CN106950528B - direction-of-arrival estimation method based on linear frequency modulation signals - Google Patents
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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
- G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
- G01S3/02—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
- G01S3/14—Systems for determining direction or deviation from predetermined direction
- G01S3/46—Systems for determining direction or deviation from predetermined direction using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
- G01S3/50—Systems for determining direction or deviation from predetermined direction using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems the waves arriving at the antennas being pulse modulated and the time difference of their arrival being measured
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Abstract
The invention discloses a linear frequency modulation signal-based direction-of-arrival estimation method.A distance between two antennas installed at an anchor node is d, and the two antennas are controlled by the same oscillator to simultaneously transmit LFM signals with different initial frequencies and the same frequency modulation rate; the tag node performs A/D sampling on a received signal received by an antenna, performs down-conversion processing on the signal, performs NDFT (normalized difference Fourier transform), generates time delay difference on a direct signal reaching the tag node through the two antennas in a line-of-sight environment, and estimates the cycle estimation AOA of the signal after the NDFT; when the multipath effect exists, different time delays exist in direct signals reaching the tag node through the two antennas, the received signal period corresponding to the time delay difference of the direct signals is solved, NDFT of multiple points is carried out on interested frequency points, the frequency corresponding to the maximum peak value is searched, and the AOA of the signal is estimated. The invention has the advantages of multipath effect resistance, high dynamic property, high precision, low hardware complexity and the like.
Description
Technical Field
The invention belongs to the field of wireless sensor technology and electronic communication systems, and relates to a direction of arrival estimation method based on linear frequency modulation signals.
Background
the positioning of an object by measuring the angle of arrival (AOA) of RF (radio frequency) signals is a frequently used mode of operation of RF positioning systems. In recent years, with the popularization of wireless networks and the development of indoor positioning technologies, Location Based Services (LBS) have received increasing attention; for example, it shows great vitality in the fields of emergency assistance, medical care, personalized information transfer, and the like. The purpose of wireless sensor network node location is to measure relevant parameters of received radio waves, such as time of arrival (TOA), time difference of arrival (TDOA), angle of arrival (AOA) or signal field strength (RSS), etc., the observation formula of the parameters is a function of the target position, the observation station estimates the position of a signal source through a proper location algorithm, and then gives absolute or relative coordinates of each node in a plane or a space according to a specific algorithm.
In the current common positioning schemes, the positioning scheme based on TOA/TDOA is common, the positioning scheme based on TOA/AOA draws more and more attention, and the positioning scheme based on AOA has been studied in a great deal. Therefore, the technology of AOA estimation of RF signals is also very important, and with the development of RF technology and the continuous emergence of new measurement technology, the angle measurement technology is also continuously perfected and developed, and generally, the AOA estimation problem can adopt a traditional method or a modern method. In the traditional method, a mechanical rotary directional antenna is adopted to realize AOA estimation, so that the interference is easy to happen, and the angular resolution ratio is low; modern methods employ array antennas, which overcome the deficiencies of conventional methods, but require a large number of antennas and are therefore expensive.
In summary, the existing AOA estimation techniques have the following disadvantages: the method based on the mechanical rotation directional antenna has the advantages of low resolution ratio and low angle estimation precision; due to the fact that multipath effects are very serious in a complex environment, the obtained AOA has large errors, and direct transit time delay difference of signals cannot be accurately obtained; the array antenna based approach requires more antennas, with higher cost and power consumption.
Disclosure of Invention
the invention discloses a direction-of-arrival estimation method based on linear frequency modulation signals, aiming at overcoming the defects of low estimation angle precision, large errors in multipath effect, large number of antennas and high cost in the existing AOA estimation technology.
The invention relates to a direction of arrival estimation method based on linear frequency modulation signals, which consists of an anchor node provided with two omnidirectional antennas and a tag node provided with one antenna;
Specifically, the distance between two antennas installed on the anchor node is d, the anchor node has a function of transmitting a chirp LFM signal, and the two antennas are controlled by the same oscillator to simultaneously transmit LFM signals with different initial frequencies and the same frequency modulation rate.
the method for estimating the direction of arrival based on the linear frequency modulation signal comprises the following steps:
The method comprises the following steps: the anchor node is provided with two antennas with the installation distance d, and transmits LFM signals with different initial frequencies and the same frequency modulation rate:
wherein: the time domain waveform of the signal transmitted by the antenna A is as follows:
The instantaneous frequency of the signal is:
fA(t)＝fat+βt, (2)
The time domain waveform of the signal transmitted by the antenna B is as follows:
The instantaneous frequency of the signal is:
fB(t)＝fb+βt, (4)
Wherein: f. ofa＞fbAnd is and
fais the initial frequency of the antenna A, fbWhich is the initial frequency of antenna B, t represents time, beta represents the fm rate,which represents the initial phase of the antenna a,represents the initial phase of antenna B;
step two: because the indoor range is small, the transmitting signal is considered to be transmitted under the condition of near field, under the LOS (line of sight) environment, the tag node carries out A/D (analog to digital) sampling on the signal received by the antenna and carries out down-conversion processing on the signal at the same time, and the sampling period is Tssampling frequencyReceive a signal of：
sRSS＝||r(t)||2+w(t), (6)
wherein: w (t) represents the received white noise, r (t) is the signal after down-conversion of the received signal of the antenna:
Wherein: p and Q represent the number of multipaths for antennas a and B,τAi、τBiRepresenting the attenuation and the time delay of antennas A and B, respectively, on the ith path, where τAi、τBiif LOS direct path delay (under LOS environment) of the antennas A and B exists, switching to the fifth step when obvious multipath effect exists in the indoor environment, and otherwise, switching to the third step;
step three: in an LOS environment, different time delays exist in direct signals reaching a tag node through two antennas, and time delay difference is generated. In this case, in equation (7), P is 1 and Q is 1, where equation (7) is substituted into equation (6), and the received signal is obtained by arranging:
Wherein:
θ is the direction of arrival AOA of the signal, d is the spacing of A, B antennas, and c represents the speed of light;
in LOS environment, the received signal is of (f)a-fb-β(τA1-τB1) Step eight) of frequency sine wave, and NDFT conversion is carried out on the received signal; then, turning to the step four;
step four: determining the period of a signal after NDFT of the signalobtaining the time delay difference tauA1-τB1Then, the direction of arrival AOA can be obtained from the formula (9) through the geometric relationship, and the step nine is carried out as shown in the formula (10);
Wherein: c denotes the speed of light and d is the distance between antennas a and B.
Step five: when multipath effect exists, different time delays exist in direct signals reaching a label node through two antennas, in the formula (7), P and Q are not 1 at the same time, the formula (7) is substituted into the formula (6), and the received signals are arranged to obtain the following expression:
wherein: tau isk＝τAi-τBjthe delay difference between the ith path of the antenna A and the jth path of the antenna B is shown, and pi is a constant.
under the multipath environment, the received signal is different from the multipath time delay taukrelated different periods (f)a-fb-βτk) The superposition of signals is useful for AOA estimation because of the strong amplitude of the direct signal, namely the direct path time delay difference tau of signals transmitted by two antennask＝τA1-τB1The corresponding signal frequencies are:
the frequency range of the key region is: (f)a-fbD/c) assuming fa-fb＝K·fs，fs> 2 β d/c, the frequency range of the region of interest is then:
(K-1)fs～(K+1)fs, (13)
setting the maximum number of points in the ranges, switching to the step eight for NDFT transformation, and then switching to the step six;
step six: NDFT variation is carried out on the signal, whether the peak value of the signal after conversion exceeds a specified threshold value is judged, if not, the step seven is carried out, otherwise, the frequency corresponding to the position where the peak value of the signal after conversion appears is searchedas fRSSSubstituting the estimated value into the formula (14) to obtain the AOA, and turning to the ninth step;
Wherein: the distance d between the antennas is less than c/2 beta Ts。
step seven: when the searched peak value does not exceed a preset threshold value, the current environment is considered to have no direct path signal, the current environment is an NLOS (non line of sight) environment, the frequency range of a key area is increased, and the fifth step is carried out to estimate suboptimal AOA;
Step eight: NDFT transformation is carried out on the signals, and FFT transformation sampling points are uniformly distributed on a unit circle, so that the method based on FFT transformation cannot realize sampling with smaller frequency intervals in the frequency of a key area; NDFT transformation sampling points are randomly distributed on a unit circle, the frequency range of a key area is set at the sampling points as many as possible, and the frequency interval is set to be the NDFT variation as small as possible, so that the time delay difference of the first arrival path in the multipath environment is estimated, and the NDFT transformation is shown as the following formula:
Wherein: z is a radical of0,z1,…,zN-1Is any different N points selected on the unit circle; equation (15) is written in matrix form (16), and NDFT transform is performed rapidly through matrix form:
S＝ZTs, (16)
Wherein: s ═ SNDFT(z0),SNDFT(z1),…,SNDFT(zN-1)]T, (17)
s＝[sRSS[0],sRSS[1],…sRSS[N-1]]T, (18)
at (K-1) fs～(K+1)fsThe number of sampling points in the range is larger than that of the sampling points at other places;
Step nine: and performing Kalman filtering processing on the AOA, eliminating the influence of noise on the system, further improving the estimation precision, and outputting the signal arrival direction AOA.
Compared with the prior art, the invention has the following beneficial effects: by adopting an LFM signal system and setting a large bandwidth, high time resolution can be obtained, so that the accuracy of AOA estimation is improved; in a multipath environment, non-uniform discrete Fourier transform is adopted, more points are sampled in the frequency range of a heavy point region, less points are sampled in other places, the frequency resolution performance is improved by observing a fine mechanism of a frequency spectrum, and AOA estimation under a multipath condition is more accurately carried out; meanwhile, the invention has the advantages of multipath effect resistance, high dynamic property, high precision, low hardware complexity and the like.
Drawings
The drawings described herein are for further explanation of the invention and are not to be construed as limiting the invention. In the drawings:
FIG. 1 is a block diagram of the hardware components of the method for estimating direction of arrival based on chirp signals according to the present invention
FIG. 2 is a graph showing the instantaneous frequencies of the transmitted signal (solid line) and the received signal (dotted line) of the chirp-based direction of arrival estimation method of the present invention
FIG. 3 is a diagram of the distribution of NDFT (b diagram) sampling points on a unit circle of a z plane in the chirp-based direction of arrival estimation method of the present invention
FIG. 4 is a flow chart of an embodiment of a method for estimating a direction of arrival based on a chirp signal according to the present invention
Detailed Description
the present invention will now be described in more detail and fully hereinafter with reference to the accompanying drawings and examples, which are set forth to illustrate, but are not to be construed as the limit of the present invention. It should be noted that, for the convenience of description, the drawings only show some but not all of the matters related to the present invention.
the invention designs a linear frequency modulation signal-based direction-of-arrival estimation method, which comprises the steps of simultaneously transmitting LFM signals through two antennas, sampling received signals by a receiving end, acquiring direct path transmission delay difference through an NDFT algorithm, and further eliminating the influence of multipath effect on a system by performing key sampling in a key region frequency range, thereby improving the accuracy of AOA estimation. The invention aims to overcome the defects of low precision and large error of the existing AOA estimation technology in a multipath environment, and designs an AOA estimation method with signal multipath effect resistance, high dynamics, high precision and low hardware complexity.
in order to solve the technical problems, the invention adopts the following technical scheme:
as shown in fig. 1, a block diagram of hardware of a method for estimating a direction of arrival based on a chirp signal is shown, and the method is composed of an anchor node provided with two omnidirectional antennas and a tag node provided with one antenna; two antennas installed on the anchor node have fixed intervals and have the function of transmitting linear frequency modulation signals LFM, and the two antennas are controlled by an oscillator to simultaneously transmit LFM signals with different initial frequencies and the same frequency modulation rate; the tag node performs A/D sampling on a received signal received by an antenna, performs down-conversion processing on the signal at the same time, and performs NDFT conversion on the preprocessed data; due to the small indoor range, the transmitted signal can be considered to propagate in the near field situation, as shown in fig. 2; under an LOS environment, different time delays exist in direct signals reaching a tag node through two antennas, time delay difference is generated, the period of the signals can be estimated after NDFT transformation, and AOA is further estimated according to a geometric relation and is shown in a formula (10); when a multipath effect exists, different time delays exist in direct signals reaching a tag node through two antennas, and the multipath signals generate complex time delays, so that received signals are obtained by superposition of multiple signals, the amplitude of the direct signals is strong, the received signal period corresponding to the time delay difference of the direct signals can be obtained according to the distance and the sampling rate of a transmitting antenna, multipoint NDFT is carried out on frequency points of a heavy point region, as shown in FIG. 3, the frequency corresponding to the maximum peak value, namely the time delay difference of the direct signals, the signal AOA is accurately estimated, filtering processing is carried out on the AOA, and the estimation precision is further improved; and when the maximum peak value does not exceed a preset threshold, the current environment is considered to have no direct signal, the current environment is a non line of sight (NLOS) environment, the frequency range of a key area needs to be expanded, and the NDFT is continuously adopted to calculate the suboptimal AOA under the NLOS environment.
Further, in this embodiment, in order to make a person skilled in the relevant art understand the method for estimating direction of arrival based on chirp signals of the present invention, a whole flow chart of the implementation of this embodiment is also provided, as shown in fig. 4, the flow of the method includes the following steps:
SS1. initialize the respective parameters: initial frequency f of LFM signal transmitted by antennas A and Ba,fbAnd the frequency modulation rate beta, the repetition period T of the LFM signal and the distance d between the two antennas, and the sampling period T of the received signalssampling frequencynumber of sampling points N, such that fa-fb＝K·fs，fsIf the peak value is more than 2 beta d/c, judging the threshold value delta of the transformed peak value;
SS2, periodically transmitting an LFM signal by the anchor node, sampling a receiving signal of the antenna by the tag node through the A/D module, performing down-conversion operation, and simultaneously storing original data obtained by sampling;
And SS3, NDFT change is carried out on the stored data, and the frequency range of the heavy spot area is subjected to heavy sampling, such as: (K-1) fs～(K+1)fsBy sampling more points at these frequency points, the resolution performance of the frequency is improved by observing the fine mechanism of the spectrum;
SS4. searching the peak of the transformed data and recording the frequency points of the peak mappingjudging whether the peak value exceeds a given threshold value: if the given threshold value delta is exceeded, the direct signal is considered to exist in the path, and the path is switched to SS5, otherwise, the direct signal does not exist in the current path, the path is a non-line-of-sight path (NLOS), the non-line-of-sight error is eliminated through a related mechanism, meanwhile, the frequency range of a key area is expanded, and the path is switched to SS3, and the suboptimal AOA in the current environment is searched;
SS5. according to the formula (12) fRSS＝fa-fb-β(τA1-τB1) The direct path delay difference (τ) after the signal has passed through antenna A, B is estimatedA1-τB1) According to formula (14)Estimating the direction of arrival (AOA) of the signal;
SS6. for estimationAnd filtering processing such as Kalman filtering, particle filtering and the like is carried out, so that the influence of noise on the system is further eliminated, and the estimation precision is improved.
After the steps are carried out, the angle between the tag node and the anchor node can be obtained in real time at the tag node, and high-precision estimation of the AOA is achieved.
The invention has low cost, can reduce the error caused by the multipath effect, has higher angle measurement precision through multiple practical experimental measurements, and is particularly suitable for the following scenes: the automobile can realize effective direction finding and positioning based on the AOA positioning system in positioning applications such as driving in an indoor parking lot and shopping of pedestrians in a shopping mall.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and those of ordinary skill in the art should understand; the invention can be modified or partially replaced, and the protection scope of the invention is not limited by the patent protection scope of the invention, and the protection scope of the invention is subject to the claims; all structural changes made by the contents of the drawings or the specification of the invention are intended to be covered by the protection scope of the patent.
Claims (1)
1. A direction of arrival estimation method based on linear frequency modulation signal, the apparatus that this method is based on is by installing anchor node of two omnidirectional antennas and installing the label node of an antenna to make up;
Specifically, the distance between two antennas installed on the anchor node is d, the anchor node has a function of transmitting Linear Frequency Modulation (LFM) signals, and the two antennas are controlled by the same oscillator to simultaneously transmit LFM signals with different initial frequencies and the same frequency modulation rate;
The method is characterized by comprising the following steps:
The method comprises the following steps: the anchor node is provided with two antennas with the installation distance d, and transmits LFM signals with different initial frequencies and the same frequency modulation rate:
Wherein: the time domain waveform of the signal transmitted by the antenna A is as follows:
The instantaneous frequency of the signal is:
fA(t)＝fa+βt, (2)
the time domain waveform of the signal transmitted by the antenna B is as follows:
the instantaneous frequency of the signal is:
fB(t)＝fb+βt, (4)
wherein：fa＞fband is and
faIs the initial frequency of the antenna A, fbwhich is the initial frequency of antenna B, t represents time, beta represents the fm rate,which represents the initial phase of the antenna a,Represents the initial phase of antenna B;
step two: because the indoor range is small, the transmitting signal is considered to be transmitted under the condition of near field, under the LOS environment, the tag node carries out A/D sampling on the signal received by the antenna and simultaneously carries out down-conversion processing on the signal, and the sampling period is TsSampling frequencythe received signals are:
sRSS＝||r(t)||2+w(t), (6)
wherein: w (t) represents the received white noise, r (t) is the signal after down-conversion of the received signal of the antenna:
Wherein: p and Q represent the number of multipaths, α, of antennas A and BAi、αBi、τAi、τBirepresenting the attenuation and the time delay of antennas A and B, respectively, on the ith path, where τA1、τB1if LOS direct path delay of the antennas A and B exists and obvious multipath effect exists in the indoor environment, the step five is carried out, and if not, the step three is carried out;
step three: under an LOS environment, different time delays exist in direct signals reaching the label node through two antennas, and time delay difference can be generated; in this case, in equation (7), P is 1 and Q is 1, where equation (7) is substituted into equation (6), and the received signal is obtained by arranging:
wherein:
Theta is the direction of arrival AOA of the signal, d is the spacing of A, B antennas, c represents the speed of light, alphaA1、αB1is the attenuation, τ, in the LOS direct path of antennas A and BA1、τB1Is LOS direct path delay of antennas A and B, and under LOS environment, the received signal has (f)a-fb-β(τA1-τB1) Step eight) of frequency sine wave, and NDFT conversion is carried out on the received signal; then, turning to the step four;
Step four: the frequency of the signal is obtained after NDFT conversion of the signalobtaining the time delay difference tauA1-τB1Then, the direction of arrival AOA can be obtained from the formula (9) through the geometric relationship, and the step nine is carried out as shown in the formula (10);
wherein: c represents the speed of light, d is the distance between antennas a and B;
Step five: when multipath effect exists, different time delays exist in direct signals reaching a label node through two antennas, in the formula (7), P and Q are not 1 at the same time, the formula (7) is substituted into the formula (6), and the received signals are arranged to obtain the following expression:
Wherein: tau isk＝τAi-τBjthe delay difference between the ith path of the antenna A and the jth path of the antenna B is represented, and pi represents a constant;
Under the multipath environment, the received signal is different from the multipath time delay taukRelated different frequencies (f)a-fb-βτk) The superposition of signals is useful for AOA estimation because of the strong amplitude of the direct signal, namely the direct path time delay difference tau of signals transmitted by two antennas1＝τA1-τB1The corresponding signal frequencies are:
The frequency range of the key region is: (f)a-fbD/c) assuming fa-fb＝K·fs，fs> 2 β d/c, the frequency range of the region of interest is then:
(K-1)fs～(K+1)fs, (13)
Setting the maximum number of points in the ranges, switching to the step eight for NDFT transformation, and then switching to the step six;
step six: NDFT conversion is carried out on the signal, whether the converted signal peak value exceeds a specified threshold value or not is judged, if not, the step seven is carried out, otherwise, the frequency corresponding to the position where the converted signal peak value appears is searchedAs fRSSsubstituting the estimated value into the formula (14) to obtain the AOA, and turning to the ninth step;
Wherein: the distance d between the antennas is less than c/2 beta Ts；
Step seven: when the searched peak value does not exceed the preset threshold value, the current environment is considered to have no direct path signal, the current environment is an NLOS environment, the frequency range of the key area is increased, and the fifth step is carried out to estimate the suboptimal AOA;
step eight: NDFT transformation is carried out on the signals, and FFT transformation sampling points are uniformly distributed on a unit circle, so that the method based on FFT transformation cannot realize sampling with smaller frequency intervals at the frequency of a key region; NDFT transformation sampling points are randomly distributed on a unit circle, the frequency range of a key area is provided with sampling points as many as possible, and NDFT transformation with the frequency interval as small as possible is arranged, so that the time delay difference of a first arrival path in a multipath environment is estimated, and the NDFT transformation is shown as the following formula:
wherein: z is a radical of0,z1,…,zN-1is any different N points selected on the unit circle, the formula (15) is written into a matrix form (16), and NDFT transformation is rapidly carried out through the matrix form:
S＝ZTs, (16)
Wherein: s ═ SNDFT(z0),SNDFT(z1),…,SNDFT(zN-1)]T, (17)
s＝[sRSS[0],sRSS[1],…sRSS[N-1]]T, (18)
At (K-1) fs～(K+1)fsThe number of sampling points in the range is larger than that of the sampling points at other places;
step nine: and performing Kalman filtering processing on the AOA, eliminating the influence of noise on the system, further improving the estimation precision, and outputting the signal arrival direction AOA.
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