a secondary antenna. Assume that the w-th antenna collects data:
Figure GDA00036357192800000710
wherein psiwThe phase sequence is obtained by phase-spreading the phase measured by the w-th antenna. SwIs the displacement sequence measured by the w-th antenna. Phi is aw,n(N is 1, 2.. times.n) is a phase obtained by phase-spreading a phase measured by the w-th antenna, and s is a phase obtained by phase-spreading a phase measured by the w-th antennaw,n(N ═ 1, 2.., N) is the displacement measured by the w-th antenna. T is the transposed symbol.
5) Take fig. 3 as an example. If the phases measured by the m-th and m-1-th antennas are to be connected together, S is first updatedm-1The value of (c):
Figure GDA0003635719280000071
wherein d isa,m,m-1＝||pa,m-pa,m-1||。Sm-1' is the updated displacement sequence.
6) The initial phase difference of the two antennas is called delta phim,m-1. Constructing a cost function
Figure GDA0003635719280000072
Where Σ · is the summation function, I is the number of measurements in the coincidence range of the two antennas,
Figure GDA0003635719280000073
and
Figure GDA0003635719280000074
indicating displacement data measured by the m-th antenna on both sides of the displacement data measured by the m-1 th antenna in the coincidence range,
Figure GDA0003635719280000075
and
Figure GDA0003635719280000076
it is its corresponding phase data.
Figure GDA0003635719280000077
The phase data measured by the m-1 th antenna is the ith in the coincidence range. (.) 2Is a squared symbol.
7) The initial phase difference can then be calculated by minimizing a cost function J:
Figure GDA0003635719280000078
wherein
Figure GDA0003635719280000079
To predict the sign, arg min is the minimum function. By calculating the initial phase difference, the problem of the initial phase difference of different antennas can be solved, and then phase combination is carried out.
8) The unwrapped phase still contains the number of ambiguous cycles. The fixed phase offset caused by r can be eliminated in the following way
φr,n＝φn-φmin
φr,nTo eliminate the phase after the phase offset is fixed. Phi is aminIs the minimum in the measured phase data. After all the above processes, we get a curve with a lowest point value of 0, which eliminates the ambiguity and preserves the shape of the curve.
9) By the above data processing we obtain the phase sequence psir＝[φ1,φ2,...,φN]TAnd the shift sequence S ═ S1,s2,...,sN]T. Then we can get the remaining distance sequence xi1,ξ2,...,ξN]TBy:
Figure GDA0003635719280000081
10) constructing a similarity function according to a Cauchy-Schwarz inequality
Figure GDA0003635719280000082
Wherein<·,·>Is inner product symbol, | ·| non-conducting phosphor2Is a 2-norm symbol.
11) Because | | | xi | non-calculation2Is invariant, the similarity function can be simplified to
Figure GDA0003635719280000083
Wherein
Figure GDA0003635719280000084
12) Then obtaining the estimated unknown quantity by the steepest descent method
Figure GDA0003635719280000091
And
Figure GDA0003635719280000092
Figure GDA0003635719280000093
where argmax is the sign of the maximum value.
13) Obtaining a label position:
Figure GDA0003635719280000094
Wherein x isa,cAnd ya,cRepresenting the x and y coordinates of the center antenna in the antenna array, respectively.

Claims (1)

1. RFID label positioning method based on motion model and synthetic aperture, setting initial labelThe starting position, i.e. the position to be located, is p0＝[x0,y0]The speed of the label is v, and the positioning steps are as follows:
(1) a motion model stage:
1) the reader performs the interrogation process at N different times, pn＝[xn,yn]Indicating the position of the tag at the nth interrogation, the relative distance between the tag and the stationary reader antenna at the nth interrogation is expressed as:
dn＝||pa-pn||
wherein p isaIs the antenna position, | | | | · | |, is the modulo function;
2) let r denote the length of the antenna perpendicular to the direction of label movement, ξnIs from dnThe length of the antenna minus r is called the residual distance, and the position of the antenna at the perpendicular point to the perpendicular line of the motion trajectory is called pvL represents p0And pvDistance between, snRepresenting the displacement of the label from the first interrogation to the nth interrogation, and obtaining a motion model:
Figure FDA0003635719270000011
(2) phase unwrapping phase
1) Using the phase offset information to locate the tag, the first step in the data processing is phase unwrapping:
Figure FDA0003635719270000012
wherein the content of the first and second substances,
Figure FDA0003635719270000013
is the measured phase, N1, 2nIs the phase after phase unwrapping, N ═ 1,2]Is a rounding function; sign (·) is a step function; through phase unwrapping, the problem of jump of measured phase data can be solved;
(3) A phase combination stage:
1) selecting one antenna as a central antenna in an antenna array, wherein an antenna close to the central antenna is called a primary antenna and the other antenna is called a secondary antenna in two antennas to be connected, and setting data collected by the w-th antenna as follows:
Figure FDA0003635719270000021
wherein psiwIs a phase sequence obtained by phase unwrapping the phase measured by the w-th antenna, SwIs the displacement sequence measured by the w-th antenna, phiw,nN is a phase obtained by phase-spreading a phase measured by the w-th antenna, and s is a phase obtained by phase-spreading a phase measured by the w-th antennaw,nN is the displacement measured by the w-th antenna, and T is the transposed symbol;
2) if the phases measured by the m-th and m-1-th antennas are to be connected together, S is first updatedm-1The value of (c):
Figure FDA0003635719270000022
wherein d isa,m,m-1＝||pa,m-pa,m-1||，Sm-1' is the updated displacement sequence;
3) the initial phase difference of the two antennas is called delta phim,m-1Constructing a cost function
Figure FDA0003635719270000023
Where Σ · is the summation function, I is the number of measurements in the coincidence range of the two antennas,
Figure FDA0003635719270000024
and
Figure FDA0003635719270000025
indicating displacement data measured by the m-th antenna on both sides of the displacement data measured by the m-1 th antenna in the coincidence range,
Figure FDA0003635719270000026
and
Figure FDA0003635719270000027
then it is its corresponding phase data;
Figure FDA0003635719270000028
the phase data measured by the (m-1) th antenna in the coincidence range is the ith; (.) 2Is a square symbol;
4) the initial phase difference is calculated by minimizing the cost function J:
Figure FDA0003635719270000029
wherein
Figure FDA00036357192700000210
For the predicted symbol, arg min is the function of solving the minimum value; the initial phase difference is obtained through calculation so as to solve the problem of the initial phase difference of different antennas and further carry out phase combination;
(4) and eliminating the fixed phase offset stage:
1) the unwrapped phase still contains a fuzzy number of cycles and the fixed phase offset caused by r is cancelled out in the following way
φr,n＝φn-φmin
φr,nIn order to eliminate the phase after the phase offset is fixed; phi is aminIs the minimum value in the measured phase data; through all the processing, a curve with the lowest point value of 0 is obtained, fuzzy information is eliminated, and the shape of the curve is reserved;
(5) and a steepest descent method positioning stage:
1) by the above dataProcessing to obtain a phase sequence psir＝[φ1,φ2,...,φN]TAnd the shift sequence S ═ S1,s2,...,sN]TFurther, the remaining distance sequence xi ═ xi [ xi ] is obtained1,ξ2,...,ξN]TBy:
Figure FDA0003635719270000031
2) constructing a similarity function according to a Cauchy-Schwarz inequality
Figure FDA0003635719270000032
Wherein<·,·>Is inner product symbol, | ·| non-conducting phosphor2Is a 2-norm symbol;
3) because | | | xi | non-calculation2Is invariant, and the similarity function is simplified to
Figure FDA0003635719270000033
Wherein
Figure FDA0003635719270000034
4) Obtaining an estimated unknown quantity by steepest descent
Figure FDA0003635719270000041
And
Figure FDA0003635719270000042
Figure FDA0003635719270000043
wherein argmax is the sign of the maximum value;
5) Obtaining a label position:
Figure FDA0003635719270000044
wherein x isa,cAnd ya,cRepresenting the x and y coordinates of the center antenna in the antenna array, respectively.
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