CN109257692B - A mobile anchor node-assisted underwater wireless sensor network positioning method - Google Patents

Abstract

The invention discloses a positioning method for an underwater wireless sensor network assisted by a mobile anchor node. The sub-regions are arranged closely in sequence, and the adjacent sub-regions are arranged in a co-edge arrangement, and the movement path in the interval and the movement path in the area are planned. The mobile anchor node-assisted underwater wireless sensor network positioning method of the present invention adopts a regular hexagon to divide the sub-areas, which can ensure the complete coverage of the target area, and the overlapping area of the coverage between the sub-areas is the smallest, which is beneficial to improve the positioning accuracy , to achieve meter-level positioning of underwater acoustic signals with low energy consumption, while reducing the amount of calculation.

Description

A mobile anchor node-assisted underwater wireless sensor network positioning method

technical field

The present invention is in the technical field of underwater wireless sensor networks, in particular to acoustic signal communication technology and underwater positioning technology, which is a mobile anchor node-assisted underwater wireless sensor network node positioning algorithm based on area determination.

Background technique

With the increasing emphasis on the ocean and the rapid development of wireless sensor network research all over the world, the research of Underwater Wireless Sensor Networks (UWSN) has received more and more attention, and the accurate location of underwater targets can be obtained. is the key to conducting underwater research. Nowadays, underwater positioning all use acoustic signals for positioning. Due to the shortcomings of underwater acoustic signals, such as high noise, low speed, limited bandwidth, multipath effect and Doppler frequency shift, and the anchor nodes are difficult to deploy and carry limited energy, etc. Therefore, the existing underwater acoustic positioning system has problems such as low positioning accuracy and large energy consumption, which cannot meet people's needs. Therefore, a new method is needed to achieve high-precision and low-energy-consuming positioning.

According to whether the anchor node can move, it can be divided into a fixed anchor node and a mobile anchor node (Mobility Anchor, MA for short). The fixed anchor nodes are generally placed manually or by ships, and their positions are determined by the placement device and the environment, such as water density and air viscosity coefficient. The mobile anchor node can be an underwater drift node, a high-speed maneuvering vehicle, etc., and can perform regular movements when its own position is known. After it moves to a certain planned position to transmit signals during the moving process, it continues to move to the next planned position, and all the planned positions are called virtual anchor nodes. Compared with the fixed anchor node, it avoids the difficulty of deployment and the waste of energy in the recovery process, and the position accuracy is higher. As a kind of underwater wireless sensor network node, the mobile anchor node is equipped with GPS navigation, which can effectively reduce the number of high-cost anchor nodes and greatly reduce the cost of underwater operations.

For the application of mobile anchor node positioning, Sichitiu ML firstly used a single mobile anchor node and Received Signal Strength Indicator (RSSI) ranging to estimate the position of the target node in 2004. For the path planning of mobile anchor nodes, D. Koutsonikolas et al. proposed three path planning methods SCAN, DOUBLESCAN and HILBERT for the first time by using the space filling line theory. These three methods belong to static path planning, and each has advantages and disadvantages in application. The SCAN path is suitable for the case where the node communication distance is small and the space filling line density is large, but the collinearity problem of the reference node is very serious; for this, the researchers further proposed the DOUBLESCAN path planning, but this doubled the positioning process. The consumption of HILBERT path planning is suitable for the situation that the node communication distance is large and the space filling line density is small. However, the above three methods all have the problem of reference node collinearity to varying degrees. In order to effectively solve this problem, R. Huang et al. proposed two methods: circular path planning and S-shaped path planning. For circular path planning, if it is necessary to completely To cover the network area, it is necessary to increase the radius of the outer great circle, thereby increasing the length of the movement path of the anchor node. If the diameter of the circle is too large, the collinearity of the reference node will also occur in the local area. In order to solve the collinearity problem of reference points, S-shaped path planning uses S-shaped curves instead of straight lines, which can achieve better results. On this basis, Han et al. introduced the equilateral triangle into the Localization with a Mobile Anchor node based on Trilateration (LMAT) algorithm to optimize the movement trajectory of the mobile anchor node, maximizing the target positioning coverage and the positioning accuracy of the target node. In the above positioning algorithm based on mobile anchor nodes, in order to enable the target node to receive enough virtual anchor node information and make the number of reference nodes meet the requirements, the coverage of adjacent virtual anchor nodes has a large overlapping area. , which not only lengthens the moving path of the MA, but also increases the energy consumption of positioning. However, blindly reducing the number of virtual anchor nodes will seriously affect the positioning accuracy based on mobile anchor nodes. Therefore, it is urgent to solve the problems of high energy consumption and low accuracy of node positioning in underwater wireless sensor networks.

SUMMARY OF THE INVENTION

In order to solve the problems of low positioning accuracy and large energy consumption in the existing underwater wireless sensor network node positioning technology, the present invention proposes a mobile anchor node-assisted underwater wireless sensor network positioning method, which can solve the above problems.

In order to solve the above-mentioned technical problems, the present invention adopts the following technical solutions to be realized:

A mobile anchor node-assisted underwater wireless sensor network positioning method, comprising the following steps:

In the path planning step, the signal coverage radius R of the mobile anchor node is determined, and the target area is divided into several regular hexagonal sub-areas with a side length of R. The sub-areas are arranged closely in sequence, and the adjacent sub-areas are arranged on the same side. , planning the movement path in the interval and the movement path in the area;

The steps of determining the sub-area where the target node is located include:

(11) The mobility anchor node moves according to the interval movement path, and moves from one sub-area to another sub-area. Each time the mobility anchor node moves to a sub-area, it transmits a request signal, and the request signal at least contains the information of the mobility anchor node. location information and time stamp information of the request signal;

(12), if the target node receives the acoustic signal sent by the mobility anchor node in the sub-region Q0, it transmits a feedback signal to the mobility anchor node, the feedback signal at least includes the time stamp information of the feedback signal, and the sub-region Q0 is the planned any subregion;

(13) The mobile anchor node sends the feedback signal to the positioning center, and the positioning center calculates the propagation delay τ ₀ of the target node, and calculates the distance D0 between the target node and the mobile anchor node according to D0=cτ ₀ , where c is speed of sound;

的关系，若D0小于

则判断目标节点位于子区域Q0中；(14), determine the radius of the inscribed circle of D0 and the sub-region

relationship, if D0 is less than

Then it is judged that the target node is located in the sub-region Q0;

The location positioning steps in the target node area include:

(21), the mobile anchor node moves according to the movement path in the sub-region Q0, and the mobile anchor node transmits a request signal when moving to a fixed position;

(22), the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and each fixed-point position;

(23) Calculate the coordinates of the target node according to the distance between the target node and each fixed point position and the coordinates of each fixed point position.

更改移动锚节点的区间移动路径，更改后的区间移动路径为：移动锚节点顺次移动至与子区域Q0相共边的若干个子区域，然后执行目标节点区间位置定位步骤，包括：Further, in step (14), if the range of D0 is greater than

Change the interval movement path of the mobile anchor node. The changed interval movement path is: the mobile anchor node sequentially moves to several sub-regions that share the same edge with the sub-region Q0, and then performs the target node interval position positioning steps, including:

(31), the mobile anchor node starts from the sub-region Q0, moves according to the changed interval movement path, and transmits a request signal every time the mobile anchor node moves into a sub-region;

(32), the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and the sub-region;

(33) Calculate the coordinates of the target node according to the distance between the target node and each sub-region and the coordinates of each sub-region.

Further, the positioning center calculates the distances D _{1 ......}

Wherein, (x _k , y _k ) are the coordinates of the sub-region that shares a side with the sub-region Q0 and transmits the feedback signal, and the value of k is 1 or 2 or 3.

Further, one of least squares method, Chan, Fang algorithm is used to solve the equation system.

Further, in step (14), in the step of determining the target node area, the modified interval movement path is: the sub-area that shares the edge with the sub-area Q0 and that the mobile anchor node passes through for the first time.

Further, the movement path within the area is planned according to the Circlus path planning.

Further, the request signal is an acoustic signal sent by broadcasting.

Further, in step (13), the propagation delay τ ₀ is the difference between the time carried in the timestamp information of the feedback signal and the time when the mobile anchor node receives the feedback signal.

Further, when the mobile anchor node moves from one sub-region to the next sub-region according to the interval movement path, the movement angle of the mobile anchor node is calculated according to the positional relationship between the two sub-regions.

Further, the acoustic signal is a linear frequency modulation signal, and the frequency is 200-300 Hz.

Compared with the prior art, the advantages and positive effects of the present invention are: the mobile anchor node-assisted underwater wireless sensor network positioning method of the present invention first selects the coverage of the mobile anchor node according to the size of the target area and the performance of the mobile anchor node. The radius R, taking into account the carrying energy of the water carrier, divides the target area when the target area is completely covered, and secondly, divides the sub-areas in a regular hexagon way, which can ensure that the target area is completely covered, and the sub-areas are covered. The overlapping area of the range is the smallest, which is beneficial to improve the positioning accuracy, realize the meter-level positioning of the underwater acoustic signal with low energy consumption, and reduce the calculation amount at the same time.

Other features and advantages of the present invention will become more apparent upon reading the detailed description of the embodiments of the present invention in conjunction with the accompanying drawings.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

1 is a flowchart of an embodiment of a mobile anchor node-assisted underwater wireless sensor network positioning method proposed by the present invention;

2 is a schematic diagram of a divided area in an embodiment of the positioning method proposed by the present invention;

3 is a schematic diagram of a sub-region inscribed circle in an embodiment of the positioning method proposed by the present invention;

4 is a schematic diagram of a circumscribed circle of a sub-region in an embodiment of the positioning method proposed by the present invention;

FIG. 5 is a schematic diagram of positioning calculation in an embodiment of the positioning method proposed by the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Embodiment 1, this embodiment proposes a mobile anchor node-assisted underwater wireless sensor network positioning method, as shown in FIG. 1 , including the following steps:

In the path planning step, the signal coverage radius R of the mobile anchor node is determined, and the target area is divided into several regular hexagonal sub-areas with a side length of R. The sub-areas are arranged closely in sequence, and the adjacent sub-areas are arranged on the same side. , plan the movement path in the interval and the movement path in the area, as shown in Figure 2, the sub-regions are arranged closely in sequence, and the adjacent sub-regions are arranged in a co-edge arrangement;

According to the size of the target area and the performance of the mobile anchor node, the coverage radius R of the mobile anchor node is selected, and the target area is divided under the condition of complete coverage of the target area, taking into account the carrying energy of the underwater vehicle.

Using a regular hexagon to divide the sub-areas can ensure that the target area is completely covered, and the overlapping area of the coverage between the sub-areas is the smallest, which is beneficial to improve the positioning accuracy, realize the meter-level positioning of the underwater acoustic signal with low energy consumption, and reduce the calculated amount.

The positioning center is a computing center set above the sea surface, which completes the search for the target area. It can realize the functions of route planning, determining the signal coverage radius R of the mobile anchor node, update and distance calculation, and can communicate with the mobile anchor node.

The steps of determining the sub-area where the target node is located include:

S11. The mobility anchor node moves according to the interval movement path, and moves from one sub-area to another sub-area. Every time the mobility anchor node moves to a sub-area, it transmits a request signal, and the request signal at least includes the location information of the mobility anchor node. and timestamp information of the request signal;

Before the mobile anchor node moves according to the planned path, the mobile anchor node reaches the specified depth according to the depth information of the target node.

S12. If the target node receives the acoustic signal sent by the mobility anchor node in the sub-region Q0, it transmits a feedback signal to the mobility anchor node. The feedback signal at least includes the time stamp information of the feedback signal, and the sub-region Q0 is any planned sub-region. ;

S13, the mobile anchor node sends the feedback signal to the positioning center, and the positioning center calculates the propagation time delay τ ₀ of the target node, and calculates the distance D0 between the target node and the mobile anchor node according to D0=cτ ₀ , where c is the speed of sound; Communication between the mobile anchor node and the positioning center can be performed by means of optical communication, wired communication, and the like.

的关系，若D0小于

则判断目标节点位于子区域Q0中；如图3所示，若D0小于

则判断目标节点M位于子区域Q0中。S14. Determine the radius of the inscribed circle of D0 and the sub-region

relationship, if D0 is less than

Then judge that the target node is located in the sub-region Q0; as shown in Figure 3, if D0 is less than

Then it is judged that the target node M is located in the sub-region Q0.

R同时为移动锚节点的信号覆盖半径，因此，若D0小于√3/2R，说明移动锚节点肯定位于该子区域内，其距离其他任一子区域的距离均超过信号覆盖半径，因此，该种情况出现的话判断目标节点位于子区域Q0中。在路径规划时，各子区域的位置信息已经确定，因此目标节点的位置相应确定，与其所在子区域的位置一致。Since the side length of the subregion of the regular hexagon is R, the radius of its inscribed circle is

R is also the signal coverage radius of the mobile anchor node. Therefore, if D0 is less than √3/2R, it means that the mobile anchor node must be located in this sub-area, and its distance from any other sub-area exceeds the signal coverage radius. Therefore, this If such a situation occurs, it is determined that the target node is located in the sub-region Q0. During path planning, the location information of each sub-area has been determined, so the location of the target node is determined accordingly, which is consistent with the location of the sub-area where it is located.

The location positioning steps in the target node area include:

S21, the mobile anchor node moves in the sub-region Q0 according to the movement path within the region, and the mobile anchor node transmits a request signal when moving to a fixed position;

S22, the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and each fixed-point position;

S23: Calculate the coordinates of the target node according to the distance between the target node and each fixed point position and the coordinates of each fixed point position.

如图4所示，若目标节点位于阴影区，则其不位于子区域Q0中，也即，目标节点所在子区域是不确定的，需要进一步确认。此种对应两种情况，一种是仍然位于子区域Q0中。另外一种情况是位于多个子区域的重叠覆盖区，且该多个子区域仅可能为子区域Q0相邻的子区域，否则定位节点接收不到位于子区域Q0时发送的请求信号，既然大概区域范围已经锁定，则无需继续按照刚开始规划的路径运动，需要修改路径，以减小计算量，因此，本定位方法还包括更改移动锚节点的规划路径，更改后的规划路径为移动锚节点顺次移动至与子区域Q0相共边的若干个子区域，然后执行信号的发射与反馈步骤和目标节点距离判定步骤，若移动锚节点按照更改后的规划路径移动过程中未接收到新的反馈信号，则判断目标节点位于子区域Q0中，也即对应第一种情况。In the target node area determination step, if the range of D0 is greater than

As shown in FIG. 4 , if the target node is located in the shaded area, it is not located in the sub-region Q0, that is, the sub-region where the target node is located is uncertain and needs to be further confirmed. This corresponds to two situations, one is still located in the sub-region Q0. Another situation is overlapping coverage areas located in multiple sub-areas, and the multiple sub-areas may only be adjacent sub-areas of sub-area Q0, otherwise the positioning node cannot receive the request signal sent when it is located in sub-area Q0, since the approximate area The range has been locked, so there is no need to continue to move according to the path planned at the beginning, and the path needs to be modified to reduce the amount of calculation. Therefore, this positioning method also includes changing the planned path of the mobile anchor node, and the modified planned path is the order of the mobile anchor node. Move to several sub-regions that share the same edge with the sub-region Q0, and then perform the signal transmission and feedback steps and the target node distance determination step. , then it is judged that the target node is located in the sub-region Q0, which corresponds to the first case.

更改移动锚节点的区间移动路径，更改后的区间移动路径为：移动锚节点顺次移动至与子区域Q0相共边的若干个子区域，然后执行目标节点区间位置定位步骤，包括：In step S14, if the range of D0 is greater than

Change the interval movement path of the mobile anchor node. The changed interval movement path is: the mobile anchor node sequentially moves to several sub-regions that share the same edge with the sub-region Q0, and then performs the target node interval position positioning steps, including:

S31, the mobile anchor node starts from the sub-region Q0, moves according to the modified interval movement path, and transmits a request signal every time the mobile anchor node moves into a sub-region;

S32, the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and the sub-region;

S33. Calculate the coordinates of the target node according to the distance between the target node and each sub-region and the coordinates of each sub-region.

Each sub-region has at most 6 adjacent sub-regions, therefore, the number of sub-regions corresponding to the modified interval movement path does not exceed 6, so the calculation amount can be greatly reduced.

Once the distance _Dk between the mobile anchor node and the target node is measured, according to geometric principles, the node to be located must be located on a circle with the location of the reference base station as the center and the distance _Dk as the radius. If the distance between the target node and the mobile anchor node in three sub-regions is known, take the position of the three sub-regions as the center of the circle and the distance between the target node and the three sub-regions as the radius to draw a circle, as shown in Figure 5.

The mobility anchor node moves according to the changed interval movement path, and it still starts to send request signals in the form of broadcast every time it moves to a sub-area. If the mobility anchor node moves according to the changed planned path, the transmitted request The signal is received by the target node, and a new feedback signal is generated. The positioning center calculates the distance D ₁ ... D _k between the target node and each sub-region in turn, and the coordinates of the target node are (x, y). Get the coordinates of the target node:

Wherein, (x _k , y _k ) are the coordinates of the sub-region that shares a side with the sub-region Q0 and transmits the feedback signal, and the value of k is 1 or 2 or 3.

In step S23, the movement path within the area is planned according to the Circlus path planning.

Wherein, in step S23, the same method as above is also used to calculate, at this time, the distance between the mobile anchor node and the target node is replaced by the distance between the mobile anchor node and the fixed-point position, and the coordinates of the sub-region are replaced by the coordinates of the fixed-point position. That's it.

Under ideal conditions, the coordinates (x, y) of the node to be located can be obtained by solving Equation 1, but due to non-line-of-sight, multipath, diffraction and reflection reasons, the distance measurement result _Dk inevitably has errors , so that the solution of the system of equations cannot be solved. In this embodiment, one of the least squares method, the Chan algorithm, and the Fang algorithm is used to solve the system of equations.

In step S14, the modified interval movement path is the sub-area that shares the edge with the sub-area Q0 and that the mobile anchor node passes through for the first time. There are at most 6 sub-areas around any sub-area that share the same edge with it. During the movement of the mobile anchor node, the first planned interval movement path movement is installed first, so it is possible that one or more adjacent shared areas of the sub-area Q0 have been reached. The sub-region of the edge, and the sub-region has not received a feedback signal, indicating that the target node is far away from the sub-region. When the feedback signal is received in the sub-region Q0, and the distance between the target node and the sub-region Q0 is judged If it is larger than the radius of its inscribed circle, the planned path needs to be changed. Just move the mobile anchor node to several adjacent sub-regions around the sub-region Q0 and transmit the request signal again. For the sub-regions that the mobile anchor node has already arrived at One or more adjacent sub-regions of the region Q0 that share the same edge can be excluded, which can reduce the amount of calculation. Therefore, the modified interval movement path is the sub-region that shares the edge with the sub-region Q0 and the mobile anchor node passes through for the first time. .

Since the location of the target node is unknown, the request signal is an acoustic signal sent by broadcasting, ensuring that when the target node is located in all directions, as long as it is within the signal coverage radius R, the request signal can be received.

In step S13, the propagation delay τ ₀ is the difference between the time carried in the time stamp information of the feedback signal and the time when the mobile anchor node receives the feedback signal.

In this embodiment, the positions of each sub-region are known. When the mobile anchor node moves from one sub-region to the next sub-region according to the interval movement path, the motion of the mobile anchor node is calculated according to the positional relationship between the two sub-regions. angle.

Because the composition of seawater is very complex, part of the sound wave is absorbed when it is transmitted, and the higher the frequency, the stronger the absorption. For the low-frequency sound wave seawater, the absorption is less. The research shows that the sound signal with a frequency of about 200Hz has less transmission loss. In the embodiment, it is preferable that the acoustic signal is a chirp signal, and the frequency is 200-300 Hz.

Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those of ordinary skill in the art within the scope of the present invention should also belong to protection scope of the present invention.

Claims (9)

1. a mobile anchor node-assisted underwater wireless sensor network positioning method, is characterized in that, comprises the following steps: In the path planning step, the signal coverage radius R of the mobile anchor node is determined, and the target area is divided into several regular hexagonal sub-areas with a side length of R. The sub-areas are arranged closely in sequence, and the adjacent sub-areas are arranged on the same side. , planning an interval movement path and an intra-area movement path; the intra-area movement path is planned according to the Circlus path planning; The steps of determining the sub-area where the target node is located include: (11) The mobility anchor node moves according to the interval movement path, and moves from one sub-area to another sub-area. Each time the mobility anchor node moves to a sub-area, it transmits a request signal, and the request signal at least contains the information of the mobility anchor node. location information and time stamp information of the request signal; (12), if the target node receives the acoustic signal sent by the mobility anchor node in the sub-region Q0, it transmits a feedback signal to the mobility anchor node, the feedback signal at least includes the time stamp information of the feedback signal, and the sub-region Q0 is the planned any subregion; (13) The mobile anchor node sends the feedback signal to the positioning center, and the positioning center calculates the propagation delay τ ₀ of the target node, and calculates the distance D0 between the target node and the mobile anchor node according to D0=cτ ₀ , where c is speed of sound; (14), determine the radius of the inscribed circle of D0 and the sub-region

relationship, if D0 is less than

Then judge that the target node is located in the sub-region Q0, if the range of D0 is greater than

Change the interval movement path of the mobile anchor node; The location positioning steps in the target node area include: (21), the mobile anchor node moves according to the movement path in the sub-region Q0, and the mobile anchor node transmits a request signal when moving to a fixed position; (22), the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and each fixed-point position; (23) Calculate the coordinates of the target node according to the distance between the target node and each fixed point position and the coordinates of each fixed point position. 2. positioning method according to claim 1, is characterized in that, in step (14), the interval movement path after modification is: mobile anchor node moves to several sub-regions that are in common side with sub-region Q0 successively, then Execute the steps of positioning the target node interval, including: (31), the mobile anchor node starts from the sub-region Q0, moves according to the changed interval movement path, and transmits a request signal every time the mobile anchor node moves into a sub-region; (32), the mobile anchor node receives the feedback signal of the target node, and sends the feedback signal to the positioning center, and the positioning center calculates the distance between the target node and the sub-region; (33) Calculate the coordinates of the target node according to the distance between the target node and each sub-region and the coordinates of each sub-region. 3. The positioning method according to claim 2, wherein the positioning center calculates the distances D1... Solve for the coordinates of the target node:

Wherein, (x _k , y _k ) are the coordinates of the sub-region that shares a side with the sub-region Q0 and transmits the feedback signal, and the value of k is 1 or 2 or 3. 4 . The positioning method according to claim 3 , wherein one of least squares method, Chan and Fang algorithms is used to solve the equation system. 5 . 5. positioning method according to claim 2 is characterized in that, in step (14), in the target node area determination step, the interval movement path after the modification is: with the sub-area Q0 is co-edge and the mobile anchor node passes through for the first time sub-region. 6 . The positioning method according to claim 1 , wherein the request signal is an acoustic signal sent by broadcasting. 7 . 7. The positioning method according to any one of claims 1-5, wherein in step (13), the propagation delay τ ₀ is the time carried in the time stamp information of the feedback signal and the time received by the mobile anchor node. The difference between the times of the feedback signal. 8. The positioning method according to any one of claims 1-5, wherein when the mobile anchor node moves from one sub-region to the next sub-region according to the interval movement path, according to the positional relationship between the two sub-regions , and calculate the motion angle of the moving anchor node. 9 . The positioning method according to claim 6 , wherein the acoustic signal is a linear frequency modulation signal, and the frequency is 200 Hz to 300 Hz. 10 .

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