CN113343728B - RFID positioning method based on nonlinear optimization method - Google Patents

RFID positioning method based on nonlinear optimization method Download PDF

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CN113343728B
CN113343728B CN202110544106.8A CN202110544106A CN113343728B CN 113343728 B CN113343728 B CN 113343728B CN 202110544106 A CN202110544106 A CN 202110544106A CN 113343728 B CN113343728 B CN 113343728B
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reader
phase
tag
antenna
cost function
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CN113343728A (en
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马永涛
付艳喜
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Tianjin University
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
    • G06K7/10—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation
    • G06K7/10009—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves
    • G06K7/10316—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves using at least one antenna particularly designed for interrogating the wireless record carriers
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K7/00—Methods or arrangements for sensing record carriers, e.g. for reading patterns
    • G06K7/10—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation
    • G06K7/10009—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves
    • G06K7/10297—Methods or arrangements for sensing record carriers, e.g. for reading patterns by electromagnetic radiation, e.g. optical sensing; by corpuscular radiation sensing by radiation using wavelengths larger than 0.1 mm, e.g. radio-waves or microwaves arrangements for handling protocols designed for non-contact record carriers such as RFIDs NFCs, e.g. ISO/IEC 14443 and 18092
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
    • H04W4/80—Services using short range communication, e.g. near-field communication [NFC], radio-frequency identification [RFID] or low energy communication
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Abstract

The invention relates to an RFID positioning method based on a nonlinear optimization method, which comprises the following steps: firstly, establishing an RFID phase model; designing a cost function in a second step: the position of the tag is fixed and unknown, and the reader antenna moves at a determined track and speed to form a synthetic aperture; constructing a cost function by utilizing the relation between the phase difference and the distance difference, converting the positioning problem into a solution optimization problem, wherein the solution corresponding to the minimum value of the cost function is the target position; and thirdly, solving the optimal solution of the optimization problem to obtain the position of the target to be positioned.

Description

RFID positioning method based on nonlinear optimization method
Technical Field
The invention relates to an RFID positioning method.
Background
The passive ultrahigh frequency RFID technology has become a model for the successful application of the Internet of things technology in industry due to the unique advantages (low power consumption, uniqueness of electronic tags and batch inventory of tag data). Radio Frequency (RF) based positioning technology has a wide potential for development in the fields of mobile computing, resource management and human-computer interaction, has attracted great research interest in the industry, and some theories have been transformed into advanced application systems.
Synthetic Aperture Radar (SAR) is a high resolution radar imaging technology, which forms a virtual antenna array by the relative motion between the target and the radar, and can replace a large-size array antenna to achieve the desired azimuth resolution. In recent years, many researches apply the SAR concept to the RFID positioning technology, and obtain more sampling information by using the relative motion between the radio frequency tag and the reader antenna, thereby achieving the positioning effect with high precision.
Disclosure of Invention
The patent provides an RFID positioning method based on a nonlinear optimization method, and aims to realize accurate positioning of an object attached with a radio frequency tag by using the existing RFID equipment. Aiming at the fact that a traditional positioning method based on time measurement is not suitable for an RFID system with narrow bandwidth, such as signal time of arrival (TOA), time difference of arrival (TDOA) and the like, the method utilizes phase information of a radio frequency label backscatter signal to establish a cost function containing position parameters, further adopts a nonlinear optimization method to solve the cost function, and finally provides position information of an object to be positioned. The technical scheme is as follows:
an RFID positioning method based on a nonlinear optimization method comprises the following steps:
first step of establishing RFID phase model
The reader controls the antenna to radiate radio frequency signals to the space, the radio frequency front end of the selected tag obtains energy from the electromagnetic field to be activated, and the tag modulates the backscatter signals through transforming input impedance so as to store the backscatter signalsThe data is sent back to the reader, and the reader demodulates the backscattering signal to obtain phase information; the distance between the reader antenna and the tag is set as d, the phase value is set as phi,
Figure BDA0003072922620000011
is a phase offset that is related to hardware factors.
Second step cost function design
The position of the tag is fixed and unknown, and the reader antenna moves at a determined track and speed to form a synthetic aperture; during the movement, the reader randomly accesses the tag and records each access time t ═ t { (t) 0 ,t 1 ,...,t i ,...,t N The sum of phase values
Figure BDA0003072922620000012
Initial position z of reader antenna 0 ＝[x 0 ,y 0 ]It is known that, assuming that the reader moves along the x-axis at a velocity v, t i The position of the reader at a moment is denoted z i ＝[x i ,y i ]＝[x 0 +v(t i -t 0 ),0]Distance between reader and tag is d i ＝||z i -z t || 2 Wherein z is t ＝[x t ,y t ]Representing the label coordinates;
the phase value φ is rewritten as:
Figure BDA0003072922620000013
wherein k represents the number of ambiguous phase cycles;
in order to eliminate hardware-dependent phase offsets
Figure BDA0003072922620000021
Taking the phase values of two adjacent times as a difference, and expressing the ith phase difference as:
Figure BDA0003072922620000022
Δφ i ＝φ i+1 -φ i
Δk i ＝k i+1 -k i
Δd i ＝d i+1 -d i ,i＝1,2,3,...,N-1
limiting the sampling interval of the antenna to be less than lambda/4 and taking pi as a standard, obtaining the following determination of delta k i The method of (1):
Figure BDA0003072922620000023
by using the relationship between the phase difference and the distance difference, a cost function is constructed as follows:
Figure BDA0003072922620000024
wherein Δ d i ＝||z i+1 -z t || 2 -||z i -z t || 2
Converting the positioning problem into a solution optimization problem, wherein a solution corresponding to the minimum value of the cost function is a target position;
and thirdly, solving the optimal solution of the optimization problem to obtain the position of the target to be positioned.
In the first step, the phase value φ may be represented by the following equation:
Figure BDA0003072922620000025
wherein x represents the wavelength of the light emitted from the light source,
Figure BDA0003072922620000026
the method comprises three parts:
Figure BDA0003072922620000027
the phase offsets brought to the sending unit of the reader, the tag unit and the receiving unit of the reader respectively.
And thirdly, solving the optimal solution of the optimization problem to obtain the position of the target to be positioned.
In the third step, the optimization problem can be solved using the gauss-newton method.
The invention provides a new solution for the indoor positioning problem. In general, the method has the following characteristics: 1. the SAR method is introduced into the RFID positioning problem, and a more sufficient measurement result is obtained by utilizing the relative motion between the reader antenna and the radio frequency tag. 2. An RFID phase-distance model based on a backscattering mechanism is established, and a distance measure which is more reliable than an energy value is obtained by utilizing phase change brought by electromagnetic wave propagation. 3. And establishing a convex optimization objective function by utilizing the corresponding relation between the phase difference and the distance difference, thereby converting the positioning problem into an optimization problem which is easy to solve. 4. The method provided by the invention is based on the existing RFID system, so that the method has higher practical value. The effectiveness of the method provided by the invention is verified by a series of simulation and experiments.
Drawings
FIG. 1 RFID backscatter coupling communication mechanism
FIG. 2 a non-linear optimization iterative process
FIG. 3 is a graph of cumulative error effect (CDF) versus other positioning methods
FIG. 4 is a graph of computational efficiency compared to other positioning methods
Detailed Description
The invention is based on the RFID backscattering coupling mechanism, and utilizes the relative movement between the reader antenna and the radio frequency tag to lead the distance between the reader antenna and the radio frequency tag to generate continuous change, meanwhile, the phase of the electromagnetic wave directly related to the distance also changes correspondingly, and the positioning of the target object is realized by measuring the change of the phase. In general, the technical scheme to be adopted by the invention is as follows: firstly, starting from a communication mechanism of the RFID, the relation between the distance and the phase is established, and theoretical support is provided for the design of a positioning scheme. Secondly, a cost function containing target position information is designed, and the positioning problem is converted into an optimal solution problem for searching the cost function. And finally, solving an optimal solution for minimizing the cost function by a nonlinear optimization method, and giving a positioning result.
The technical route is as follows:
1. the reader antenna is fixed on the mobile platform to form relative motion between the antenna and the radio frequency tag. The reader continuously polls the radio frequency tag and receives a backscattering signal of the radio frequency tag in the moving process. The received signal is demodulated to obtain phase information reflecting the propagation distance of the electromagnetic wave.
2. In order to eliminate phase deviation introduced by hardware factors, the phase difference mode is used for replacing direct use of the phase to position, a cost function containing target position information is designed according to the corresponding relation between the phase difference and the distance difference, and the solution of minimizing the cost function is the target position to be positioned.
3. The optimal solution of the cost function is iteratively solved through a nonlinear optimization method, such as a least square method (LMS), so that a positioning result is given.
The method specifically comprises the following steps:
RFID phase model
Passive RFID systems use backscatter radio for communication. The tag is not equipped with a battery and relies on energy derived from the signal transmitted by the reader. The basic process of the work is as follows: the reader controls the antenna to radiate a radio frequency signal to the space, the radio frequency front end of the selected tag obtains energy from the electromagnetic field to be activated, the tag modulates the backscatter signal through transforming input impedance so as to send the stored data back to the reader, and the reader demodulates the backscatter signal to obtain phase information.
Assuming that the distance between the reader antenna and the tag is d, the total propagation distance is 2d since the electromagnetic wave signal undergoes a round trip from transmission to reception. The phase is related to the hardware characteristics of the system in addition to the propagation distance. In general, the phase can be represented by:
Figure BDA0003072922620000031
wherein x represents the wavelength of the light emitted from the light source,
Figure BDA0003072922620000032
for the phase shift related to hardware factors, it can be considered as a constant, including three parts:
Figure BDA0003072922620000033
Figure BDA0003072922620000034
the phase offsets brought to the sending unit of the reader, the tag unit and the receiving unit of the reader respectively.
2. Cost function design
In a SAR scenario, the tag position is fixed and unknown, and the reader antenna moves with a determined trajectory and velocity to form a synthetic aperture. During the movement, the reader randomly accesses the tag and records each access time t ═ t { (t) 0 ,t 1 ,...,t i ,...,t N The sum of phase values
Figure BDA0003072922620000035
Assume reader antenna initial position z 0 ＝[x 0 ,y 0 ]The movement speed is v, then t i The position of the antenna at the moment of time can be denoted z i ＝z 0 +v(t i -t 0 ) And the distance between the antenna and the label is as follows:
d i ＝||z i -z t || 2 (2)
wherein z is t Is the coordinates of the tag to be located.
As shown in equation (1), the measured phase is a true phase value, and is subjected to a modulo operation of 2 pi, so that the true distance cannot be reflected. For simplicity, we rewrite it as:
Figure BDA0003072922620000041
in order to eliminate hardware-dependent phase offsets
Figure BDA0003072922620000042
We differentiate between two adjacent phase values, for example, the ith phase difference can be expressed as:
Figure BDA0003072922620000043
Δφ i ＝φ i+1 -φ i
Δk i ＝k i+1 -k i
Δd i ＝d i+1 -d i ,i＝1,2,3,...,N-1 (4)
as shown in the above equation, it is also necessary to determine Δ k in order to obtain the relationship between the phase difference and the distance difference i We describe determining Δ k below i The method of (1).
According to the triangle rule, the difference value of the distance between the label and the two adjacent antenna sampling positions is not larger than the distance between the two antenna sampling positions, therefore, if the sampling interval of the limited antenna is smaller than lambda/4, the phase change should be smaller than pi theoretically, therefore, the following determined delta k is obtained by taking pi as a standard i The method of (1):
Figure BDA0003072922620000044
then determining Δ k i Then, by using the relationship between the phase difference and the distance difference, a cost function is constructed as follows:
Figure BDA0003072922620000045
wherein Δ d i ＝||z i+1 -z t || 2 -||z i -z t || 2
So far, the positioning problem is converted into a solution optimization problem, and a solution corresponding to the minimum value of the cost function is the target position.
3. Solving for an optimal solution
The optimal solution corresponding to the above optimization problem can be expressed as:
Figure BDA0003072922620000046
there are many solutions to the nonlinear optimization problem, and here we use the classical gaussian-newton method to solve it, and the specific solution steps are as follows:
(1) initial estimate x for a given solution (1) When k is 1;
(2) solving the system of equations
Figure BDA0003072922620000051
Get delta (k) ；
Wherein r is k Satisfy the requirement of
Figure BDA0003072922620000052
A k Jacobian matrix as vector r in z t ＝x (k) The value of (c).
(3) Put x (k+1) ＝x (k) +δ (k) ；
(4) If | x (k+1) -x (k) If | < epsilon, stopping iteration to obtain a positioning result z t ＝x (k+1) (ii) a Otherwise, setting k to be k +1, and turning to the step (2);
so far, the solution of the optimization problem obtained after iteration is the position of the target to be positioned.

Claims (3)

1. An RFID positioning method based on a nonlinear optimization method comprises the following steps:
first step of establishing RFID phase model
The reader controls the antenna to radiate radio frequency signals to the space, the radio frequency front end of the selected tag obtains energy from an electromagnetic field to be activated, the tag modulates backscattering signals through transforming input impedance so as to send the stored data back to the reader, and the reader demodulates the backscattering signals to obtain phase information; setting the distance between the reader antenna and the tagA distance of d and a phase value of
Figure FDA0003733567650000011
Figure FDA0003733567650000012
Is a phase offset related to hardware factors;
second step cost function design
The position of the tag is fixed and unknown, and the antenna of the reader moves at a determined track and speed to form a synthetic aperture; during the movement, the reader randomly accesses the tag and records each access time t ═ t { (t) 0 ,t 1 ,...,t i ,...,t N A sum of phase values
Figure FDA0003733567650000013
Initial position z of reader antenna 0 ＝[x 0 ,y 0 ]Given that the reader moves along the x-axis at a velocity v, t is known i The position of the time reader is denoted z i ＝[x i ,y i ]＝[x 0 +v(t i -t 0 ),0]Distance between reader and tag is d i ＝||z i -z t || 2 Wherein z is t ＝[x t ,y t ]Representing the label coordinates;
phase value
Figure FDA0003733567650000014
Rewriting into:
Figure FDA0003733567650000015
wherein k represents the number of ambiguous phase cycles;
in order to eliminate hardware-dependent phase offsets
Figure FDA0003733567650000016
The phase values of two adjacent times are used as a difference, the ith phase difference representsComprises the following steps:
Figure FDA0003733567650000017
Δφ i ＝φ i+1 -φ i
Δk i ＝k i+1 -k i
Δd i ＝d i+1 -d i ,i＝1,2,3,...,N-1
limiting the sampling interval of the antenna to be less than lambda/4 and taking pi as a standard to obtain the following determination of delta k i The method comprises the following steps:
Figure FDA0003733567650000018
by using the relationship between the phase difference and the distance difference, a cost function is constructed as follows:
Figure FDA0003733567650000019
wherein Δ d i ＝||z i+1 -z t || 2 -||z i -z t || 2
Converting the positioning problem into a solution optimization problem, wherein a solution corresponding to the minimum value of the cost function is a target position;
and thirdly, solving the optimal solution of the optimization problem to obtain the position of the target to be positioned.
2. The RFID positioning method based on non-linear optimization method of claim 1, wherein in the first step, the phase value φ is represented by the following equation:
Figure FDA0003733567650000021
wherein x represents the wavelength of the light emitted from the light source,
Figure FDA0003733567650000022
comprises three parts:
Figure FDA0003733567650000023
the phase offsets brought to the sending unit of the reader, the tag unit and the receiving unit of the reader respectively.
3. The RFID positioning method based on nonlinear optimization method as claimed in claim 1, wherein in the third step, the optimization problem is solved using Gaussian-Newton method.
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