A concealed communication method combining unmanned aerial vehicle and intelligent reflective surface

Abstract

The present invention provides a covert communication method combining a drone and an intelligent reflective surface, and belongs to the field of wireless communication networks. The method specifically refers to transmitting covert information between a source and a destination when the exact location of an eavesdropper is unknown, and determining the minimum probability of error detection and the optimal threshold value of the detection power. Under the condition that the source and the destination are most likely to be discovered, and under the condition that the concealment constraints are met, the method of jointly optimizing the source transmission power, the horizontal position of the drone and the intelligent reflective surface, and the phase shift of the intelligent reflective surface is adopted to maximize the covert transmission rate. The present invention provides a reference value method for how to achieve secure transmission of covert communication and maximize the transmission concealment rate.

Description

Technical Field

[0001] The present invention belongs to the technical field of user information security of wireless communication networks, and relates to a strategy of combining a drone and an intelligent reflective surface to counter active monitoring by malicious eavesdroppers. Specifically, when the exact position of the eavesdropper is unknown, covert information is transmitted between a source and a destination, and the maximum covert transmission rate is achieved by jointly optimizing the source transmission power, the horizontal positions of the drone and the intelligent reflective surface, and the phase shift of the intelligent reflective surface. Background Art

[0002] With the gradual advancement of aviation technology, the use of UAVs (Unmanned Aerial Vehicles, UAVs) has gradually expanded to dangerous environments where it is difficult for humans to perform tasks directly, including disaster relief, surveillance, data collection, etc. Benefiting from the higher flight altitude, UAVs can establish a line-of-sight channel for air-to-ground communications, which has significantly improved performance compared to traditional ground communications with non-line-of-sight channels. With flexible deployment and high mobility, UAV-assisted air-to-ground networks can be deployed on demand to provide wireless connections for disconnected devices, especially in areas without communication infrastructure and in emergency situations. With UAV-assisted wireless networks, coverage and connectivity can be significantly improved. However, the high-quality line-of-sight channel for air-to-ground communications increases the possibility of confidential information being eavesdropped or attacked. Therefore, the security of UAV communication systems is of vital importance.

[0003] Due to the unpredictable nature of wireless channels, encryption techniques will also fail when facing adversaries with intelligent anti-encryption capabilities and powerful computing power, while physical layer security cannot guarantee the amount of information that can be eavesdropped. In future wireless networks, such as military applications and emergencies such as national events, higher security levels are usually required to avoid the possibility of eavesdropping and shield the existence of wireless transmissions. In this case, encryption and physical layer security methods cannot work. Therefore, more and more research is devoted to solving the security problems of wireless communications. The emerging covert communication is an attractive technology that can hide information transmission from being discovered by eavesdroppers, which provides strong security protection for various security-sensitive critical applications. For the analysis of the concealment of wireless systems, Bash et al. established the square root law in the additive white Gaussian noise channel. Since then, research on the concealment performance of wireless communications has increased.

[0004] Despite the many advantages of drone-assisted covert networks, there are still some challenges. One of the key issues is that the line of sight (LoS) link between drones and ground users is easily blocked, resulting in performance degradation. To solve this problem, intelligent reflective surfaces (IRS) have emerged as a powerful tool and have begun to be widely used in wireless communications. Intelligent reflective surfaces can intelligently adjust the incident signal through some reflective elements to enhance the transmission signal. The significant advantage of intelligent reflective surfaces is the reconfiguration of the propagation environment, which can provide better compatibility for transceivers. Combining intelligent reflective surfaces with drones has obvious advantages. On the one hand, when the wireless channel between the drone and the ground user is blocked, the intelligent reflective surface can assist the drone in establishing a new channel to improve the communication quality and expand the air-ground coverage. On the other hand, compared with the fixed-position intelligent reflective surface, the combination of drones and intelligent reflective surfaces has better flexibility and can achieve full-angle reflection in the air, further expanding the range of wireless transmission. In addition, the intelligent reflective surface requires low power, and its combination with drones can further save energy consumption.

[0005] In order to further ensure privacy and security, it is imperative to study the covert communication of drones and smart reflective surfaces. The present invention proposes a covert communication method combining drones and smart reflective surfaces to maximize the covert transmission rate under the condition that the eavesdropper's position is uncertain. The specific scheme is shown in the schematic diagram. Figure 1 As shown in Figure 2. Different from the existing work, we use drones and smart reflective surfaces as relays, and the uncertainty in the communication system is introduced by the background noise of the eavesdropper. a , the positions of the drone and the smart reflective surface, and the phase shift θ of the smart reflective surface i Optimize and maximize the concealed transmission rate R while satisfying the concealment constraint. Summary of the invention

[0006] In view of the problems existing in the prior art, the present invention provides a covert communication method using a combination of a drone and an intelligent reflective surface to achieve signal transmission between a transmitter (Alice) and a receiver (Bob) without being detected by an eavesdropper (Willie), wherein the exact location of Willie is unknown. The most unfavorable scenario for both communicating parties is constructed by setting Willie to the optimal detection power threshold value, and on this basis, P is jointly optimized. a , the position and phase shift of the drone and the smart reflector θ i , to ensure that the concealed transmission rate R is maximized under the condition of satisfying the concealment condition constraints.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] A covert communication method combining a drone and an intelligent reflective surface comprises the following steps:

[0009] The first step is to set up the following:

[0010] 1) In a wireless communication network, a ground transmitter (Alice) transmits confidential information to a ground receiver (Bob), and there is a potential eavesdropper (Willie) on the ground with an uncertain location who attempts to intercept and eavesdrop on legitimate transmissions. At the same time, due to the serious obstacles to long-distance communication between Alice and Bob, there is no direct communication link between the two. Therefore, drones and smart reflective surfaces are used as mobile relays to improve the performance of the communication system, where the smart reflective surface is carried by the drone.

[0011] 2) In this system model, the flight altitude of the UAV and the smart reflective surface is fixed at H, and the horizontal coordinates of the UAV and the smart reflective surface are Bob's coordinates are expressed as The estimated Willie position is expressed as The radius of the circle in Willie's potential area is C W Due to the line-of-sight characteristics of the space-to-ground channel, it is assumed that the space-to-ground channel gain satisfies the free space path loss model, and the channel power gain at each reference distance of 1m is expressed as β 0 For the channels from the drone and the smart reflector to Alice and Willie, consider the Rayleigh fading channel Where d and μ are the distance between users and the path loss index, respectively, and g is a complex Gaussian distribution that follows independent and identical distribution. The diagonal matrix of the smart reflector is Where N is the number of reflective elements in the smart reflective surface, and i = 1, 2, 3, ..., N. Define h uw 、h au 、h aw They represent the channel gains from the drone and smart reflective surface to Willie, from Alice to the drone and smart reflective surface, and from Alice to Willie respectively.

[0012] 3) Alice’s information transmission power P a Not exceeding its upper power limit P max , the information transmission rate R is not lower than the threshold value R that can be correctly demodulated th .

[0013] 4) Willie is at its detection position q WIn the above, the power of the signal received by Willie is used to determine whether the communication between Alice and Bob is taking place: when the signal power received by Willie is higher than the preset detection power threshold, it is considered that Alice and Bob are communicating as the source and the destination; when the signal power received by Willie is less than or equal to the preset detection power threshold, it is considered that Alice and Bob are not communicating as the source and the destination. There is a certain probability that the above judgment made by Willie is correct. When Willie's judgment is correct, it means that the communication between Alice and Bob is eavesdropped, and the covert communication is forced to end; when Willie's judgment is wrong, it indicates that the covert communication between Alice and Bob is successful and has not been eavesdropped. Willie's minimum error detection probability ξ can be obtained by calculation. * , and determine the threshold value T of Willie's optimal detection power * The optimization problem considers Willie to adopt the minimum error detection probability ξ * and the threshold value T of the optimal detection power * This is the best case for Willie detection, but the worst case for covert communication.

[0014] 5) In the worst case constructed in step 4), by properly arranging the information transmission power P a , the position and phase shift of the drone and the smart reflector θ i , so that the covert transmission rate R is maximized, while ensuring that the constraints of covert communication are met, that is, ξ * ≥1-ε, where ε is the concealment parameter.

[0015] In the second step, according to the specific settings of the first step, the threshold value T of Willie's optimal detection power is calculated and analyzed. * and the minimum error detection probability is ξ * , which maximizes the probability that Willie correctly detects whether Alice and Bob are communicating, where the optimal detection power threshold value T * As shown in formula (1), the minimum error detection probability is ξ * As shown in formula (2):

[0016]

[0017]

[0018] In the formula, v = |h uw Θh au +h aw | 2, represents the channel gain at Willie; ρ、r w They represent the rated noise power, noise uncertainty coefficient, and Willie's signal-to-noise ratio respectively.

[0019] The concealed transmission rate is shown in formula (3):

[0020]

[0021] In the formula, h ub and h au They represent the channel gains from the drone and smart reflective surface to Bob, and from Alice to the drone and smart reflective surface respectively; Θ represents the diagonal matrix element of the smart reflective surface; σ b represents the noise power at Bob;

[0022] In the third step, the optimization target is the concealed transmission rate. According to the model, the following optimization problem can be constructed:

[0023]

[0024] In this optimization problem, C1 is the signal transmission concealment requirement. C2 is the transmission power constraint requirement at Alice, P max is the corresponding maximum transmission power. C3 gives the phase shift constraint of the smart reflector, indicating that the phase shift of each reflector unit can be adjusted in the range of [0,2π], which is feasible.

[0025] The fourth step is to design an algorithm to solve the optimization problem:

[0026] Since C1 in this problem is a non-convex hidden constraint, the optimization problem is difficult to solve directly. Next, we first determine the maximum allowable transmit power P a Then, the optimal phase shift θ of the smart reflector is derived i , optimal locations for drones and smart reflective surfaces.

[0027] 1) Transmitting power P a With phase shift θ i Optimization

[0028] First, the optimal position q of the drone and the smart reflector is fixed. Then, according to C1 in formula (2) and formula (4), the maximum allowable transmission power P can be derived. a1 In addition, the transmission power at Alice is also limited by C2 in formula (4). Therefore, the optimal transmission power P a * Should meet:

[0029] P a * =min(Pmax ,P a1 ) (5)

[0030] Next, the optimal phase shift θ of the smart reflector is analyzed. i From formula (2), we can observe that ξ * With phase shift θ i This means that the optimal θ can be derived by maximizing the covert transmission rate R i In order to maximize the objective function in formula (4), there is always an optimal solution θ under the condition that the phase shift constraint C3 is satisfied. i Because the different beams reflected in the smart reflector should be coherently superimposed at the receiver, the optimal phase shift θ i * Should meet:

[0031] θ i * =θ φ +arg(h ub )+arg(h au ) (6)

[0032] In the formula, θ φ represents an arbitrary phase shift in [0,2π].

[0033] 2) Optimization of the optimal position of the drone and the smart reflective surface

[0034] When optimizing the best position q of the drone and the smart reflector, the transmission power P needs to be fixed. a * With phase shift θ i * It is noted that the concealed transmission rate in formula (3) can be further expressed as:

[0035]

[0036] In the formula, for the convenience of calculation, Β=|β 0 | 2 N 2 , where β 0 represents the channel power gain per 1m reference distance, N represents the number of reflective elements; d UB and d AU Represent the distances from the drone and the smart reflective surface to Bob and Alice respectively.

[0037] Therefore, the objective function can be simplified as:

[0038]

[0039] In this case, ignoring the numerator in the objective function, formula (8) can be further expressed as:

[0040]

[0041] Analyzing formula (9), we can get the optimal position of the UAV and the intelligent reflective surface, namely:

[0042]

[0043] Where q represents the position of the UAV and the smart reflective surface; q B represents Bob's coordinates; H represents the flight altitude of the drone and the smart reflective surface.

[0044] The optimal position q of the drone and the smart reflective surface can be obtained by calculation * Should meet:

[0045]

[0046] In the formula, Indicates the ratio coefficient between the height and distance of the drone and the smart reflective surface; d AB Represents the distance from Alice to Bob.

[0047] This paper studies a covert communication method using drones and intelligent reflective surfaces as mobile relays. The advantages of drones and intelligent reflective surfaces are fully utilized to maximize the covert transmission rate while meeting the concealment requirements. The optimal detection threshold T is derived. * And obtain the minimum error detection probability ξ * , which is the worst case when Alice and Bob communicate covertly. Then, optimize Alice's transmission power P a , the optimal horizontal position of the UAV-IRS and the phase shift θ of the IRS i , which maximizes the covert transmission rate while ensuring that the end-to-end error detection probability is greater than the minimum limit. Numerical results demonstrate the effectiveness of this approach in covert communications.

[0048] The beneficial effects of the present invention are:

[0049] The present invention uses drones and intelligent reflective surfaces as mobile relays in the air to achieve covert communication between the source and the destination, and considers the deployment scheme of maximizing the transmission concealment rate by reasonably designing the signal transmission power, the optimal position of the drone and the intelligent reflective surface, and the phase shift of the reflective element under the worst communication conditions. The present invention provides a reference value method for how to achieve secure transmission of covert communication and maximize the transmission concealment rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Schematic diagram of a covert communication network assisted by drones and smart reflective surfaces.

[0051] Figure 2 The impact of changes in the reflective elements of the smart reflective surface on the covert transmission rate.

[0052] Figure 3 The impact of changes in the reflective elements of the smart reflective surface on the transmission power.

[0053] Figure 4 The effect of the horizontal position of the drone and the smart reflective surface on the covert transmission rate.

[0054] Figure 5 Trajectory diagram of the drone and smart reflective surface as Bob moves.

[0055] Figure 6 The effect of transmit power on covert transmission rate.

[0056] Figure 7 The impact of the distance between Alice and Willie on the covert transmission rate under different schemes.

[0057] Figure 8 Impact of concealment parameters on concealed transmission rate under different schemes. DETAILED DESCRIPTION

[0058] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0059] In order to better understand the above technical solution, a specific analysis is given below in conjunction with the accompanying drawings and specific implementation methods.

[0060] The present invention is a method for realizing covert communication by combining a drone and an intelligent reflective surface. The communication method realizes information transmission between a source and a destination by jointly optimizing transmission power, optimal positions of the drone and the intelligent reflective surface, and phase shift of the reflective element, and maximizes the covert transmission rate under the condition of satisfying the concealment constraint. The method comprises the following steps:

[0061] The first step is to make the following specific settings:

[0062] 1) The initial positions of the signal source Alice and the signal sink Bob are fixed, namely signal source A (0, 0) and signal sink B (400, 300), and the height of the drone and the smart reflective surface is H = 100m;

[0063] 2) Channel power gain β at a reference distance of 1m 0 , noise power, path loss index, rated noise power, and the maximum transmit power are set to: β 0 =-30dB, μ=3, P max =36dBm;

[0064] In the second step, based on the settings in the first step, the threshold value T of Willie's optimal detection power is obtained by calculating formulas (1) and (2): * and the minimum probability of false detection ξ * In the case of different reflective elements N and concealment parameters ∈, the influence on the concealed transmission rate R in formula (3) is considered and analyzed:

[0065] first, Figure 2 The relationship between the covert transmission rate R and the number of reflective elements N under different ∈ is analyzed in , where the UAV and the smart reflective surface are located at the optimal position and random position, respectively. The optimal horizontal position is obtained based on the calculation, and the random position of the UAV and the smart reflective surface is set to O(260,190)m. For these two positions, the optimal phase shift is applied to achieve coherent signal superposition for the legitimate receiver. It can be concluded that the covert transmission rate R increases with the increase of N, which is due to the increase in the number of reflective elements leading to the improvement of the channel gain at the receiving end. In addition, it can be observed that the performance at the optimal position is significantly better than that at the random position, which shows that it is necessary to optimize the position of the UAV and the smart reflective surface.

[0066] Next, in Figure 3 The transmission power P of the signal source at different locations is compared in a The relationship between the number of reflective elements N, where ∈ is 0.01 and 0.02 respectively. The random position is set to O(200, 190, 100) meters. The results show that the transmission power P of the source a It decreases with the increase of N, which means that the concealment requirement becomes more relaxed. In addition, the transmission power of the optimal position is lower than that of the random position, that is, optimizing the position of the UAV and the smart reflective surface can effectively save the transmission power.

[0067] The third step is to determine the optimization problem as shown in formula (4), and analyze the impact of the hidden transmission rate R of the optimized target in formula (4) when the UAV and the intelligent reflective surface are located in different positions.

[0068] Figure 4 The relationship between the horizontal position of the UAV and the smart reflective surface and the concealed transmission rate is given in , where ∈ = 0.01, N = 100. The horizontal position of the UAV and the smart reflective surface refers to the horizontal distance from the source (Alice) to it. In addition, set d AB=400m, the results show that the covert transmission rate does not increase or decrease monotonically when the UAV and the smart reflective surface move. When the UAV and the smart reflective surface are at the optimal position, the covert transmission rate is the largest. When the UAV and the smart reflective surface are away from the optimal position, the covert transmission rate decreases. This is because the distance between the source or destination and the UAV and the smart reflective surface is relatively long, which leads to a decrease in channel gain. Therefore, the optimal horizontal position of the UAV and the smart reflective surface should balance the covert transmission rate and concealment requirements. On the one hand, the optimal position should ensure that the reflection path loss from the UAV and the smart reflective surface to the source or destination is small. On the other hand, it should be easier to meet the concealment requirements at the optimal position.

[0069] exist Figure 5 The trajectory diagram of the drone and the smart reflective surface as the destination (Bob) moves is shown in , where ∈ = 0.01, N = 100, and the eavesdropper is located in a potential area with a radius of 20m centered at (125, -180)m. Since the height of the drone and the smart reflective surface is H = 100 meters, it can be observed that as Bob moves horizontally, the optimal horizontal position of the drone and the smart reflective surface is determined according to the position coefficient ζ * (τ) moves in the same trend.

[0070] also, Figure 6 The relationship between the concealed transmission rate and the transmission power is shown, where ∈ = 0.01, and the random positions of the drone and the smart reflector are O(100, 80) m. The optimal phase shift algorithm is used for both the optimal position and the random position. It can be seen that the concealed transmission rate increases with the transmission power P a In addition, in P a =13dBm, the covert transmission rate of N = 200 is improved by about 0.62bit / Hz compared with N = 100. At the same time, the optimized position can achieve significant performance gain compared with the random position. The results verify the effectiveness of the proposed joint design of transmit power, phase shift and horizontal position in the UAV and smart reflective surface assisted covert communication network.

[0071] The fourth step is to derive the maximum allowable transmission power P by solving the optimization problem algorithm. a , the optimal phase shift θ of the smart reflector i , the best position of the drone and the smart reflective surface, where the optimization results are formulas (5), (6), and (11). We further analyze the distance d between Alice and Willie. AW For the optimization of the target concealed transmission rate R, set d AW The movement varies between 180m and 260m.

[0072] exist Figure 7The variation of the covert transmission rate with the distance between Alice and Willie in 8 different cases is analyzed, where the random position is set to O(100, 80) m and the optimal position is determined according to formula (11). First, it is observed that the covert transmission rate increases with d AW This is because d AW The increase of leads to a decrease in the channel gain from Alice to Willie, which affects Willie's detection performance of covert communication. Then, comparing the changes in the concealment requirement curve from ∈ = 0.01 to ∈ = 0.02, it can be found that in N and d AW Under the same conditions, the covert transmission rate increases significantly. From the results, when the UAV and the smart reflective surface are located at the optimal position and N = 200, ∈ = 0.02, the best performance of the covert transmission rate can be achieved.

[0073] To further investigate the impact of the concealment requirement, Figure 8 The relationship between the covert transmission rate and the concealment parameter ∈ for different H is shown, where N = 100 and the random position is O(180,120)m. The results show that the covert transmission rate increases with the increase of the concealment parameter ∈. On the other hand, the covert transmission rate decreases with the decrease of H.

[0074] The above-described embodiments merely express the implementation methods of the present invention, but they cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A covert communication method combining a drone and an intelligent reflective surface, It is characterized in that The following steps are involved: The first step is to set up the following: 1) In a wireless communication network, a ground transmitter Alice transmits confidential information to a ground receiver Bob, and there is a potential eavesdropper Willie on the ground with an uncertain location who attempts to intercept and eavesdrop on legitimate transmissions; drones and smart reflective surfaces are used as mobile relays, where the smart reflective surface is carried by the drone; 2) In the system model, the flight altitude of the UAV and the smart reflective surface is fixed at H, and the horizontal coordinates of the UAV and the smart reflective surface are Bob's coordinates are expressed as The estimated Willie position is expressed as The radius of the circle in Willie's potential area is C W Due to the line-of-sight characteristics of the air-to-ground channel, it is assumed that the air-to-ground channel gain satisfies the free space path loss model. The channel power gain at a reference distance of 1 m is expressed as β 0 ; For the channels from the drone and the smart reflector to Alice and Willie, consider the Rayleigh fading channel Where d and μ are the distance between users and the path loss index, respectively, and g is a complex Gaussian distribution that follows independent and identical distribution; the diagonal matrix of the smart reflector is θ i ∈[0,2π), where N is the number of reflection elements in the smart reflection surface, and i=1,2,3,…,N; define h uw 、h au 、h aw They represent the channel gains from the drone and smart reflective surface to Willie, from Alice to the drone and smart reflective surface, and from Alice to Willie respectively; 3) Alice’s information transmission power P a Not exceeding its upper power limit P max , the information transmission rate R is not lower than the threshold value R that can be correctly demodulated th ; 4) Willie is at its detection position q W In the above example, the power of the signal received by Willie is used to determine whether the communication between Alice and Bob is in progress: 5) In the worst case constructed in step 4), by properly arranging the information transmission power P a , the position and phase shift of the drone and the smart reflector θ i , so that the covert transmission rate R is maximized, while ensuring that the constraints of covert communication are met, that is, ξ * ≥1-ε, where ε is the concealment parameter; In the second step, according to the specific settings of the first step, the threshold value T of Willie's optimal detection power is calculated and analyzed. * and the minimum error detection probability is ξ * , which maximizes the probability that Willie correctly detects whether Alice and Bob are communicating, where the optimal detection power threshold value T * As shown in formula (1), the minimum error detection probability is ξ * As shown in formula (2): where \(v = |h uw \Theta_h au +h aw |\) 2 , representing the channel gain at Willie; \(\rho, r w represent the rated noise power, noise uncertainty factor, and signal-to-noise ratio of Willie, respectively; The concealed transmission rate is shown in formula (3): In the formula, h ub and h au They represent the channel gains from the drone and smart reflective surface to Bob, and from Alice to the drone and smart reflective surface respectively; Θ represents the diagonal matrix element of the smart reflective surface; σ b represents the noise power at Bob; In the third step, the optimization target is the concealed transmission rate. According to the model, the following optimization problem can be constructed: In this optimization problem, C1 is the requirement for signal transmission concealment; C2 is the constraint requirement for the transmission power at Alice, and P max is the corresponding maximum transmission power; C3 gives the phase shift constraint of the intelligent reflecting surface, indicating that the phase shifts of the reflecting elements can be adjusted within the range of [0, 2π], which is practical; The fourth step is to design an algorithm to solve the optimization problem: First determine the maximum allowed transmit power P a ; Then, the optimal phase shift θ of the smart reflector is derived i , the best position of the drone and the smart reflective surface; the specific steps are: 1) Transmitting power P a With phase shift θ i Optimization First, the optimal position q of the UAV and the smart reflector is fixed. Then, according to C1 in formula (2) and formula (4), the maximum allowable transmission power P can be derived. a1 ; In addition, the transmission power at Alice is also limited by C2 in formula (4); therefore, the optimal transmission power P a * Should meet: P a * =min(P max ,P a1 ) (5) Next, the optimal phase shift θ of the smart reflector is analyzed. i ; From formula (2), we can observe that ξ * With phase shift θ i This means that the optimal θ can be derived by maximizing the covert transmission rate R i ; In order to maximize the objective function in formula (4), there is always an optimal solution θ under the condition that the phase shift constraint C3 is satisfied. i ; Because the different beams reflected in the smart reflector should be coherently superimposed at the receiver, the optimal phase shift θ i * Should meet: i i * =θ φ +arg(h ub )+arg(h au ) (6) In the formula, θ φ represents an arbitrary phase shift in [0,2π]; 2) Optimization of the optimal position of the drone and the smart reflective surface When optimizing the best position q of the drone and the smart reflector, the transmission power P needs to be fixed. a * With phase shift θ i * unchanged; note that the concealed transmission rate in formula (3) can be further expressed as: In the formula, for the convenience of calculation, Β=|β 0 | 2 N 2 , where β 0 represents the channel power gain per 1m reference distance, N represents the number of reflective elements; d UB and d AU Represent the distances from the drone and the smart reflective surface to Bob and Alice respectively; Therefore, the objective function can be simplified as: In this case, ignoring the numerator in the objective function, formula (8) can be further expressed as: Analyzing formula (9), we can get the optimal position of the UAV and the intelligent reflective surface, namely: Where q represents the position of the UAV and the smart reflective surface; q B represents Bob's coordinates; H represents the flight altitude of the drone and the smart reflective surface; The optimal position q of the drone and the smart reflective surface can be obtained by calculation * Should meet: In the formula, Indicates the ratio coefficient between the height and distance of the drone and the smart reflective surface; d AB Represents the distance from Alice to Bob.

2. According to claim 1, a covert communication method combining a drone and an intelligent reflective surface, It is characterized in that Step 4) described in the first step is specifically as follows: when the signal power received by Willie is higher than the preset detection power threshold value, it is considered that Alice and Bob are communicating as the source and destination; when the signal power received by Willie is less than or equal to the preset detection power threshold value, it is considered that Alice and Bob are not communicating as the source and destination; there is a certain probability that the above judgment made by Willie is correct. When Willie's judgment is correct, it means that the communication between Alice and Bob is eavesdropped, and the covert communication is forced to end; when Willie's judgment is wrong, it indicates that the covert communication between Alice and Bob is successful and has not been eavesdropped; Willie's minimum error detection probability ξ can be obtained by calculation. * , and determine the threshold value T of Willie's optimal detection power * ; This optimization problem considers Willie to use the minimum error detection probability ξ * and the threshold value T of the optimal detection power * This is the best case for Willie detection, but the worst case for covert communication.

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