CN101131432A - Positioning method for wireless radio frequency recognition system and device thereof - Google Patents
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Abstract
The invention is the location method in wireless radio-frequency recognition system. By the receiving signal transmission time extending of detected label and the weighted least square TOA location method, it gets a group of estimating position set; then by the signal strength of the detected label and the reference label and the RSSI location method, it gets a primary estimating position. According to the primary estimating position, it judges the distance of non stadia error in the sets and estimates the error size to renew the weighted matrix, then it estimates the position according to the weighted matrix to get the finial position. The method has combined the TOA and the RSSI position method, so it has inhibited the influence of NLOS error and improved the RFID system location precision.
Description
Technical Field
The present invention relates to the field of wireless communication technologies, and in particular, to a method and an apparatus for positioning a radio frequency identification system in wireless communication under NLOS (non line of sight) conditions.
Background
With the popularization and development of mobile computing devices, wireless positioning technology attracts more and more attention, RFID (Radio Frequency Identification) is a technology that uses Radio Frequency signals to realize contactless information transmission through spatial coupling (alternating magnetic field or electromagnetic field) and achieves Identification through transmitted information, and an RFID system mainly comprises two parts: the RFID system is suitable for mass deployment due to low cost of the label, and has good application prospect in the position positioning technology.
The existing positioning technology of the RFID system is mostly as follows: 1. the propagation time of a wireless signal or referred to as time of arrival (TOA) information of the signal is converted into a distance between the reader/writer and the tag, a circle is drawn with the coordinates of the reader/writer as the center of the circle and the distance converted from the propagation time of the signal as the radius, and the intersection point of the circles formed by the plurality of readers/writers is the position of the RFID tag, as shown in fig. 1. Secondly, the Signal Strength of the Signal is obtained from the Signal Received by the reader/writer, and the position of the tag is determined based on the Signal Strength by using a characteristic that the attenuation of the radio wave is approximately in inverse proportion to the square of the propagation distance by using an RSSI (Received Signal Strength Indicator) algorithm (Signal Strength ranging algorithm).
In the above two methods, in an NLOS environment, due to the reflection and scattering phenomena, the information propagation delay estimated by the reader/writer includes the delay of the direct signal and the additional delay caused by reflection or scattering, and the signal energy received by the reader/writer is also affected, thereby causing inaccurate positioning.
Disclosure of Invention
The invention aims to provide a positioning method and a positioning device of a radio frequency identification system, which are used for effectively eliminating errors caused by NLOS (non line of sight) propagation and improving the positioning accuracy of an RFID (radio frequency identification) system.
In order to achieve the purpose, the invention adopts the following technical scheme:
a positioning method of a radio frequency identification system comprises the following steps:
presetting the number of readers-writers, the position coordinates of the readers-writers, the number of reference labels, the position coordinates of the reference labels and undetermined error value parameters between the readers-writers and the labels to be detected;
acquiring a set of estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal time delay of the to-be-detected label and the error value;
obtaining a first estimated position of the label to be detected according to the label to be detected, the receiving power of each reference label signal and the position coordinate of the reference label;
selecting a second estimated position closest to the first estimated position from the set, obtaining a median of the first estimated position and the second estimated position, and obtaining the error value according to the median;
and updating the weighting matrix according to the error value, and acquiring the position coordinate of the label to be detected according to the updated weighting matrix.
A location device in a radio frequency identification system, comprising:
the parameter setting module is used for setting the number of the reference tags, the position coordinates of the reference tags, the number of the readers and the position coordinates of the readers;
the estimated position set module is used for acquiring and obtaining a set of estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal delay of the to-be-detected label and the undetermined error value parameter between the reader-writer and the to-be-detected label;
the first estimation position module is used for acquiring a first estimation position of the to-be-detected label according to the position coordinates of the reference label and the receiving power of the to-be-detected label and the signals of the reference labels;
an error elimination module, configured to select a second estimated position closest to the first estimated position from the set, and obtain the error value according to a median value of the first estimated position and the second estimated position;
and the estimation position set module updates the weighting matrix according to the error value obtained by the error elimination module, and obtains the position coordinate of the label to be detected according to the updated weighting matrix.
A radio frequency identification system, comprising:
the parameter setting module is used for setting the number of the reference tags, the position coordinates of the reference tags, the number of the readers and the position coordinates of the readers;
the signal processing module is used for synchronizing and demodulating the signals of the to-be-detected label and each reference label received by each reader-writer to acquire the identification information of the labels, the receiving power of the signals and the signal time delay;
the estimated position set module is used for acquiring a set of estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal delay of the to-be-detected label and the undetermined error value parameter between the reader-writer and the to-be-detected label;
the first estimation position module is used for acquiring a first estimation position of the to-be-detected label according to the position coordinates of the reference label and the receiving power of the to-be-detected label and the signals of the reference labels;
an error elimination module, configured to select a second estimated position closest to the first estimated position from the set, and obtain the error value according to a median value of the first estimated position and the second estimated position;
and the estimation position set module updates the weighting matrix according to the error value obtained by the error elimination module and obtains the position coordinate of the label to be detected according to the updated weighting matrix.
According to the positioning method and the device of the radio frequency identification system, the signal strength information and the signal time delay TOA information of the signal received by the reader-writer are simultaneously considered, so that the two factors can be combined, the measurement error under the NLOS environment is effectively eliminated, and the positioning accuracy of the RFID system is improved. In addition, the parameter setting scheme of the invention can carry out different settings according to different environments of the positioning system, and has strong adaptability to the dynamic change of the environment, so that the obtained positioning information is more accurate and reliable.
Drawings
FIG. 1 is a schematic diagram of TOA technology-based positioning in NLOS environment;
FIG. 2 is a schematic diagram of a reader/writer and a tag according to the present invention;
FIG. 3 is a flow chart illustrating a positioning method according to the present invention;
FIG. 4 is a schematic diagram of a radio frequency identification system according to the present invention;
FIG. 5 is a graph comparing the performance of the positioning scheme of the embodiment of the present invention and the Taylor series expansion algorithm to eliminate the NLOS error in the TOA distance value.
Detailed Description
Referring to fig. 1, which is a positioning schematic diagram based on the TOA technology in the NLOS environment in the prior art, radio waves emitted by a tag to be tested are received by a reader/writer 1, a reader/writer 2, a reader/writer 3, and a reader/writer 4, respectively, and according to the time when each reader/writer receives the signal and the timestamp loaded in the emitted signal, the propagation delay of the signal from the tag to be tested to the reader/writer, that is, TOA information is obtained, and the known radio wave propagation speed is c (that is, 3 × 10 8 m/s) to convert the distance between the label to be detected and the reader-writer, taking the reader-writer as the center of circle and the distance converted according to the propagation delay as the center of circleThe radius is made into a circle, and the intersection point of the circles is the position of the label to be detected. However, in the NLOS environment, due to the reflection and scattering phenomena, the determined propagation time includes the direct-emitting time and the additional time delay caused by the reflection and scattering, and therefore, the distance determined according to the propagation time delay is not the actual distance between the tag to be measured and the reader-writer, which causes inaccurate positioning, and in the case of an error time delay of 1 μm, a positioning error of 300m will be caused.
Based on this, as shown in fig. 3, in order to reduce the positioning inaccuracy caused by the NLOS error, the positioning method of the radio frequency system of the present invention mainly includes the following steps:
s301: presetting the number and position coordinates of reference tags required to be used as reference positions and the number and position coordinates of readers, wherein the number and position of the set reference tags and the number and position coordinates of the readers are different according to different geographic environments where the current positioning system is located, and the readers and the reference tags are uniformly distributed under the condition that the wireless environment is uniform, for example, when any point in a positioning environment range can receive wireless signals; under the condition of uneven wireless environment, for example, when more irregular objects exist in the positioning environment, wireless signal blind areas may exist, according to the actual situation, the reader-writer and the reference tag may be unevenly distributed;
s302: the method comprises the steps that base band signals sent from a tag to be detected and each reference tag are synchronized and demodulated to obtain signal propagation delay t, tag identification information and receiving power Prec of each base band signal, and as the tags add time stamps to the signals when the signals are transmitted, the signal propagation delay t, namely TOA information, can be easily obtained according to the time stamps carried by the base band signals and the time for receiving the signals, and the receiving power Prec can be determined according to the amplitude of the received signals;
s303: presetting undetermined error value parameters between each reader-writer and a label to be detected, and obtaining a set of estimated positions of the label to be detected and a corresponding weighting matrix according to signal propagation delay t of the label to be detected received by each reader-writer, position coordinates of each reader-writer and undetermined error difference value parameters (the undetermined error value parameters simultaneously consider two conditions of LOS propagation and NLOS propagation between the reader-writer and the label to be detected, namely when the reader-writer and the label to be detected are LOS propagation, the error value is 0, and when the reader-writer and the label to be detected are NLOS propagation, the error value is larger than zero), wherein the set of the estimated positions and the weighting matrix corresponding to each estimated position can be obtained by adopting a weighted least square TOA algorithm;
s304: obtaining a first estimated position of the label to be detected according to the label to be detected received by each reader-writer, the receiving power Prec of each reference label and the position coordinates of each reference label, wherein the first estimated position can be obtained by adopting a k-neighbor algorithm;
s305: selecting a second estimated position which is closest to the first estimated position result from the set, and judging distance values with non-line-of-sight errors in the set, wherein the mode of judging which distance values have non-line-of-sight errors is as follows: in the weighting matrix corresponding to the second estimation position, the distance value of the reader-writer corresponding to the element 0 is the distance value with NLOS error, namely the error value corresponding to the reader-writer is larger than zero;
s306: calculating a median value of the first estimation position and the second estimation position, calculating an estimation value of an error value according to the median value, wherein the estimation value of the error value can be calculated by a square difference between an actual distance value and an average value and the actual distance value, and updating a weighting matrix according to the error value;
s307: the position coordinates of the tag to be detected, that is, the final positioning position of the tag to be detected, are obtained according to the updated weighting matrix, and the final positioning position of the tag to be detected may be obtained by using a weighted least squares TOA algorithm according to the updated weighting matrix, that is, by constructing a new weighting matrix according to the error value and according to the error value estimated in step S306, and by using the new weighting matrix, obtaining a final estimated position by using a weighted least squares TOA algorithm.
The positioning method of the radio frequency identification system is combined with two positioning modes, namely a first estimation position of an RSSI positioning method is used for acquiring a second estimation position which is closer to the first estimation position from an estimation position set of a TOA positioning method, error estimation under the NLOS environment is obtained through the two estimation positions so as to update a weighting matrix, and positioning is estimated again according to the updated weighting matrix.
In step S303, when the position estimation is performed by using weighted least squares TOA estimation (WLS TOA), the weighting method used may be: the weighting coefficient of the LOS distance value is not 0, and the weighting value of the NLOS distance value is 0, so that the distance value under the NLOS state is invalid, and a more accurate positioning position is obtained. Since all cases are considered, then in this set of estimated positions there must be one estimated position that is closest to the true position.
In step S304, when the k-proximity algorithm is used to perform estimation and positioning for obtaining, the weighting factor of the reference tag may be
Wherein i, j and q are integers, i and j are not more than q, respectively represent the ith and jth adjacent reference tags, E j The euclidean distance representing the received signal strength between the tag under test and the jth reference tag.
In the above-mentioned estimation positioning method of the present invention, a specific embodiment of the present invention is described in detail below.
1. Firstly, according to the needs of the actual environment, setting the number and the positions of reference tags which need to be used as reference positions, and setting the number and the positions of readers/writers, wherein the positions can be the reference tags or the position coordinates of the readers/writers, in this embodiment, the number of readers/writers is at least 3, and under different conditions, different settings can be made according to the environment and the geographical position of the current system, so that at least 3 LOS paths can be assumed to exist, an example of the reference tag position setting is shown in fig. 2, a plurality of reference tags are uniformly distributed around the readers/writers, so that the number of reference tags around each tag to be positioned is as equal as possible, and thus under the condition that the total number of reference tags is unchanged, the problem that the positioning accuracy is reduced due to the lack of enough reference tags in some places is avoided; the above situation is the situation that the wireless signal can be received in the located area, and for the situation that the wireless signal blind area exists, the positions of the reference tag and the reader-writer need to be reasonably set to ensure that no blind area exists.
2. Receiving baseband signals sent by the label to be tested and the reference label, and connecting the sampled basebandThe received signal is synchronized and demodulated to obtain the signal propagation delay t and the received power P rec Extracting the ID number of the label to identify different labels;
3. acquiring a set of estimated positions according to the signal propagation delay t of the tag to be detected received by each reader-writer and the position of each reader-writer, wherein in the embodiment, a weighted least square estimation algorithm based on time of arrival (TOA) is adopted to acquire the set of estimated positions and a corresponding weighted matrix;
when the tag to be tested emits a radio signal, the radio signal can be captured by a plurality of readers-writers, and because the number of the readers-writers in the RFID positioning system is at least 3 and no upper limit exists, it can be assumed that at least 3 readers-writers and the tag to be tested have line of sight (LOS) paths in TOA information received by all the readers-writers, and simultaneously, the condition can be met as much as possible when the readers-writers are arranged in the direction, so that 3 LOS paths can be obtained.
In this embodiment, the basic method of the weighted least square TOA algorithm is to perform two-step weighted least square linear estimation on a nonlinear equation set established by the signal delay estimated by the reader.
Firstly, when the weighted least square estimation of the first step is carried out, two weighted values are adopted, namely, for the distance value under the LOS environment, a certain weighted value is adopted, namely, the weighted coefficient is not 0, and for the distance value under the NLOS environment, the weighted value is 0 (indicating that the distance value of the reader-writer is invalid), wherein for the distance value of the same reader-writer, two conditions that the distance value is the LOS distance value and the NLOS distance value are considered at the same time, in this condition, different weighted matrixes are constructed by utilizing the weighted values, and because all conditions are considered, a matrix necessarily exists in the weighted matrixes, namely, elements on the diagonal line of the matrix are completely matched with the distance values of the reader-writer with the LOS path and the distance values of the NLOS path (namely, the LOS distance values are properly weighted, the NLOS distance values are weighted to be 0), and after all estimation results and the weighted matrixes corresponding to the estimation results are obtained, the estimation results and the weighted matrixes can be stored.
In this step, since it is not known in advance which distance values have NLOS errors, the number of M readers/writers and at least three LOS are shared(M represents the number of readers/writers) and each weighting method obtains an estimation result, and each estimation result corresponds to a weighting matrix.
For any reader-writer M (M is more than or equal to 1 and less than or equal to M), under the condition of considering NLOS, the distance value between the reader-writer M and the label to be tested is as follows:
Wherein d is m ＝ct m Is an actual distance value (c is the electromagnetic wave propagation velocity, t) m For propagation delay), (x) m ，y m ) Is the coordinate of the mth reader-writer, (x, y) is the coordinate of the label to be measured,
for reader-writer m squared to origin distance, NLOS m Is the error between the label to be tested and the receiver m caused by NLOS propagation, and under the LOS condition, the value is 0, n m Is an error introduced by the reader-writer time delay estimation, and the value is 0 under the LOS condition. [ equation 1]The following form may be rewritten:
Wherein K = x 2 +y 2 . Thus, the parameter to be estimated is θ = [ x, y, K =] T And [ formula 2]]Can be written in the form of a linear matrix:
h＝Hθ+N Err - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - [ formula 3]
Wherein:
J＝ε T w ε - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -formula 4]
Wherein the weighting matrix W = C -1 ＝(a 2 I) -1 Using [ formula 2]]And [ formula 3]]And [ formula 4 ]]A WLS estimate of θ can be obtained:
The covariance matrix of (a) is:
Wherein D = [ D ] 10 …，d M0 ]I M×M ，d m0 Representing the real distance between the mth reader and the label to be detected;
the covariance matrix, which is an estimate of the delay, when LOS is present,
in the presence of NLOS, order
Wherein d is m0 Is the true distance, s, between the mth reader and the tag to be positioned m Is the standard deviation of the distance value of the mth reader/writer, assuming that the distance values of the respective readers/writers are independent of each other and obey normal distribution.
After all the weighting matrices and their corresponding estimation results are obtained, they can be stored in a register, and of the estimation results of all the weighting methods, the estimation result of each weighting method is recorded as
Since all possible weighting modes are considered, the method is characterized in that
The set of estimated values must include an estimated value closest to the true position of the tag to be detected, and the weighting method corresponding to the estimated value is optimal, and the optimal estimated value can be obtained by joint analysis with the initial estimated position obtained as described below.
4. Acquiring a first estimated position of the tag to be detected according to Received Signal Strength Indicator (RSSI) information of the tag to be detected and each reference tag, wherein in the embodiment, the first estimated position is acquired by adopting a k-neighbor algorithm according to the RSSI information;
the relationship of the transmission power of the tag-to-reader with the propagation distance under the line-of-sight condition can be expressed by Friss equation: p rec ＝P EIRP G reader (l/4pd) 2 Wherein, l is carrier wave length, d is reader-writer and labelIn addition, the RFID reader-writer can identify a plurality of different received signal power levels, so that the Friss formula is converted according to the power received by the RFID reader-writer, and the distance information corresponding to the power can be obtained.
The number of the reference tags serving as reference positions in the system is assumed to be N, the reference tags can be active tags or passive tags, the active tags can actively transmit signals and can simultaneously identify a plurality of target systems, the passive tags can work only by providing energy from the outside, when the tags enter a working area of the system and receive specific electromagnetic waves, induction current can be generated by a coil, and the tags are powered by a rectifying circuit.
The reader-writer continuously scans the tags in the recognizable range to obtain the receiving power information thereof, and the measured signal intensity vector of the tags is defined as S = (S) 1 ，S 2 ，…，S M ) In which
And the strength level of the signal sent by the label to be tested and received by the ith reader-writer is represented. Definition of parametersThe received signal strength vector of the test tag at the reader-writer is s =(s) 1 ，s 2 ，…，s M ) Wherein s is i Indicating the signal strength level of the tag received by the reader/writer i. The Euclidean distance of the signal strength between the label to be measured and the reference label j obtained in each measurement is defined as
E defines the positional relationship between the reference tag and the tag to be measured, according to which the reference tag closest to the tag to be measured has the smallest E j Value, for N reference tags, the tag under test has E vector E = (E) 1 ，E 2 ，…，E N )。
The reference label with the closest distance to the E vector value can be obtained by comparing the E vector value of the label to be detected, because the E vector only reflects the position distance relation of the reference label, in order to form the coordinate information of the label to be detected, q nearest reference label coordinates are used for forming the position information of the label to be detected, any integer between 3 and N can be selected for q, the more the q values are, the higher the positioning precision is, and the method depends on the actual positioning environment. Weighting the coordinates (x, y) of the label to be detected by the coordinates of each neighbor to obtain:
Wherein, w i The weighting factors of the ith neighbor reference label, and the sum of the weighting factors is 0. The addition ofWeight factor w i The selection criteria of (a) are: the closer the signal strength of the reference tag to the tag to be tested, the larger the corresponding weighting factor. For simplicity, we have chosen here the weighting factors as:
Recording the estimated position result obtained by the RSSI positioning method as Z rssi I.e. the first estimated position, with values:
5. eliminating NLOS error distance value and obtaining final positioning position
The result set obtained from the weighted least squares TOA according to the foregoing description
In (1), there must be a solution close to the true value; and according to the RSSI positioning method, a solution close to the true value is obtained. Based on the solution obtained from the RSSI positioning method, fromTo select an optimal weighting matrix
The selection mode is as follows:
Where { w } denotes the set of all weighting matrices, and k denotes the different weighting matrices. Remember and
the set of the corresponding reader-writers with the middle weight value of 0 is R N I.e. the set consisting of NLOS readers is R N And the number of the LOS readers is L, the set consisting of the LOS readers is R L The number of the compounds is M-L. Notation of [ equation 5]And [ formula 9 ]]Estimated position of the obtained positioning resultHas a median value of (x) temp ，y temp ) Then the median has been compared closer to the true location information.
Let q be t ＝[x temp ，y temp ] T Set R N The estimation of the NLOS propagation error of each element in (1) is denoted as disN l (1. Ltoreq. L. Ltoreq. L), which can be obtained by the following formula:
Then the covariance matrix of the disN is:
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - [ formula 11]
Wherein:
(x nl ，y nl ) Indicating the location coordinates of the ith reader with NLOS error.
Let the NLOS propagation error of the reader/writer with LOS path be 0 and the estimated error thereof be 0, so that
The estimated value of NLOS error of all readers is shown, and the covariance matrix is as follows:
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - [ formula 12]
The estimated value obtained using equation 10 can be used to update the distance value with NLOS propagation error, and then the updated distance value is used to estimate θ again based on equation 5 using a weighted least squares TOA positioning algorithm. The weighting matrix at this time is:
- - - - - - - - - - - - - - - - - - - - - - - - - [ formula 13]
The updated covariance matrix is:
cov(θ new )＝[H T W new H] -1 H T W new H[H T W new H] -1 - - - [ formula 14]
According to [ formula 10]][ formula 12 ]]And [ equation 13]Can obtain the final position estimation based on
The covariance matrix is:
Wherein: h 0 ＝[x，y，0.5]I，θ 0 =[x，y]I。
With respect to the manner in the above-described embodiment, a specific analysis is made below with respect to the performance thereof.
The TOA estimation result can be approximated as a gaussian variable of variance C, and the estimated vector parameter t = [ t ] 1 ，t 2 ，t 3 ，...，t M ] T The mean value is, due to the presence of NLOS
Wherein:
d′＝[((x 1 -q F (1)) 2 +(y 1 -q F (2)) 2 ) 1/2 ，...，((x M -q F (1)) 2 +(y M -q F (2)) 2 ) 1/2 ] T - - - - [ formula 16]
t′＝d′/c
Order to
CRLB (Cramer-Radon) allowance of unbiased estimation quantity and vector parameterA lower bound is allowed to be placed on the variance of each element of the estimator. Assuming that t satisfies Gaussian distribution
Wherein
As a mean, C is an mxm covariance matrix. Then the probability density likelihood function for the TOA estimate is:
If the PDF (Probability Density Function) p (t; t') satisfies the "positive then" condition, i.e.:
Where the mathematical expectation is taken for p (t; t'), then any unbiased estimate
Must satisfy:
Where I (t') is the Fisher information matrix of M, defined by the formula:
Where the derivative is calculated at the true value of t ', the mathematical expectation is taken for p (t; t').
To calculate [ I -1 (t′)] ij And calculating a first derivative:
- - - [ formula 20]
Because the covariance matrix C is irrelevant to t', an unbiased estimated quantity is obtained
To obtain theta F The CRLB for the coordinate estimate is:
Wherein
t i Representing true propagation of tags and readers under test
Delaying;
when cov (disN) =0, that is, when there is no NLOS propagation error, it can be obtained
When cov (disN) ≠ 0, it indicates that NLOS error exists, according to the assumed condition that M-L is more than or equal to 3 and the [ formula 6]][ formula 11 ]]Equation 13][ formula 15 ]]Cov (disN) ≈ 0 may be considered. Therefore, the covariance of the estimated position obtained by the positioning method is close to CRLB, thereby effectively reducing NLOS error and improving positioning precision.
According to the above embodiment, a specific simulation analysis is performed as follows, and the specific simulation conditions are as follows: the RFID radio frequency is selected to be 800MHz. Assuming that M =8 readers are provided, the positions thereof are (x 1=0, y1= 0), (x 2= -2, y2= 6.5), (x 3=4, y3= 6), (x 4= -2, y4= 4), (x 5=3, y5= 4.5), (x 6= -5, y6= 4.5), (x 7=2, y 7= 4.5), respectively= 3), (x 8= -4, y8= 2). The number of nearest neighbors in the RSSI algorithm q =4, the coordinates of the 16 reference labels are respectively (0, 3), (0, 5), (-2, 3.5), (-2, 5.5), (-3, 5) (3, 5), (-3, 3), (1.5, 2), (-1, 4.5), (-1, 2.5), (1, 2.5). Hypothesis d in the simulation m ＝d m0 +n m + NL × rand, where rand results in [0,1]The random number in the interval, NL, is the maximum NLOS propagation error. The relationship between the time delay estimation error value of the reader-writer and the radio frequency f is as follows: n =0.1 × (1 GHz/f). Let the last 4 reference tags and the tag under test have the same NLOS value, in this simulation, the signal fading caused by penetrating obstacles is not taken into account.
Fig. 5 shows a graph of a relationship between a maximum NLOS Error (NL) and an average positioning Error MSE (Mean Square Error) of TOA measurements (which are converted into distance measurements, referred to as distance values) of 100 test sample points on a moving track of a moving tag (100 times of independent positioning average results) according to the method proposed in the embodiment of the present invention (RSSI partial results are also given) and the taylor series expansion algorithm in the static positioning method. The application method of the Taylor series expansion algorithm comprises the following steps: in the case of an initial value being drawn, a taylor series expansion is used to solve the set of nonlinear positioning equations determined from the time of arrival values. The convergence parameter is given as 0.1 meter, the taylor series expansion algorithm uses the same NLOS model as in the present embodiment, the NLOS model given by each simulation cycle obeys a uniform distribution between 0-NL, and the number of readers with LOS propagation is 3. During simulation, the real coordinates of the label to be tested are (2.2, 4.5), (-1.5, 3), (0, 2), (1.5, 3.5) and (0, 4), respectively, and the probability that the label to be tested is located at the five coordinates in the simulation is equal.
Where CRLB is given by [ equation 22 ]. It can be seen that when there is NLOS interference and the caused propagation error is large, the result obtained by taylor series expansion algorithm increases linearly, the RSSI positioning method has a certain ability of resisting NLOS error, but the positioning accuracy under low error influence is not ideal, but the positioning scheme proposed by the present invention can effectively estimate the position of the mobile station under the condition that there are at least three LOS paths, and when the NLOS propagation error increases significantly, the variance of the estimation result of the method of the present invention is basically the same. Since there are at least 3 LOS paths, the influence of equation 6 tends towards 0, and the positioning scheme of the present invention approaches the CRLB limit in case of high estimated error variance, which better approximates the true TOA distance value.
Referring to fig. 4, a schematic diagram of a radio frequency identification system of the present invention includes a positioning device and a signal processing module.
A signal processing module 411, configured to synchronize and demodulate a received baseband signal, and obtain a power Prec of a wireless signal sent by a tag, a tag ID number, and a signal propagation delay value t, where the signal processing module 411 specifically includes: a baseband synchronization module 401 for obtaining time synchronization between the reader and the tag, a propagation delay extraction module 402 for obtaining a propagation delay value t of a signal received by the reader, a tag identification code acquisition module 403 for extracting identification information of each tag, that is, a tag ID number, and a reception power acquisition module 404 for obtaining a power Prec of a wireless signal sent by each tag.
The positioning device includes:
a parameter setting module 406, configured to set the number and the positions of the reference tags that need to be used as reference positions, and the number and the positions of the readers, where the setting of the positions of the readers and the reference tags may be different according to different geographic environments where the current system is located;
an estimated position set module 405, configured to set parameters of an undetermined error value between a reader-writer and a tag to be detected, and obtain a set of estimated positions of the tag to be detected and a weighting matrix corresponding to each estimated position according to a signal propagation delay of the tag to be detected received by each reader-writer, a position of each reader-writer, and the parameters of the error value to be determined, where an acquisition manner of the set may be: obtaining by adopting weighted least squares (TOA) estimation according to the propagation delay of the tag to be detected received by each reader-writer, wherein the set of the estimated positions can be a set of estimated positions considering all conditions;
a first estimated position module 407, configured to obtain a first estimated position of the tag to be detected by using a k-nearest neighbor algorithm according to the receiving power of the tag to be detected and each reference tag received by each reader, and the position of each reference tag, where in the k-nearest neighbor algorithm, a weighting factor for the reference tag is,
wherein j denotes the jth adjacent reference tag, E j Representing the Euclidean distance of the received signal strength between the label to be detected and the jth reference label;
an error elimination module 408, configured to select a second estimated position closest to the first estimated position from the estimated position set, obtain a distance value between the reader and the tag to be detected according to the signal delay of the tag to be detected, and obtain the error value according to a median value between the first estimated position and the second estimated position and the distance value;
and the tag positioning module 409 is configured to update the weighting matrix according to the error value, and re-estimate the position of the to-be-determined tag to obtain a final positioning position of the to-be-determined tag.
The apparatus may further include a register module 410 for storing a set of estimated positions obtained by the estimated position set module.
In addition, the reference tag can be an active tag or a passive tag, the active tag can actively transmit signals and can identify a plurality of target systems at the same time, the passive tag can work only by providing energy from the outside, when the tag enters a working area of the system and receives specific electromagnetic waves, the coil generates induced current, and the tag is powered by the rectifying circuit. According to different requirements of practical application, different types of labels can be selected, passive labels can be selected under the condition of short-distance identification, and active labels can be selected under the condition of long-distance identification.
According to the positioning device of the radio frequency identification system, when a tag to be detected needs to be positioned and estimated, according to the geographic environment of the current system and comprehensively considering other factors, the number and the position of a reader-writer and a reference tag are set by a parameter setting module 406, the reference tag can be an active tag or a passive tag, then a signal processing module 411 synchronizes and demodulates a received signal received by each reader-writer, obtains a propagation delay t, a tag identification ID number and a received power Prec, and transmits t and ID to an estimated position set module 405, transmits ID and Prec to a first position estimation module 407, an estimated position set module 405 obtains a set of estimated positions and corresponding weighted matrixes according to the received t and ID and the number and the position of the reader-writer set by the parameter setting module 406, and stores the set and the weighted matrixes into a register 410, transmits the set and the weighted matrixes to an error elimination module 408 by the register or directly to the error elimination module 408, the initial position estimation module 407 updates the estimated position set and the estimated error value of the estimated position set by the initial position estimation module 408 according to the received tag number and the received Prec, and the estimated position set of the weighted matrixes 406, and the estimated error value of the estimated position estimated error matrix obtained by the initial position estimation module 408, and the estimated error value obtained by the estimated error estimation module 408, and the estimated error estimation module 408, a final estimated position is obtained.
It should be noted that the above detailed description is only a detailed description of possible embodiments of the present invention, and two or more specific labels may be applied without departing from the spirit of the present invention.
Claims (14)
1. A positioning method of a radio frequency identification system is characterized by comprising the following steps: presetting the number of readers-writers, the position coordinates of the readers-writers, the number of reference labels, the position coordinates of the reference labels and undetermined error parameter between the readers-writers and the labels to be detected;
acquiring a set of estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal time delay of the to-be-detected label and the to-be-determined error difference value parameter;
obtaining a first estimated position of the to-be-detected label according to the to-be-detected label and the receiving power of each reference label signal and the position coordinate of the reference label;
selecting a second estimated position from the set that is closest to the first estimated position, and obtaining the error value from a median of the first estimated position and the second estimated position;
and updating the weighting matrix according to the error value, and acquiring the position coordinate of the label to be detected according to the updated weighting matrix.
2. The method of claim 1, wherein the step of determining the position of the rfid tag comprises: the error value comprises a non line of sight error value NLOS between the reader and the to-be-detected reader and a time delay estimation error value of the reader.
3. The method of claim 1, wherein: and acquiring a set of the estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position by adopting a weighted least square algorithm.
4. The method of claim 1, wherein: and obtaining a first estimated position of the label to be detected through a k-proximity algorithm.
5. The method of claim 1, wherein the step of determining the position of the rfid tag comprises: the number of reference tags is at least 3.
6. The method of claim 1, wherein the step of determining the position of the rfid tag comprises: the reference tag is an active tag or a passive tag.
7. The method of claim 1, wherein: the process of obtaining the error value specifically includes: obtaining a distance value between the reader with the error value and the tag to be tested according to the signal time delay of the tag to be tested, and obtaining the error value according to the median value and the distance value; the reader/writer with the error value is as follows: and the reader corresponding to the zero element in the weighting matrix corresponding to the second estimation position.
8. The method of claim 1, wherein: in the k-neighbor algorithm, the weighting factor of the reference label is
Wherein i and j are integers less than or equal to q and represent the ith and jth adjacent reference label, E i 、E j Respectively representing the euclidean distances of the received power between the tag to be measured and the ith and jth reference tags.
9. The method of any one of claims 1 to 8, wherein: the reader and the reference tags are uniformly distributed under the condition that the wireless environment is uniform.
10. A positioning device in a radio frequency identification system, comprising: the method comprises the following steps:
the parameter setting module is used for acquiring the position coordinates of each reference label and the position coordinates of each reader-writer;
the estimated position set module is used for acquiring and obtaining a set of estimated positions of the to-be-detected label and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal time delay of the to-be-detected label and the to-be-determined error value parameter between each reader-writer and the to-be-detected label;
the first estimation position module is used for acquiring a first estimation position of the to-be-detected tag according to the position coordinates of the reference tags and the receiving power of the to-be-detected tag and the signals of the reference tags;
an error elimination module, configured to select a second estimated location from the set that is closest to the first estimated location, and obtain the error value according to a median value of the first estimated location and the second estimated location;
and the estimation position set module updates the weighting matrix according to the error value obtained by the error elimination module and obtains the position coordinate of the label to be detected according to the updated weighting matrix.
11. The location apparatus of claim 10, wherein: the device also comprises a register module used for storing the set of the estimated positions obtained by the estimated position set module and the weighting matrix corresponding to each estimated position.
12. The positioning device of claim 10, wherein: the reader and the reference tags are uniformly distributed under the condition that the wireless environment is uniform.
13. The positioning device of the rfid system according to claim 10, 11 or 12, wherein: the reference tag is an active tag or a passive tag.
14. A radio frequency identification positioning system, characterized by: the method comprises the following steps:
the parameter setting module is used for acquiring the position coordinates of each reference label and the position coordinates of each reader-writer;
the signal processing module is used for synchronizing and demodulating the signals of the to-be-detected label and each reference label received by each reader-writer to acquire the identification information of the labels, the receiving power of the signals and the signal time delay;
the estimated position set module is used for acquiring a set of the estimated positions of the tags to be detected and a weighting matrix corresponding to each estimated position according to the position coordinates of each reader-writer, the signal time delay of the tags to be detected and the parameters of the undetermined error values between the reader-writer and the tags to be detected;
the first estimation position module is used for obtaining a first estimation position of the to-be-detected label according to the position coordinates of the reference labels and the receiving power of the to-be-detected label and the signals of all the reference labels;
an error elimination module, configured to select a second estimated location from the set that is closest to the first estimated location, and obtain the error value according to a median value of the first estimated location and the second estimated location;
and the estimation position set module updates the weighting matrix according to the error value obtained by the error elimination module, and obtains the position coordinate of the label to be detected according to the updated weighting matrix.
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