Reconfigurable intelligent surface-based passive positioning system and positioning precision optimization method

Abstract

The invention discloses a passive positioning system based on a reconfigurable intelligent surface and a positioning precision optimization method. According to the system, on the basis that a sight distance link exists between a target and a reconnaissance station, an additionally introduced RIS is used as a positioning reference point, passive cross positioning of the detected target is achieved, and therefore the positioning precision is improved; in addition, in a special scene (such as a dense city scene or a mountainous area channel scene) in which the sight distance link between the reconnaissance station and the detection target is blocked, a plurality of cascade links can be formed through a plurality of RISs, so that the cross positioning of the detection target is realized, the positioning effect of the system on the target is ensured, and the applicability and the stability of the system are enhanced. According to the precision optimization method, the SNR is maximized by optimizing the position and phase shift of the RIS in the system, so that the purpose of improving the positioning precision is achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wireless communication and passive positioning, and particularly relates to a passive positioning system based on a reconfigurable intelligent surface and a positioning precision optimization method. BACKGROUND

[0002] Reconfigurable Intelligent Surfaces (RIS) is a new type of wireless communication technology that does not require any changes to wireless devices or networks, but only adds RIS on the transmission path to control and optimize signal propagation. RIS is composed of multiple passive reflection elements, each of which is composed of passive devices such as capacitors and resistors. These passive reflection elements have the ability to individually reflect the original transmitted signal, with the advantages of low energy consumption and low cost. By controlling their amplitude or phase, these elements can achieve different functions such as signal amplification, attenuation, deflection, focusing and scattering.

[0003] Passive positioning can be divided into different working principles, such as direction finding cross positioning method based on angle of arrival, positioning method based on time difference of signal arrival observation station, and positioning method based on received signal strength of arrival. In the application of angle of arrival passive positioning, the noise measurement equation is highly nonlinear with the unknown target position. There are currently a variety of different passive positioning schemes. However, although existing research has made some progress in the field of passive positioning, it still faces great challenges in dealing with complex environments such as high building density areas, mountainous terrain or scenes with severe multipath effects:

[0004] 1) In traditional passive positioning systems, when there is a line-of-sight link between the target and the reconnaissance station, the system mainly relies on a single line-of-sight link for signal measurement and positioning. This single signal path lacks sufficient spatial information, making it impossible to form effective cross positioning, limiting the improvement of positioning accuracy, especially in tasks that require high-precision positioning.

[0005] 2) In complex environments, the signal quality of traditional passive positioning systems is easily affected by factors such as path loss, multipath interference and environmental noise. These factors result in a low signal-to-noise ratio (SNR) when the signal arrives at the reconnaissance station, thereby limiting the accuracy of the system's positioning. Due to the lack of dynamic channel optimization means, the system is difficult to maintain stable high-precision positioning performance in different scenarios.

[0006] 3) In dense urban scenarios or complex environments such as mountainous areas, the line-of-sight link between the target and the reconnaissance station is often easily blocked, causing the signal to be difficult to directly reach the reconnaissance station, thereby making the system unable to obtain effective measurement information. The problem of blockage and signal loss in these special environments seriously affects the coverage ability of the system and the reliability of target positioning. SUMMARY

[0007] In order to solve the above problems existing in the prior art, the present application provides a passive positioning system based on a reconfigurable intelligent surface and a positioning accuracy optimization method.

[0008] The technical problem to be solved by the present application is solved by the following technical scheme:

[0009] The present application provides a passive positioning system based on a reconfigurable intelligent surface, comprising: a reconnaissance station, N RIS, a detection target, wherein the detection target and each RIS, each RIS module and the reconnaissance station, and the detection target and the reconnaissance station all have a link;

[0010] Wherein each RIS is used to reflect the signal emitted by the detection target to the reconnaissance station; the reconnaissance station is used to receive the signal directly emitted by the detection target and the signal reflected by the N RIS, and to position the detection target based on the signal directly emitted by the detection target and the signal reflected by the N RIS, or the reconnaissance station is used to receive the signal reflected by the N RIS and to position the detection target based on the signal reflected by the N RIS.

[0011] The present application also provides a positioning accuracy optimization method, applied to the above-mentioned system, wherein each RIS has a physical position, and each RIS comprises a plurality of array-arranged reflecting units, each reflecting unit having a phase shift, the method comprising:

[0012] S1, obtaining the physical position range, the phase shift range, the preset position interval and the preset phase shift interval corresponding to each RIS; each range is composed of a range lower limit value and a range upper limit value;

[0013] S2, for each RIS, each time a physical position is taken out from the physical position range corresponding to the RIS according to the preset position interval corresponding to the RIS, and a phase shift is taken out from the phase shift range corresponding to the RIS according to the preset phase shift interval corresponding to the RIS, and then based on the physical positions and phase shifts of the N RIS taken out this time, a SNR corresponding to the system this time is calculated, and thus the physical position range and phase shift range of the N RIS are traversed, and a plurality of SNRs are obtained;

[0014] S3, selecting the maximum SNR from the plurality of SNRs, and taking the physical position and phase shift of the N RIS corresponding to the maximum SNR as optimal configuration parameters;

[0015] S4, deploying the N RIS in the system according to the optimal configuration parameters for positioning the detected target, wherein the positioning error of the detected target by the reconnaissance station is negatively correlated with the SNR of the system.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] The present application provides a passive positioning system based on reconfigurable intelligent surface and a positioning accuracy optimization method. By introducing RIS into the passive positioning system, the cascaded link is increased (equivalent to increasing the reconnaissance station) under the line-of-sight condition, so that cross positioning of the detected target can be realized, and the positioning accuracy of the system is improved. In addition, the present application takes SNR as an index to measure positioning accuracy, and realizes the maximization of SNR by optimizing the phase shift and physical position of RIS, further improving the positioning accuracy. Finally, in a complex special environment, the present application realizes cascaded reflection through the deployment of multiple RIS, so that the positioning of the detected target can be realized when the line-of-sight link between the reconnaissance station and the detected target is blocked, thereby enhancing the applicability and stability of the system.

[0018] The present application will be further described in detail below in combination with the drawings and specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a cross positioning schematic diagram of a passive positioning system based on reconfigurable intelligent surface provided by an embodiment of the present application applied in a general scene;

[0020] Figure 2 is a cross positioning schematic diagram of a passive positioning system based on reconfigurable intelligent surface provided by an embodiment of the present application applied in a special scene;

[0021] Figure 3 is a flowchart of a positioning accuracy optimization method provided by an embodiment of the present application. DETAILED DESCRIPTION

[0022] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0023] Firstly, in order to solve the problem that the positioning accuracy of the traditional passive positioning system is limited in complex environment, the RIS is introduced into the passive positioning system. On the basis that there is a line-of-sight link between the target and the reconnaissance station, the cross positioning of passive positioning is realized by using the cascade link provided by the RIS. At this time, the RIS plays the role of a new reconnaissance station, effectively increases the positioning reference point, and improves the positioning ability of the system. Secondly, due to the increase in the number of reconnaissance stations, the positioning accuracy is improved. In addition, the application also designs a positioning accuracy optimization method for measuring the positioning accuracy by SNR. By optimizing the position and phase shift of the RIS, the SNR is maximized, thereby improving the positioning accuracy. Finally, the application proposes to use RIS to enhance the applicability of the system in special scenarios (such as dense urban scenarios or mountain pass scenarios). In these special scenarios, due to the fact that the line-of-sight link between the reconnaissance station and the detection target may be blocked, the application forms multiple cascade links through the deployment of multiple RIS, thereby realizing cross positioning and ensuring the positioning effect of the system on the target, overcoming the problem of limited line-of-sight conditions in complex environments.

[0024] Firstly, the application provides a passive positioning system based on a reconfigurable intelligent surface, comprising: a reconnaissance station, N RIS, a detection target. The detection target and each RIS, each RIS module and the reconnaissance station, and the detection target and the reconnaissance station all have a link. Each RIS is used to reflect the signal emitted by the detection target to the reconnaissance station. The reconnaissance station is used to receive the signal directly emitted by the detection target and the signal reflected by the N RIS, and to position the detection target based on the signal directly emitted by the detection target and the signal reflected by the N RIS; or, the reconnaissance station is used to receive the signal reflected by the N RIS and to position the detection target based on the signal reflected by the N RIS. Exemplarily, when the reconnaissance station positions the detection target, an existing cross positioning method can be used, and the specific principle of the cross positioning method will not be described here.

[0025] In some embodiments, when N is a positive integer greater than or equal to 1, the reconnaissance station is used to receive the signal directly emitted by the detection target and the signal reflected by the N RIS, and to position the detection target based on the signal directly emitted by the detection target and the signal reflected by the N RIS. In this case, the system can be applied to a general scenario where the link (i.e. the line-of-sight link) between the detection target and the reconnaissance station is not blocked, in order to improve the positioning accuracy. Specifically, the signal emitted by the detection target has two paths: one is directly transmitted to the reconnaissance station, and the other is reflected to the reconnaissance station through the RIS. The reconnaissance station realizes cross positioning by receiving the direct signal from the target and the RIS reflected signal, and then accurately determines the position of the target.

[0026] In some embodiments, N is a positive integer greater than or equal to 2; the reconnaissance station is configured to receive N RIS reflected signals, and detect the position of the target based on the N RIS reflected signals. In this case, the system can be applied to the special scene of detecting the blockage of the link (i.e. the line-of-sight link) between the target and the reconnaissance station (for example, dense urban scene or mountain pass scene, etc.). Because in this scenario, the reconnaissance station and the target cannot maintain line-of-sight due to the obstruction of buildings or mountains, the traditional direct transmission radiation source positioning method is difficult to achieve effective positioning. Therefore, multiple RISs are introduced to reflect the signals emitted by the target, so that the signals can reach the reconnaissance station. Specifically, the signals emitted by the target are reflected by multiple RISs and finally received by the reconnaissance station. The reconnaissance station realizes cross positioning by receiving signals reflected by multiple RISs, and then accurately determines the position of the target.

[0027] In the present application, the positioning error of the reconnaissance station for the target is negatively related to the SNR of the system. Specifically, in the system, each RIS has a physical position, and each RIS includes a plurality of arrayed reflecting units, each reflecting unit having a phase shift. For example, the phase shifts of the reflecting units in each RIS are the same. The physical position of each RIS is the physical position coordinate of the central reflecting unit of the RIS, and the physical position coordinate refers to the position coordinate in a coordinate system common to the RIS, the reconnaissance station and the target in the system. The SNR of the system is related to the phase shift of the reflecting units in each RIS and the physical position of each RIS, and the positioning accuracy can be further improved by adjusting the physical position of the RIS and the phase shift of the reflecting units in the RIS to maximize the SNR of the signal received by the reconnaissance station. Specifically, in the optimization calculation process, the specific optimization problem can be formulated as:

[0028]

[0029]

[0030] wherein the RIS is composed of M reflecting units, m∈{1,…,M}, and the phase shift of each reflecting unit is controlled by b bits. Therefore, the phase shift of each reflecting unit can be tuned to one of 2 b total slots T represents the number of slots into which the entire communication process is divided, and represents that the system divides the entire communication period into a plurality of small time units for processing. For each time slot t, the value range is t∈{1,…,T}. The phase shift matrix of the i-th RIS in the t-th time slot is represented as wherein is the phase shift of the m-th reflecting element of the i-th RIS, i∈{1,…,I}, I represents the total number of RISs in the system, for example, I=2. a set of discrete values of the physical location coordinates of each RIS is: (i.e., the phase shift of each reflecting element is selected from the set), where λ(t) represents the signal-to-noise ratio of the system at time slot t. p1 and p2 represent the three-dimensional coordinate vectors of the physical locations of the first RIS and the second RIS, respectively. a set of discrete values of the physical location coordinates of each RIS is: S represents the number of intervals, and 0 represents the coverage radius of the detected target (i.e., the physical location coordinates of each RIS are selected from the set). In the specific optimization, after the value of SNR is calculated at each time slot, the RIS controller can dynamically adjust the physical location of the RIS and configure the phase shift of each reflecting element to obtain a set of configuration parameters. Then, the SNR value of the system at the next time slot under the configuration parameters is calculated. After that, the physical location of the RIS is dynamically adjusted and the phase shift of each reflecting element is configured to obtain a set of configuration parameters. This process is repeated several times to obtain a set of configuration parameters that maximize the SNR of the system. This set of configuration parameters is the optimal location coordinates and optimal phase shift configuration of the RIS. At this time, the location of the detected target obtained by the cross-location of the reconnaissance station is the most accurate, greatly improving the positioning accuracy and reliability.

[0031] To characterize the influence of SNR on positioning accuracy, the Cramér-Rao Lower Bound (CRLB) of array angle estimation is introduced: where λ is the carrier wavelength, and L is the effective aperture of the reconnaissance station antenna array. According to the formula, the azimuth angle estimation variance is inversely proportional to λ(t) (i.e., the integrated SNR). The larger the SNR, the more accurate the angle measurement. Further, by incorporating multi-time slot observations into the Fisher information matrix and taking the inverse of its trace, the lower bound of two-dimensional position estimation is obtained: where the constant c1 depends on the array geometry and scene layout and is independent of SNR. This formula further indicates that the higher the cumulative SNR, the smaller the theoretical lower limit of the target position mean square error (MSE) pos . As can be seen from the above two formulas, the positioning error MSE pos is strictly inversely proportional to λ(t). Therefore, by optimizing the physical location and phase shift matrix of the RIS to maximize the SNR in the system design, it is equivalent to minimizing the lower bound of the positioning error, thereby effectively improving the positioning accuracy of the system.

[0032] For example, when N is 1, the expression of the SNR of the system at the tth time slot is as follows:

[0033]

[0034] where λ(t) denotes the SNR of the system at the t-th time slot, t ∈ {1, …, T}, T denotes the total time slots, P denotes the transmission power of the detection target, σ 2 denotes the noise power, h R,B denotes the channel gain between the scout station and the RIS, H denotes the conjugate transpose symbol, h R,v (t) denotes the channel gain between the RIS and the detection target, h B,v (t) denotes the channel gain between the detection target and the scout station, θ t denotes the angle between the detection target and the RIS relative to the direction between the detection target and the scout station. ρ is the median of the average path gain at a reference distance of 1 meter, K R,B is the Rician factor, a is the path loss exponent, d R,B denotes the distance between the scout station and the RIS, is the LoS component, is the array response of the channel gain between the scout station and the RIS, f c is the carrier frequency, τ R,B denotes the delay of the link from the scout station to the RIS, j represents the imaginary unit. It is assumed that the physical position coordinates of the scout station are (x B , y B , z B ), the physical position coordinates of the RIS are (x R , y R , z R ), and the physical position coordinates of the detection target v are (x v (t), y v (t), z v (t)). The distance between the scout station and the RIS can be determined according to (x B , y B , z B ) and (x R , y R , z R ).

[0035] The expression of the channel gain h B,v (t) between the detection target and the scout station is as follows:

[0036]

[0037] where τ B,v (t) denotes the delay of the link between the scout station and the detection target v, f m (t) denotes the Doppler shift generated when the detection target v moves relative to the scout station, d B,v ((t) denotes the distance between the scout station and the detection target v.

[0038] For example, when N = 2 and the link between the target and the scout is not blocked, the expression of the SNR of the system in the tth time slot is as follows:

[0039]

[0040] wherein, denotes the channel gain between the scout and the first RIS, H denotes the conjugate transpose symbol, denotes the channel gain between the first RIS and the target, denotes the channel gain between the scout and the second RIS, denotes the channel gain between the second RIS and the target, θ (1)t denotes the angle between the target and the first RIS relative to the direction between the target and the scout, (2)t denotes the angle between the target and the second RIS relative to the direction between the target and the scout.

[0041] The channel gain between the scout and the first RIS is expressed as follows:

[0042]

[0043] wherein, a is the path loss exponent, denotes the distance between the scout and the upper RIS, is the LOS component, is the channel gain array response between the scout and the first RIS, denotes the delay of the link from the scout to the first RIS.

[0044] The channel gain between the first RIS and the target is expressed as follows:

[0045]

[0046] wherein, is the array response of the channel gain between the first RIS and the target v, denotes the delay of the link from the first RIS to the target v, denotes the Doppler shift generated when the target v moves relative to the first RIS, denotes the distance between the first RIS and the target v, K R,v is the Rician factor.

[0047] The channel gain between the scout and the second RIS The expression of is as follows:

[0048]

[0049] wherein, denotes the distance between the scout station and the second RIS, is the LOS component, denotes the delay of the link from the scout station to the second RIS.

[0050] The channel gain between the second RIS and the detection target The expression of is as follows:

[0051]

[0052] wherein, denotes the delay of the link from the second RIS to the detection target v, denotes the Doppler shift generated when the detection target v moves relative to the second RIS, denotes the distance between the second RIS and the detection target v.

[0053] Exemplarily, when N is 2, and the link between the detection target and the scout station is blocked, the expression of the SNR of the system in the tth time slot is as follows:

[0054]

[0055] Exemplarily, Figure 1 is a cross positioning schematic diagram of a reconfigurable intelligent surface-based passive positioning system applied in a general scenario. As shown in Figure 1 , in this scenario, the left scout station and the right detection target maintain line-of-sight connection (i.e., the link is not blocked), and the signal can be directly transmitted from the detection target to the scout station. In order to further improve the positioning accuracy, two RISs are introduced to reflect the signal. The signal transmitted by the detection target is transmitted to the scout station after being reflected by the first RIS and the second RIS, respectively. By receiving the two signals reflected by the RIS and the direct signal, the scout station can accurately calculate the position of the detection target by using the cross positioning method. In this process, the position of the RIS and the phase shift configuration of each reflection unit can be flexibly adjusted according to the requirements, so as to maximize the SNR received by the scout station, and further optimize the positioning accuracy. Here, the joint use of the two RISs effectively increases the positioning reference points, and further improves the positioning ability of the system.

[0056] Exemplarily, Figure 2 is a cross positioning schematic diagram of a reconfigurable intelligent surface-based passive positioning system applied in a special scenario. As shown in Figure 2As shown, in this scenario, there is a line-of-sight (LoS) blockage (e.g., dense buildings or mountains) between the scout station and the detection target, which prevents the signal from being transmitted directly from the detection target to the scout station. In this case, the RISs present in the system play a key role. In the special scenario, although the LoS connection is blocked, the RISs can still reflect the signal emitted by the detection target, so that the signal can be transmitted to the scout station through the RISs. Through the paths reflected by multiple RISs, the signal received by the scout station is recovered, thereby realizing the cross positioning of the target. This mechanism ensures that the scout station can accurately locate the detection target even in complex terrain conditions. In this process, the positions of the RISs and the phase shift configurations of each reflecting unit can be optimized according to actual needs to maximize the SNR of the signal received by the scout station. Here, the role of the RISs in the special scenario is to overcome the LoS blockage problem, ensure signal transmission, and improve positioning accuracy, thereby realizing accurate positioning of the target.

[0057] Secondly, the application also provides a positioning accuracy optimization method, which is applied to the above-mentioned system. In the system, each RIS has a physical position, and each RIS includes a plurality of array-arranged reflecting units, each reflecting unit having a phase shift. The method is used to optimize the positioning accuracy of the system by optimizing the physical position and phase shift of each RIS in the system. As shown, the method includes: Figure 3

[0058] S1, obtaining the physical position range, phase shift range, preset position interval and preset phase shift interval corresponding to each RIS; each range is composed of a range lower limit value and a range upper limit value.

[0059] Here, the physical position range corresponding to each RIS refers to the value range of the physical position of the RIS, and the preset position interval corresponding to each RIS refers to the value interval when the value is taken from the physical position range corresponding to the RIS; the phase shift range corresponding to each RIS refers to the value range of the phase shift of the reflecting unit of the RIS, and the preset phase shift interval corresponding to each RIS refers to the value interval when the value is taken from the phase shift range corresponding to the RIS. It should be noted that the physical position range, phase shift range, preset position interval and preset phase shift interval corresponding to each RIS can be set according to actual needs, and the smaller the preset position interval and the preset phase shift interval, the more the available physical positions and phase shifts of the RIS.

[0060] ​S2, for each RIS, taking one physical position from the physical position range corresponding to the RIS according to the preset position interval corresponding to the RIS and taking one phase shift from the phase shift range corresponding to the RIS according to the preset phase shift interval corresponding to the RIS, and then calculating one SNR corresponding to the system this time based on the physical position and the phase shift of the N RIS taken this time, and so on until the physical position range and the phase shift range of the N RIS are traversed, and then a plurality of SNRs are obtained.

[0061] In some embodiments, for each RIS, when taking a value from the physical position range corresponding to the RIS for the first time and when taking a value from the phase shift range corresponding to the RIS for the first time, the lower limit value of the physical position range can be taken as the physical position taken this time, and the lower limit value of the phase shift range can be taken as the phase shift taken this time. In other embodiments, for each RIS, when taking a value from the physical position range corresponding to the RIS for the first time and when taking a value from the phase shift range corresponding to the RIS for the first time, the sum of the lower limit value of the physical position range and the preset position interval can be taken as the physical position taken this time, and the sum of the lower limit value of the phase shift range and the preset phase shift interval can be taken as the phase shift taken this time.

[0062] S3, selecting the maximum SNR from the plurality of SNRs, and taking the physical position and the phase shift of the N RIS corresponding to the maximum SNR as the optimal configuration parameters.

[0063] S4, deploying the N RIS in the system according to the optimal configuration parameters for positioning the detection target, wherein the positioning error of the detection target by the reconnaissance station is negatively correlated with the SNR of the system.

[0064] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like means that the specific features or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in the specification.

[0065] In the specification, the word "comprising" does not exclude other components or steps, and "one" or "a" does not exclude a plurality. Some measures are described in different embodiments, but this does not mean that these measures cannot be combined to produce good results.

[0066] The above is further detailed description of the present application in combination with specific preferred embodiments, and cannot be deemed as limitation of the specific implementation of the present application to these descriptions. For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, and all should be deemed as falling within the protection scope of the present application.

Claims

1. A passive location system based on reconfigurable intelligent surfaces, characterized in that, The system comprises: a scout station, N RISs, and a detection target, wherein each of the detection target and each RIS module has a link with the scout station, and the detection target has a link with each RIS module; wherein each RIS is configured to reflect the signal transmitted by the detection target to the scout station; the scout station is configured to receive the signal directly transmitted by the detection target and the signal reflected by the N RISs, and perform positioning of the detection target based on the signal directly transmitted by the detection target and the signal reflected by the N RISs, or the scout station is configured to receive the signal reflected by the N RISs and perform positioning of the detection target based on the signal reflected by the N RISs.

2. The system of claim 1, wherein, N is a positive integer greater than or equal to 1; the scout station is configured to receive the signal directly transmitted by the detection target and the signal reflected by the N RISs, and perform positioning of the detection target based on the signal directly transmitted by the detection target and the signal reflected by the N RISs.

3. The system of claim 1, wherein, N is a positive integer greater than or equal to 2; the scout station is configured to receive the signal reflected by the N RISs and perform positioning of the detection target based on the signal reflected by the N RISs.

4. The system of claim 1, wherein, The positioning error of the detection target by the scout station is negatively correlated with the SNR of the system.

5. The system of claim 4, wherein, Each RIS has a physical position, and each RIS comprises a plurality of arrayed reflecting units, each reflecting unit having a phase shift; the SNR of the system is related to the phase shift and the physical position of the RIS.

6. The system of claim 2, wherein, When N is 1, in the tth time slot, the expression of the SNR of the system is as follows: wherein λ(t) represents the SNR of the system at the tth time slot, t is in the range of t ∈ {1, …, T}, T represents the total time slots, P represents the transmission power of the detection target, σ 2 represents the noise power, h R,B represents the channel gain between the reconnaissance station and the RIS, H represents the conjugate transpose symbol, h R,v (t) represents the channel gain between the RIS and the detection target, h B,v (t) represents the channel gain between the detection target and the reconnaissance station, θ t represents the angle between the detection target and the RIS relative to the direction between the detection target and the reconnaissance station.

7. The system of claim 3, wherein, When N is 2, and the link between the detection target and the scout station is blocked, in the tth time slot, the expression of the SNR of the system is as follows: wherein λ(t) represents the SNR of the system at the tth time slot, t is in the range of t∈{1,…,T}, T represents the total time slots, P represents the transmission power of the detection target, σ 2 represents the noise power, represents the channel gain between the reconnaissance station and the first RIS, H represents the conjugate transpose symbol, represents the channel gain between the first RIS and the detection target, represents the channel gain between the reconnaissance station and the second RIS, represents the channel gain between the second RIS and the detection target, θ (1)t represents the angle between the detection target and the first RIS relative to the direction between the detection target and the reconnaissance station, θ (2)t represents the angle between the detection target and the second RIS relative to the direction between the detection target and the reconnaissance station.

8. The system of claim 3, wherein, When N is 2, and the link between the detection target and the scout station is not blocked, in the tth time slot, the expression of the SNR of the system is as follows: wherein λ(t) represents the SNR of the system at the tth time slot, t is in the range of t ∈ {1, …, T}, T represents the total time slots, P represents the transmission power of the detection target, σ 2 represents the noise power, h B,v (t) represents the channel gain between the detection target and the reconnaissance station, represents the channel gain between the reconnaissance station and the first RIS, H represents the conjugate transpose symbol, represents the channel gain between the first RIS and the detection target, represents the channel gain between the reconnaissance station and the second RIS, represents the channel gain between the second RIS and the detection target, θ (1)t represents the angle between the detection target and the first RIS relative to the direction between the detection target and the reconnaissance station, θ (2)t represents the angle between the detection target and the second RIS relative to the direction between the detection target and the reconnaissance station.

9. The system of claim 5, wherein, The phase shift of each reflecting unit in each RIS is the same.

10. A method of optimizing positioning accuracy, characterized by The method is applied to the system of any one of claims 1-9, wherein each RIS has a physical position, and each RIS comprises a plurality of arrayed reflecting units, each reflecting unit having a phase shift, and the method comprises: S1, obtaining the physical position range, the phase shift range, the preset position interval and the preset phase shift interval corresponding to each RIS; each range is composed of a range lower limit value and a range upper limit value; S2, for each RIS, each time, a physical position is taken out from the physical position range corresponding to the RIS according to the preset position interval corresponding to the RIS, and a phase shift is taken out from the phase shift range corresponding to the RIS according to the preset phase shift interval corresponding to the RIS, and then, based on the physical positions and phase shifts of the N RISs taken out this time, a SNR corresponding to the system this time is calculated, and thus, when the physical position range and the phase shift range of the N RISs are traversed, a plurality of SNRs are obtained. S3, selecting the maximum SNR from the plurality of SNRs, and taking the physical positions and phase shifts of the N RIS corresponding to the maximum SNR as optimal configuration parameters; S4, deploying the N RIS in the system according to the optimal configuration parameters for positioning a detection target, wherein the positioning error of the detection target by the reconnaissance station is negatively correlated with the SNR of the system.

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