CN107589407B - Multi-sending and multi-receiving through-wall thunder imaging back-front-back wall oscillation once multi-path restraining method - Google Patents
Multi-sending and multi-receiving through-wall thunder imaging back-front-back wall oscillation once multi-path restraining method Download PDFInfo
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Abstract
The invention discloses a multi-sending and multi-receiving through-wall mine imaging back-front-back-wall oscillation one-time multi-path restraining method. After obtaining an original radar image, binarizing the original radar image, extracting central values of all targets in the original image, calculating direct propagation distance and multipath propagation distance of target electromagnetic waves, calculating a normalized multipath error coefficient and an error threshold value, thereby judging the target of the original image, finally rejecting the multipath false target, realizing multipath inhibition, effectively identifying the real target in the radar image and the two front and back wall oscillation one-time multipath false targets, the loss of a real target can not be caused while the multipath false target is inhibited, two different multipath forms in one-way multipath can be perfectly considered, the method has the advantages of comprehensively and completely inhibiting the multipath, avoiding omission, ensuring the practical effect of the through-wall radar in the detection environment of the closed building and providing powerful guarantee for operators to make correct decisions.
Description
Technical Field
The invention belongs to the technical field of through-wall radar imaging, and particularly relates to a method for inhibiting front-back wall oscillation once multipath after multiple-shot and multiple-shot through-wall radar imaging.
Background
The through-wall radar imaging technology mainly utilizes electromagnetic waves to penetrate through barriers such as building walls and the like to detect, position and identify static or moving targets in a closed building and simultaneously image the layout of the building, and has great application value in the fields of anti-terrorism, public security law enforcement, disaster rescue and the like. In an ideal situation, when the through-wall radar detects a closed space, the transmitted electromagnetic wave signal returns to the receiver only through the backscattering of the target, so that the echo signal received by the receiver only comprises the echo signal of the target; in practical application, however, the signal is likely to be received after being refracted or reflected many times by walls, ceilings or floors and other objects in the building during the propagation process, so that the received echo signal not only contains the echo component and noise of the target, but also contains the echo components of multiple paths, which may cause the occurrence of a false target of multiple paths in radar imaging.
According to different conditions of multipath, in the MIMO through-the-wall radar, multipath propagation is carried out only during transmission, and a multipath false target directly returns to a receiving antenna from a target during receiving, or electromagnetic waves are directly propagated to the target from the receiving antenna during transmission, and multipath returning to the receiving antenna only through multipath during receiving is called multipath; multipath, which propagates via multipath paths simultaneously upon reception and transmission, is referred to as two-way multipath.
The front-back wall oscillation one-time multipath is a typical one-way multipath common in the application of a Multiple Input Multiple Output (MIMO) through-wall radar, and a multipath route of the multipath is possibly present in a transmitting antenna-target stage and possibly present in a target-receiving antenna stage, so that during focusing imaging, the multipath is split into two high-brightness areas, and under the condition of no prior information, two multipath false targets can cause false alarms and are judged as two real targets, thereby seriously affecting the imaging quality of the through-wall radar. Therefore, through-wall imaging in closed buildings is important to suppress such multipath decoys.
Many universities and research institutions both at home and abroad have many researches on multipath interference suppression methods. The university of electronic technology proposes a multi-path suppression method based on sub-aperture imaging (z.x.li, y.jia, et al, "a novel approach of multi-path rendering based sub-adaptation imaging in through-wall-Radar imaging", IEEE Radar Conference,2013.), which explains the characteristics of multi-path, that is, the multi-path position moves along with the movement of an antenna array, but the target position is always unchanged, and the purpose of multi-path suppression can be achieved only by multiplying and fusing Radar imaging of different antenna arrays. The university of veravah in the united states proposes a multipath inhibition method (setliur P, Alli G, Nuzzo l. multipath extraction in through-wall-corrugated imaging via point spread functions [ J ]. IEEE Transaction on image processing.2013.) based on a point spread function, which reversely extrapolates a multipath false target and associates the multipath false target with a corresponding real target, and superimposes the amplitude value of the multipath false target on the real target in turn, thereby improving the signal-to-noise ratio of the target position while inhibiting the multipath amplitude. The university of national defense science and technology proposes a multipath inhibition method based on compressed sensing (j.wang, p.wang, y.li, q.song and z.zhou, "AMultipath suppression technique for Through-the-wall Radar," ieee international conference on Ultra-wide, 2013.), and Radar echoes can be regarded as the mapping of a target space to an environment transfer function, so that under the condition of a known building environment, the transfer function can be reconstructed, the Radar echoes are subjected to minimum norm inversion, a real Radar image is reconstructed, and multipath false targets are avoided.
In the method, when multipath is inhibited, multipath modeling does not consider the characteristics of multipath in a transmitting path and a receiving path in the MIMO radar, and the multipath is represented as two separate targets when imaging, but only considers and processes the condition of a single path of a type of multipath. Therefore, the method for suppressing the one-way multipath interference of the MIMO through-wall radar in the closed space has important value in the field of through-wall radar imaging.
Disclosure of Invention
The invention aims to: in order to solve the problems in the prior art, the invention provides a multi-shot multi-path restraining method for multiple-shot multi-shot wall-through thunder to achieve one-shot back-and-forth wall oscillation.
The technical scheme of the invention is as follows: a multi-sending and multi-receiving through-wall lightning imaging back and front wall oscillation once multi-path restraining method comprises the following steps:
A. constructing a front wall and rear wall oscillation multipath model, and processing a received signal by adopting a subsequent projection imaging method to obtain an original radar image;
B. recording an imaging space between the first layer wall and the second layer wall as an area 1, recording an imaging space behind the second layer wall as an area 2, and performing 8-connected domain extraction on the original radar image in the step A to obtain a binary radar image;
C. respectively marking the targets in the region 1 and the region 2, extracting barycentric coordinates of all the targets and taking the barycentric coordinates as coordinates of each corresponding target;
D. respectively calculating the direct path propagation distance corresponding to each target in the area 2 and the two types of multipath propagation distances corresponding to each target in the area 1;
E. calculating a normalized multipath error coefficient according to the direct path propagation distance and the two types of multipath propagation distances in the step D;
F. calculating an error threshold value according to the normalized multipath error coefficient in the step E;
G. performing multi-path judgment on each target in the area 2 according to the normalized multi-path error coefficient in the step E and the error threshold value in the step F;
H. and G, processing the binary radar image in the step B according to the multipath judgment result in the step G to obtain a radar image after multipath inhibition.
Further, the building of the front wall and rear wall concussion multipath model in the step a is to set the Target (x) position as the position of the Targettar,ytar) The mth transmitting array element coordinate is (x)tm,ytm) The nth receiving array element coordinate is (x)n,yn) The wall thickness of the front wall is d, the dielectric coefficient is epsilon, and the coordinate of the rear wall is ybThe transmitting signal is s (t), the receiving signal is r (t), and the electromagnetic wave propagation path is a transmitting antenna, a target and a receiving antenna.
Further, the received signal in the step A is represented as
r(t)＝αs(t-τtar)+β1s(t-τm1)+β2s(t-τm2)
Wherein, α1,β2Scattering coefficients, tau, for the target and multipath path 1, multipath path 2, respectivelytar,τm1,τm2The echo delays of the target and the multipath path 1 and the multipath path 2 respectively.
Further, the step C marks the targets in the region 1 and the region 2, respectively, extracts barycentric coordinates of all the targets and uses the barycentric coordinates as coordinates of each corresponding target, specifically, marks the target in the region 1 as [ Pf1,Pf2,...PfK]Where K is the total number of targets in region 1 and the K-th target barycentric coordinate is Pfk(xfk,yfk) (ii) a Label the target of region 2 as [ P ]b1,Pb2,...PbT]Where T is the total number of targets in zone 2 and the coordinates of the center of gravity of the tth target is Pbt(xbt,ybt)。
Further, the calculation formula for calculating the direct path propagation distance corresponding to each target in the area 2 in the step D is
Wherein M and N are respectively the number of antenna transmitting and receiving array elements and gammat,mnThe electromagnetic wave propagation distance corresponding to the mth transmitting antenna and the nth receiving antenna is set for the tth target.
Further, the calculation formula for calculating the two types of multipath propagation distances corresponding to each target in the area 1 in the step D is
Where γ' is the electromagnetic wave propagation distance corresponding to the multipath path 1, and γ ″ is the electromagnetic wave propagation distance corresponding to the multipath path 2.
Further, the calculation formula of the normalized multipath error coefficient in the step E is
Wherein, λ'i-j,λ″i-jRespectively, the ith target P in the region 1fiAnd the target P in the area 2bjNormalized error coefficients corresponding to multipath path 1 and multipath path 2.
Further, the calculation formula of the error threshold value in the step F is
Wherein epsiloni′,εi"respectively corresponding normalized error coefficients λ'i-j,λ″i-jThe error threshold value of (2).
Further, the step G of performing multipath decision on each target in the area 2 according to the normalized multipath error coefficient in the step E and the error threshold value in the step F specifically includes the following sub-steps:
g1, setting the initial iteration number k to 1, and selecting one target judgment error coefficient vector minimum value from the region 1
Whether or not there is
And is
If not, reselecting the next target; if so, the next step is carried out；
G2, judgment subscript akWhether or not it is equal to subscript bk(ii) a If so, the target is not multipath; if not, the corresponding subscript a in the area 2k,bkTwo objects of
Is the kth target P of region 1fkA multipath of (a);
g3, judging whether the iteration times K are less than K; if yes, returning to the step G1; if not, the judgment is finished.
Further, the step H of processing the binarized radar image in the step B according to the multipath decision result in the step G specifically includes setting the pixel points of the corresponding area to 0 in the binarized radar image according to the target group in the step G which is decided as the multipath for the area 2, and then multiplying the binarized radar image and the original radar image by the pixel points in a one-to-one correspondence manner.
The invention has the beneficial effects that: after obtaining an original radar image, the invention binarizes the original radar image through threshold detection, extracts the central values of all targets in the original image, calculates the direct propagation distance and the multipath propagation distance of target electromagnetic waves based on the geometric principle of multipath propagation, and calculates the normalized multipath error coefficient and the error threshold value, thereby judging the target of the original image to be a real target or a multipath false target, and finally eliminates the multipath false target to realize multipath inhibition, thereby effectively identifying the real target in the radar image and two multipath false targets oscillated by front and back walls, avoiding the loss of the real target while inhibiting the multipath false target, simultaneously perfectly considering two different multipath forms in multipath one-way, comprehensively and completely inhibiting the multipath without omission, and ensuring the practical effect of the through-wall radar in the closed building detection environment, and provides powerful guarantee for operators to make correct decisions.
Drawings
Fig. 1 is a flow diagram of a multi-shot multi-receive through-wall lighting method for one-time multi-path suppression of front and rear wall oscillation after imaging.
Fig. 2 is a schematic diagram of target path coordinates in an embodiment of the present invention.
Fig. 3 is a schematic diagram of front-back wall joint multipath coordinates in the embodiment of the present invention.
FIG. 4 is a diagram of a simulation scenario in an embodiment of the present invention.
Fig. 5 is a schematic diagram of an antenna array according to an embodiment of the present invention.
Fig. 6 is a diagram illustrating the result of raw radar imaging in an embodiment of the present invention.
Fig. 7 is a schematic diagram of a binarized radar image in an embodiment of the present invention.
Fig. 8 is a schematic diagram of a binarized image after multipath suppression in an embodiment of the present invention.
Fig. 9 is a schematic diagram of a radar image after multipath mitigation in an embodiment of the invention.
FIG. 10 is a diagram illustrating the results of raw radar imaging in another embodiment of the present invention.
Fig. 11 is a schematic diagram of a binarized radar image according to another embodiment of the present invention.
Fig. 12 is a schematic diagram of a radar image after multipath mitigation in another embodiment of the present invention.
Fig. 13 is a schematic diagram of a radar image after multipath mitigation in another embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Fig. 1 is a schematic flow chart of a multi-shot multi-receive through-wall lighting method for one-shot multi-path suppression of back-and-front wall oscillation according to the present invention. The invention provides a method for inhibiting front-back wall oscillation once multipath after MIMO through-wall radar imaging in a closed building space, which comprises the following steps:
A. constructing a front wall and rear wall oscillation multipath model, and processing a received signal by adopting a subsequent projection imaging method to obtain an original radar image;
B. recording an imaging space between the first layer wall and the second layer wall as an area 1, recording an imaging space behind the second layer wall as an area 2, and performing 8-connected domain extraction on the original radar image in the step A to obtain a binary radar image;
C. respectively marking the targets in the region 1 and the region 2, extracting barycentric coordinates of all the targets and taking the barycentric coordinates as coordinates of each corresponding target;
D. respectively calculating the direct path propagation distance corresponding to each target in the area 2 and the two types of multipath propagation distances corresponding to each target in the area 1;
E. calculating a normalized multipath error coefficient according to the direct path propagation distance and the two types of multipath propagation distances in the step D;
F. calculating an error threshold value according to the normalized multipath error coefficient in the step E;
G. performing multi-path judgment on each target in the area 2 according to the normalized multi-path error coefficient in the step E and the error threshold value in the step F;
H. and G, processing the binary radar image in the step B according to the multipath judgment result in the step G to obtain a radar image after multipath inhibition.
In step a, the electromagnetic wave oscillates back and forth between the front wall and the rear wall once from the transmitting antenna, and finally the multipath formed by the reflection of the target to the receiving antenna, or the multipath formed by the oscillation of the electromagnetic wave once between the front wall and the rear wall after the transmitting antenna reaches the receiving antenna is called front wall and rear wall oscillation multipath.
The invention constructs a front wall and rear wall vibration multipath model, and particularly sets the position of a Target to be Target (x)tar,ytar) The mth transmitting array element coordinate is (x)tm,ytm) The nth receiving array element coordinate is (x)n,yn) The wall thickness of the front wall is d, the dielectric coefficient is epsilon, and the coordinate of the rear wall is ybThe transmitting signal is s (t), the receiving signal is r (t), and the electromagnetic wave propagation path is a transmitting antenna, a target and a receiving antenna. Fig. 2 is a schematic diagram of target path coordinates in the embodiment of the present invention.
Fig. 3 is a schematic diagram of front and back wall joint multipath coordinates in the embodiment of the present invention.Due to the existence of the rear wall, front and rear wall oscillation multipath exists in practical situations. If the transmitted signal is s (t), the echo signal received by the receiving antenna should be the sum of the electromagnetic wave propagating back to the receiving antenna in the target path, the multipath path 1, and the multipath path 2. Setting target echo time delay as tautarEcho delay of multipath path 1 is taum1Echo delay of multipath path 2 is taum2Then the received signal r (t) is
r(t)＝αs(t-τtar)+β1s(t-τm1)+β2s(t-τm2)
Wherein, α1,β2Scattering coefficients, tau, for the target and multipath path 1, multipath path 2, respectivelytar,τm1,τm2The echo delays of the target and the multipath path 1 and the multipath path 2 respectively.
And processing the received signal by adopting a subsequent projection imaging method to obtain an original radar image.
For a region to be detected with a plurality of targets, a simulation scene is shown in fig. 4, a coordinate system zero point is located at the center of the front surface of the front wall, 2 transmitting antennas and 8 receiving antennas are linearly placed at the position ten meters away from the front wall, an antenna distribution diagram is shown in fig. 5, a transmitting signal of a radar is a stepped frequency signal with center frequency and bandwidth, and frequency is stepped. The front wall has a thickness of 0.24m and the front surface of the rear wall is located at z 10m, both walls having an equal relative permittivity of 8.6. The three targets are located at (2.0,4.0) m, (-3.0,6.0) m, and (-1.0,18.0) m, respectively, and the original radar image is shown in fig. 6.
It can be clearly found from the image that in the rear area of the second wall, the oscillation multipath of the two front and rear walls of the target 1 is very clear, and whether the through-wall radar is a real target or not can not be judged under the condition of no prior information during actual imaging of the through-wall radar.
In step B, according to the front wall and rear wall oscillation multipath model, if there is a multipath of this type, its position is located behind the second wall, and its corresponding target is between the first wall and the second wall. The present invention refers to the imaging space between the first wall and the second wall as region 1 and the imaging space behind the second wall as region 2.
And preprocessing the obtained radar image by using MATLAB, and extracting an 8-connected domain of the image by using an im2bw function to obtain a binary radar image, wherein the binary radar image comprises all targets (including real targets and multipath false targets) obtained by imaging. Fig. 7 is a schematic diagram of a binarized radar image according to an embodiment of the present invention.
In step C, the invention marks the targets in the area 1 and the area 2 respectively according to the positions of the targets, extracts barycentric coordinates of all the targets and takes the barycentric coordinates as the coordinates of each corresponding target, specifically, marks the target in the area 1 as [ P ]f1,Pf2,...PfK]Where K is the total number of targets in region 1 and the K-th target barycentric coordinate is Pfk(xfk,yfk) (ii) a Label the target of region 2 as [ P ]b1,Pb2,...PbT]Where T is the total number of targets in zone 2 and the coordinates of the center of gravity of the tth target is Pbt(xbt,ybt)。
All the coordinates of the barycenter of the target located in the area 1 are detected and distinguished from the position y of the back wall being 10m, where the coordinates of the barycenter of the target located in the area 1 are Pf1＝(2.03,4.00)m,Pf2The target barycentric coordinates of region 2 are P (-3.02,6.00) mb1＝(1.44,13.85)m,Pb2＝(3.15,13.65)m,Pb3＝(-0.98,18.02)m。
In step D, the invention calculates the direct path propagation distance corresponding to each target in the area 2, and the calculation formula is
Wherein M and N are respectively the number of antenna transmitting and receiving array elements and gammat,mnThe electromagnetic wave propagation distance corresponding to the mth transmitting antenna and the nth receiving antenna is set for the tth target. When the m-th transmitting antenna coordinate T is knownmN th receiving antenna coordinate RnFront wall relative dielectric constant εrWith thickness d, γ can be calculated by geometric theorem according to the path shown in FIG. 2t,mn。
Calculating the direct echo distance of two targets in the region to obtain
The invention calculates two kinds of multipath propagation distances corresponding to each target in the area 1, and the calculation formula is
Where γ' is the electromagnetic wave propagation distance corresponding to the multipath path 1, and γ ″ is the electromagnetic wave propagation distance corresponding to the multipath path 2. Similarly, the result can be calculated according to the theorem of geometry.
Calculating two types of primary multi-path distances of front and rear wall oscillation of the target in the area 1:
in step E, since there are always two general paths for the front and rear wall oscillation multipaths, it can be assumed that the two multipaths always occur simultaneously. For a single target P of region 1fiThe invention defines it with the object P of the area 2bjThe normalized multipath error coefficient of (c) is:
wherein, λ'i-j,λ″i-jRespectively, the ith target P in the region 1fiAnd the target P in the area 2bjNormalized error coefficients corresponding to multipath path 1 and multipath path 2.
Then there are two error coefficient vectors for each target in region 1:
λi′＝[λ′i-1,λ′i-2,...λ′i-T]
λi″＝[λ″i-1,λ″i-2,...λ″i-T]
for a single multipath path, there is at most one multipath for each area 1 target, and only the area 2 target corresponding to the minimum value in the error coefficient vector is likely to be the multipath for the area 1 target.
Multiple targets have multiple error coefficient vectors. Finding out the minimum value of the coefficient vector corresponding to each target, which is respectively:
calculating the normalized multipath error coefficients of two targets in the area 1 and three targets in the area two respectively:
to obtain Pf1The two error coefficient vectors of (2) are:
λ1′＝[λ′1-1,λ′1-2,λ′1-3]＝[0.0012,0.0027,0.1209]
λ1″＝[λ″1-1,λ″1-2,λ″1-3]＝[0.0025,0.0013,0.1703]
Pf2the error coefficient vector of (a) is:
λ2′＝[λ′2-1,λ′2-2,λ′2-3]＝[0.1132,0.1129,0.0386]
λ2″＝[λ″2-1,λ″2-2,λ″2-3]＝[0.1141,0.1139,0.0378]
for two targets in the area 1, respectively calculating the minimum value of error coefficient vectors of two kinds of multipath as:
in step F, the position offset of a certain target in the image from the theoretical multipath should be kept within a range resolution unit, so that the target can be determined to be a multipath false target.
The theoretical range resolution of the through-the-wall radar is
Where c is the speed of light and B is the bandwidth of the electromagnetic wave, since the multipath path used in calculating the error coefficient is a two-way distance, the maximum allowable position offset should be 2 × Δ R, and the threshold ε of a multipathmThe method comprises the following steps:
wherein epsiloni′,εi"respectively corresponding normalized error coefficients λ'i-j,λ″i-jThe error threshold value of (2).
The bandwidth of electromagnetic wave used in simulation is 1GHz, so the distance resolution is
Therefore, the corresponding error coefficient threshold can be calculated as:
ε1′＝0.0062,ε′2＝0.0055,ε1″＝0.0062,ε″2＝0.0054
in step G, since there are always two general paths for the front and back wall oscillation multipath, it can be assumed that the two multipath always occur simultaneously, and therefore, for the same target, the minimum value of the two corresponding coefficient vectors should be smaller than the threshold at the same time to determine that it is a group of multipath.
The multipath decision of each target in the area 2 according to the normalized multipath error coefficient in the step E and the error threshold value in the step F specifically comprises the following sub-steps:
g1, setting the initial iteration number k to 1, and selecting one target judgment error coefficient vector minimum value from the region 1
Whether or not there is
And is
If the multipath does not exist, the target does not have the multipath, and the next target is reselected; if yes, carrying out the next step;
g2, judgment subscript akWhether or not it is equal to subscript bk(ii) a If yes, indicating that the positions of the two types of multipath are superposed and are not in accordance with the actual condition, and judging that the target is not multipath; if not, the corresponding subscript a in the area 2 is describedk,bkTwo objects of
Is the kth target P of region 1fkA multipath of (a);
g3, traversing all the targets in the region 1, and judging whether the iteration number K is less than K; if yes, returning to the step G1; if not, the judgment is finished.
For region 1 object 1, Pf1Existence of min1′＝λ″1-1＝0.0012＜ε1′,min1″＝λ″1-2＝0.0013＜ε1And min1′＝λ′1-1The corresponding zone 2 subscript is 1, and the corresponding zone 2 target is Pb1，min1″＝λ″1-2The corresponding zone 2 subscript is 2, and the corresponding zone 2 target is Pb2Can determine Pb1And Pb2Two multipaths of object 1.
For region 1 object 2, i.e. Pf2Is min'2＝λ′2-3＝0.0386＞ε′2,min″2＝λ″2-3＝0.0378＞ε″2I.e. both are greater than the threshold value, the target Pf2There are no corresponding multipaths in region 2.
Therefore P isb1、Pb2Is a target Pf1The front and rear walls of (1) oscillate once for multipath, and the other objects are real existing objects.
In step H, the invention processes the binary radar image in step B according to the multi-path judgment result in step G, specifically, according to the target group which is judged to be multi-path in step G to the area 2, the pixel point of the corresponding area is set to be 0 in the binary radar image, and then the binary radar image and the original radar image are multiplied one by one in a pixel point-to-one correspondence manner, thereby obtaining the radar image after multi-path inhibition.
In the binary radar image, the corresponding region is set to zero, and the suppressed binary image is shown in fig. 8.
The binary image is multiplied by corresponding pixel points of the original image, so that a final multipath suppression image can be obtained, as shown in fig. 9.
The multi-shot multi-receive wall-penetrating radar imaging method of the present invention for multi-shot multi-path suppression of back-and-front wall oscillation will be further described below with an embodiment of measured data.
The experimental scene is the same as the simulation scene, the single target is respectively positioned at (2.3,7.6) m, and the coordinate system, the antenna array element and the wall body parameter adopt the same parameters as those adopted in MATLAB simulation. The original radar image is shown in fig. 10.
The image is binarized to obtain a binarized image as shown in fig. 11.
All the coordinates of the barycenter of the target located in the area 1 are detected and distinguished from the position y of the back wall being 10m, where the coordinates of the barycenter of the target located in the area 1 are Pf1The target barycentric coordinates of region 2 are each P (2.31,7.56) mb1＝(2.96,17.11)m,Pb2＝(1.15,17.40)m，。
Calculating the direct echo distance of the target in the region 2 to obtain
Calculating two types of multi-path distances of front and back oscillation walls of the target in the area 1:
calculating normalized multipath error coefficients of the target in the area 1 and the target in the area 2 respectively;
to obtain Pf1The two error coefficient vectors of (2) are:
λ1′＝[λ′1-1,λ′1-2]＝[0.0015，0.0056]
λ1″＝[λ″1-1,λ″1-2]＝[0.0056，0.0023]
for Pf1And respectively calculating the minimum value of error coefficient vectors of two kinds of multipath as follows:
the bandwidth of electromagnetic wave used in simulation is 1GHz, so the distance resolution is
Therefore, the corresponding error coefficient threshold can be calculated as:
ε1′＝0.0054,ε1″＝0.0054
for region 1 object 1, Pf1Existence of min1′＝λ′1-1＝0.0015＜ε1′,min1″＝λ″1-2＝0.0023＜ε1And min1′＝λ′1-1The corresponding zone 2 subscript is 1, and the corresponding zone 2 target is Pb1，min1″＝λ″1-2The corresponding zone 2 subscript is 2, and the corresponding zone 2 target is Pb2Can determine Pb1And Pb2Two multipaths of object 1.
Therefore P isb1、Pb2Is a target Pf1Front and rear wall oscillation primary multipath, Pf1Is a real existing target. In the binary radar image, the multipath corresponding area is set to zero, and the suppressed binary image is obtained as shown in fig. 12.
The binary image and the original image are multiplied by corresponding pixel points, so that a final multipath suppression image can be obtained, as shown in fig. 13.
According to simulation and actual measurement results, the method for restraining the front and rear wall oscillation primary multipath after MIMO through-wall radar imaging can effectively eliminate paired front and rear wall primary multipath false targets in the image, obtain a correct detection result, and verify the correctness and effectiveness of the method.
It will be appreciated by those of ordinary skill in the art that the embodiments described herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited embodiments and examples. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.
Claims (9)
1. A multi-sending and multi-receiving through-wall lightning imaging back and front wall oscillation once multi-path restraining method is characterized by comprising the following steps:
A. constructing a front wall and rear wall oscillation multipath model, and processing a received signal by adopting a subsequent projection imaging method to obtain an original radar image;
B. recording the front wall as a first layer wall and recording the rear wall as a second layer wall; recording an imaging space between the first layer wall and the second layer wall as an area 1, recording an imaging space behind the second layer wall as an area 2, and performing 8-connected domain extraction on the original radar image in the step A to obtain a binary radar image;
C. respectively marking the targets in the region 1 and the region 2, extracting barycentric coordinates of all the targets and taking the barycentric coordinates as coordinates of each corresponding target;
D. respectively calculating the direct path propagation distance corresponding to each target in the area 2 and the two types of multipath propagation distances corresponding to each target in the area 1;
E. calculating a normalized multipath error coefficient according to the direct path propagation distance and the two types of multipath propagation distances in the step D; the calculation formula of the normalized multipath error coefficient in the step E is
Wherein, λ'i-j,λ″i-jRespectively, the ith target P in the region 1fiAnd the target P in the area 2bjNormalized error coefficients corresponding to multipath path 1 and multipath path 2, M being the number of transmitting antennas, N being the number of receiving antennas,
the i-th object which is the multipath path 1 corresponds to the electromagnetic wave propagation distance of the m-th transmitting antenna and the n-th receiving antenna,
the i-th object which is the multipath path 2 corresponds to the electromagnetic wave propagation distance of the m-th transmitting antenna and the n-th receiving antenna,
an electromagnetic wave propagation distance corresponding to the mth transmitting antenna and the nth receiving antenna for the jth target;
F. calculating an error threshold value according to the normalized multipath error coefficient in the step E;
G. performing multi-path judgment on each target in the area 2 according to the normalized multi-path error coefficient in the step E and the error threshold value in the step F;
H. and G, processing the binary radar image in the step B according to the multipath judgment result in the step G to obtain a radar image after multipath inhibition.
2. The method of claim 1, wherein the step a of constructing the front wall and rear wall concussion multipath model sets a Target (x) position as the Target positiontar,ytar) The mth transmitting array element coordinate is (x)tm,ytm) The nth receiving array element coordinate is (x)n,yn) The wall thickness of the front wall is d, the dielectric coefficient is epsilon, and the coordinate of the rear wall is ybThe transmitting signal is s (t), the receiving signal is r (t), and the electromagnetic wave propagation path is a transmitting antenna, a target and a receiving antenna.
3. The method of claim 2, wherein the received signal in step a is represented as a multipath signal after multiple-shot wall-mine imaging back-and-forth wall oscillation
r(t)＝αs(t-τtar)+β1s(t-τm1)+β2s(t-τm2)
Wherein, α1,β2Scattering coefficients, tau, for the target and multipath path 1, multipath path 2, respectivelytar,τm1,τm2The echo delays of the target and the multipath path 1 and the multipath path 2 respectively.
4. The method of claim 1, wherein the step C marks the targets in the area 1 and the area 2, respectively, extracts barycentric coordinates of all the targets and uses the barycentric coordinates as the coordinates of each corresponding target, specifically, marks the target in the area 1 as [ Pf1,Pf2,...PfK]Where K is the total number of targets in region 1 and the K-th target barycentric coordinate is Pfk(xfk,yfk) (ii) a Label the target of region 2 as [ P ]b1,Pb2,...PbT]Where T is the total number of targets in zone 2 and the coordinates of the center of gravity of the tth target is Pbt(xbt,ybt)。
5. The method of claim 4, wherein the direct path propagation distance corresponding to each target in region 2 is calculated in step D according to the formula
Wherein M and N are respectively the number of antenna transmitting and receiving array elements and gammat,mnThe electromagnetic wave propagation distance corresponding to the mth transmitting antenna and the nth receiving antenna is set for the tth target.
6. The method for multi-transmit multi-receive through-wall radar imaging back-front-back wall oscillation once multi-path mitigation in claim 4, wherein the calculation formula for calculating the two types of multi-path propagation distances corresponding to each target in the region 1 in the step D is
Where γ ' is the electromagnetic wave propagation distance corresponding to multipath Path 1, γ ' is the electromagnetic wave propagation distance corresponding to multipath Path 2, γ 'j,mnThe jth target of the multi-path 1 corresponds to the electromagnetic wave propagation distance, γ ″, between the mth transmitting antenna and the nth receiving antennaj,mnThe jth target of the multipath path 2 corresponds to the electromagnetic wave propagation distance between the mth transmitting antenna and the nth receiving antenna, j being 1,2, …, K.
7. The method of claim 1, wherein the error threshold in step F is calculated by the following equation
Wherein Δ R represents the theoretical distance resolution, ε ', of the through-wall radar'i,ε″iAre respectively corresponding normalized error coefficients lambda'i-j,λ″i-jThe error threshold value of (2).
8. The method for multi-transmit multi-receive through-wall radar imaging back-front-back-wall oscillation once multi-path mitigation of claim 7, wherein the step G of multi-path decision for each target in the area 2 according to the normalized multi-path error coefficient in the step E and the error threshold value in the step F specifically comprises the following sub-steps:
g1, setting the initial iteration number k to 1, and selecting one target judgment error coefficient vector minimum value from the region 1
Whether or not there is
And is
If not, reselecting the next target; if yes, carrying out the next step;
g2, judgment subscript akWhether or not it is equal to subscript bk(ii) a If so, the target is not multipath; if not, the corresponding subscript a in the area 2k,bkTwo objects of
Is the kth target P of region 1fkA multipath of (a);
g3, judging whether the iteration times K are less than K; if yes, returning to the step G1; if not, judging to end;
where K is the total number of targets in region 1.
9. The method for multi-shot multi-receiver through-wall radar imaging after front-back wall oscillation once multi-path suppression according to claim 1, wherein the step H of processing the binarized radar image in the step B according to the multi-path judgment result in the step G is to set the pixel points of the corresponding area to 0 in the binarized radar image and to multiply the binarized radar image and the original radar image by pixel points in a one-to-one correspondence manner.
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