RIS-assisted satellite-ground security communication method with interference

Abstract

The invention discloses an RIS-assisted satellite-ground safety communication method with interference, and belongs to the field of satellite-ground safety communication. The invention provides a satellite-ground safety communication system for RIS assistance. The satellite-ground safety communication system comprises a satellite transmitter, an RIS, an unmanned aerial vehicle (UAV), a receiver and a plurality of EVEs. And the UAV is used for transmitting harmful interference signals. With safety rate maximization as a target, constraint conditions comprise a satellite transmitting power constraint CR1 and an RIS constant modulus constraint CR2, and a safety communication system optimization problem is constructed; by taking safety rate maximization as an optimization target, the worst receiver can also realize safety communication; effective emission of a signal transmitter is ensured through satellite emission power constraint CR1, and signal reflection is ensured through RIS constant modulus constraint CR2. According to the method, the RIS constant modulus constraint CR2 and the objective function are subjected to non-convexity processing, the optimization problem of the security communication system after non-convexity is jointly optimized, the maximum security rate is obtained, and it is ensured that the multi-user security communication system can still communicate reliably and safely under the condition of noise uncertainty.

Description

Technical Field

[0001] The invention relates to a RIS-assisted satellite-to-ground safety communication method with interference, belonging to the field of satellite-to-ground safety communication. Background Art

[0002] In recent years, the rapid development of high-speed communication technology has significantly promoted the development of satellite-to-ground communication systems. These systems have achieved seamless data transmission and information exchange between spacecraft and ground stations. However, with the widespread application of satellite-to-ground communications in civil, military and commercial activities, the sensitive data transmitted by these systems are also threatened by cyber attacks and other malicious activities. It is crucial to ensure the security of satellite-to-ground communication systems. Existing methods such as encryption technology, identity authentication, artificial intelligence and physical security measures have their own advantages and disadvantages. For example, encryption may introduce delays and be vulnerable to decryption attacks, identity authentication may be forged and attacked, and physical security measures, although effective, still have avoidance risks. Reconfigurable smart surface (RIS) technology, as a cutting-edge technology in recent years, can dynamically adjust its surface structure to control electromagnetic wave propagation, effectively eliminate signal obstacles and multipath interference, and improve signal quality and coverage. Active RIS has the ability to accurately control the amplitude, phase and direction of the signal. Although the cost and energy consumption are high, it has significant advantages over traditional antenna systems. Although the cost and energy consumption of passive RIS are low, the accuracy is slightly inferior. Existing research mainly focuses on the application of drones as receivers or relays, while this study innovatively proposes to use drones as interference sources to explore the impact of drone interference on system performance, expanding the understanding of drone capabilities and their potential impact in RIS-assisted communication systems. Summary of the invention

[0003] The following technical defects of the existing MIMO covert communication method are aimed at: (a) the problem of multi-eavesdropping network construction and multi-user collaborative optimization under resource constraint environment; (b) the shortcomings of low secure transmission rate and poor resource utilization in the traditional RIS-based satellite-to-ground secure communication system. The main purpose of the present invention is to provide a RIS-assisted satellite-to-ground secure communication method under UAV interference, which is beneficial to the unique performance of RIS in controlling reflected signals, adding a RIS module to the multi-user covert communication composed of a satellite transmitter, multiple receivers eavesdroppers EVE and one or more user receivers, with the goal of maximizing the secure transmission rate. The constraints include RIS constant modulus constraint CR2 and Alice transmission power constraint CR1, constructing a secure communication system optimization problem, and realizing secure and efficient transmission of the satellite-to-ground communication system according to the optimization result of the secure communication system optimization problem.

[0004] The purpose of the present invention is achieved through the following technical solutions.

[0005] The present invention discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, comprising the following steps:

[0006] Step 1: The secure communication system includes a satellite transmitter, RIS, UAV, receiver, and multiple EVEs. UAV is used to transmit harmful interference signals and build a secure communication system model. RIS is used to reflect signals; users and EVEs jointly receive signals transmitted by Alice and reflected by RIS; among them, Willie is used to detect whether Alice is sending useful signals. The secure communication system model includes the channel model of the satellite-to-ground communication system, the channel model from satellite to user, the channel model from satellite to EVE, the channel model from satellite to user and EVE, and the channel model from UAV to user.

[0007] Considering rain attenuation and free path loss, the channel model of the satellite-to-ground communication system is expressed as

[0008]

[0009] The channel model from satellite to user in the satellite-to-ground secure communication system based on RIS is expressed as

[0010]

[0011] The satellite-to-EVE channel model of the RIS-based satellite-to-ground secure communication system is expressed as

[0012]

[0013] Under non-ideal conditions, the channel model from satellite to user and EVE in the RIS-based satellite-to-ground secure communication system is expressed as

[0014]

[0015] The channel model from UAV to user in RIS-based satellite-to-ground secure communication system is expressed as

[0016]

[0017] Where: C L represents the free space loss, λ s is the carrier wavelength satellite ground channel, ξ is the rain attenuation power loss, f is the overall satellite channel ξ rain attenuation, Φ[n] is the phase matrix of RIS, N R is the number of RIS units, h SU is the direct channel from the satellite user, H SR is the RIS channel from the satellite, h RU For direct connection channels from users, is the channel from the satellite to the i-th EVE, is the channel from RIS to the ith EVE, h u is the overall channel from the satellite to the user, is the ideal channel from satellite to user, Δh u is the channel error from satellite to user, is the total channel from the satellite to the i-th EVE, is the ideal channel from the satellite to the ith EVE, is the channel error from the satellite to the i-th EVE, H VR is the interference channel from the drone to the user, h v It is the communication channel between the drone and RIS.

[0018] Step 2: The satellite transmitter transmits a signal through wireless communication and modulates the baseband signal onto a carrier signal; the power of the satellite transmitter satisfies constraint condition CR1, and the power constraint condition CR1 means that the total power of the satellite transmitter at the current moment is less than the fixed constraint power.

[0019] The transmission signal s is expressed as

[0020] s[n]=w[n](x[n]+n s [n])

[0021] Where: the signal is represented by x, and w is the beamforming data that meets the transmit power constraint requirements.

[0022] Step 3: The user receiver simultaneously receives the signal sent by the satellite transmitter, the interference signal transmitted by the UAV, and the signal reflected by the RIS.

[0023] The signal received by the receiver is expressed as

[0024] y U [n] = h u s[n]+h ν s in [n]+z l [n]

[0025] in: It is the noise received by EVE.

[0026] Step 4: The EVE receiver receives the signal sent by the satellite and the signal reflected by the RIS;

[0027] The signal received by the i-th EVE is expressed as shown in formula (3);

[0028]

[0029] in: is the noise received by the user.

[0030] Step 5: For the RIS-assisted secure communication system, with the goal of maximizing the secure rate, the constraints include the satellite transmit power constraint CR1 and the RIS constant modulus constraint CR2, and construct a secure communication system optimization problem; by taking the maximization of the secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication; the satellite transmit power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal.

[0031] The objective function of the secure communication system optimization problem is as follows:

[0032]

[0033]

[0034]

[0035] max R s

[0036] w[n],Φ[n]

[0037] st||w[n]|| 2 ≤P u

[0038]

[0039] Where: CR1 is the maximum transmission power constraint of the satellite, P u is the maximum transmission power of the transmitter; R2 is the RIS amplitude constant modulus constraint.

[0040] Step 6: According to Jensen's inequality, when considering the constraints, the RIS constant modulus constraint CR2 and the objective function are both non-convex problems that are difficult to solve directly. For the single-user passive RIS security communication system, the single-user active RIS security communication system, and the multi-user non-ideal security communication system, non-convex methods are constructed to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function, and the non-convex security communication system optimization problem is obtained. The non-convex security communication system optimization problem is jointly optimized to obtain the maximum security rate of the security communication system, ensuring that the multi-user security communication system can still communicate reliably and securely under noise uncertainty.

[0041] ① For the single-user passive RIS secure communication system, the precoding parameter w and RIS phase parameter are optimized using an iterative optimization algorithm.

[0042] Step ①.1, fix the RIS phase parameter to optimize the precoding parameter w, and use the LogSumExp function to transform the objective function of the secure communication system optimization problem into

[0043]

[0044] The KKT method is used to solve the optimal solution of the precoding parameter w, thereby obtaining the maximum safe rate.

[0045] Use step ①.2 and the precoding parameter w obtained in step ①.1 to construct the optimal base station transmit beamformer, fix the receive beamformer, and use the projected gradient ascent method to process the RIS constant modulus constraint CR2 problem to solve the optimal RIS phase parameter.

[0046] Step ①.3: Based on step ①.1 and step ①.2, the optimization problem of the secure communication system is repeatedly iterated to jointly optimize the base station transmit beamformer and the RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, the iteration is terminated, and the jointly optimized base station transmit beamformer and RIS reflection signal matrix are output, that is, the optimal base station transmit beam and RIS reflection parameters are obtained to maximize the secure rate. Otherwise, continue to iterate.

[0047] ② For the single-user active RIS secure communication system, the precoding parameter w and RIS phase and amplitude parameters are optimized using an iterative optimization algorithm.

[0048] Step ②.1, fix the RIS phase parameter to optimize the precoding parameter w, use the LogSumExp function, and transform the objective function of the secure communication system optimization problem into

[0049]

[0050] The KKT method is used to solve the optimal solution of the precoding parameter w, thereby obtaining the maximum safe rate.

[0051] Step ②.2: Based on the precoding parameter w obtained in step ②.1, construct the optimal base station transmit beamformer, fix the receive beamformer, and fix the reflection amplitude to use the projected gradient ascent iterative solution method to solve the RIS constant modulus constraint CR2 problem.

[0052] Step ②.3: Based on the RIS phase parameters obtained in step ②.2, the Lagrange multiplier method is used to further optimize the amplitude parameters of the RIS, and the derivative iterative solution method is used at a fixed angle to deal with the amplification parameter constraint problem in the RIS reflection coefficient optimization.

[0053] Step ②.4: Based on step ②.1, step ②.2, and step ②.3, iterate repeatedly to jointly optimize the base station transmit beamformer and RIS parameters; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS parameters to maximize the safety rate. Otherwise, continue to iterate.

[0054] ③ For multi-user non-ideal secure communication systems, an iterative optimization algorithm is used to optimize the precoding parameter w and RIS phase and amplitude parameters.

[0055] Step ③.1. Optimize the precoding parameter w by fixing the RIS phase parameter, and use the genetic algorithm GA method to solve the precoding parameter w. In the genetic algorithm GA method, each entity is regarded as a potential solution to the optimization problem. In each iteration, the fitness score of the entity is calculated, and then the entities are sorted. The entity with the highest score is selected as the parent to generate new individuals. The strategy adopted is roulette wheel selection or elite selection. A one-point crossover operator is used to exchange value segments between two parent entities of the same generation, thereby generating two new offspring. A bit flip mutation operator is also used to introduce mutations. Repeat the iterative genetic algorithm GA until the genetic algorithm GA reaches convergence. After reaching convergence, the optimal individual within the population is determined through continuous iteration and population evolution.

[0056] Step ③.2: Based on the optimal transmit beamformer obtained in step ③.1, fix the receive beamformer and use the branch and bound method to solve the RIS constant modulus constrained CR2 problem. By dividing the RIS constant modulus constrained CR2 problem into smaller sub-problems (branches) and solving sub-problems that do not meet the predetermined criteria (boundaries), the RIS constant modulus constrained CR2 problem is deconvexified. The branch and bound method is applicable to both passive and active RIS scenarios.

[0057] Step ③.3, based on the repeated iterations of step ③.1 and step ③.2, jointly optimize the base station transmit beamformer and RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS reflection signal matrix, that is, obtain the optimal base station transmit beam and RIS reflection parameters to maximize the safety rate. Otherwise, continue to iterate.

[0058] Beneficial effects:

[0059] 1. The present invention discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, which is used in a RIS-assisted satellite-to-ground secure communication system. The RIS-assisted satellite-to-ground secure communication system includes a satellite transmitter, a RIS, an unmanned aerial vehicle (UAV), a receiver, and multiple EVEs. The UAV is used to transmit harmful interference signals and build a secure communication system model. The secure communication system model adds a RIS module to the multi-user covert communication composed of a satellite transmitter, multiple receiver eavesdroppers (EVEs), and one or more user receivers, which is conducive to the construction of a multi-eavesdropping network and multi-user collaborative optimization under a resource-constrained environment.

[0060] 2. The present invention discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference. For a RIS-assisted secure communication system, the goal is to maximize the secure rate. Constraints include satellite transmission power constraint CR1 and RIS constant modulus constraint CR2, and a secure communication system optimization problem is constructed. By taking the maximum secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication. The satellite transmission power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal. The present invention can solve the shortcomings of low secure transmission rate and poor resource utilization in the traditional RIS-based satellite-to-ground secure communication system.

[0061] 3. The present invention discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, taking into account active RIS, passive RIS, and non-ideal situations, respectively constructing non-convex methods to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function, and jointly optimizing the non-convex optimization problem of the secure communication system by combining KKT conditions, LogSumExp functions, genetic algorithms, Lagrange multipliers, derivation, branch and bound method, and alternating optimization algorithms to obtain the maximum secure rate of the secure communication system, thereby ensuring that the multi-user secure communication system can still communicate reliably and securely under the condition of noise uncertainty. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 It is a single-user structural block diagram of a RIS-assisted satellite-to-ground safety communication system with interference described in the present invention;

[0063] Figure 2 It is a multi-user structural block diagram of a RIS-assisted satellite-to-ground safety communication system with interference described in the present invention;

[0064] Figure 3 It is a performance curve comparison diagram of the average secure transmission rate of the method described in the present invention under the condition of single user as the satellite transmission power changes and the traditional method;

[0065] Figure 4 It is a performance curve comparison diagram of the average secure transmission rate under multi-user conditions of the method of the present invention as a function of satellite transmission power and the traditional method;

[0066] Figure 5 It is a performance curve comparison diagram of the average secure transmission rate of the method described in the present invention under the condition of single user as the number of RIS units changes with the traditional method;

[0067] Figure 6 The present invention is a flowchart of the method for performing secure communication in different example situations. DETAILED DESCRIPTION

[0068] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in combination with specific implementation examples and with reference to the accompanying drawings.

[0069] Example 1

[0070] A three-dimensional coordinate system is established, and the RIS is configured as a uniform rectangular array (URA). The number of RIS reflector units is set to 6, with one user in the single-user scenario and four users in the multi-user scenario, as well as four eavesdroppers (EVE). The coordinates of the satellite, drone, and RIS are (0,200000) meters, (200,100) meters, and (95,10) meters, respectively. In addition, the users and eavesdroppers are located in a circular area with a radius of 1 meter, with the users distributed around (100,2) and the eavesdroppers distributed around (80,2). The path loss of the signal with distance is set to 20 dB per meter, and the rain attenuation of the satellite is set to 3 dB. It is assumed that the antenna gain of the user is 10 dBi.

[0071] For single-user passive RIS secure communication systems, such as Figure 6 As shown, the present embodiment discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, and the specific implementation steps are as follows:

[0072] Step 1: The secure communication system includes a satellite transmitter, RIS, UAV, receiver, and multiple EVEs. UAV is used to transmit harmful interference signals and build a secure communication system model. RIS is used to reflect signals; users and EVEs jointly receive signals transmitted by Alice and reflected by RIS; among them, Willie is used to detect whether Alice is sending useful signals. The secure communication system model includes the channel model of the satellite-to-ground communication system, the channel model from satellite to user, the channel model from satellite to EVE, the channel model from satellite to user and EVE, and the channel model from UAV to user.

[0073] Considering rain attenuation and free path loss, the channel model of the satellite-to-ground communication system is expressed as

[0074]

[0075] The channel model from satellite to user in the satellite-to-ground secure communication system based on RIS is expressed as

[0076]

[0077] The satellite-to-EVE channel model of the RIS-based satellite-to-ground secure communication system is expressed as

[0078]

[0079] Under non-ideal conditions, the channel model from satellite to user and EVE in the RIS-based satellite-to-ground secure communication system is expressed as

[0080]

[0081] The channel model from UAV to user in RIS-based satellite-to-ground secure communication system is expressed as

[0082]

[0083] Where: C L represents the free space loss, λ s is the carrier wavelength satellite ground channel, f is the overall satellite channel ξ rain attenuation, Φ[n] is the phase matrix of RIS, N R is the number of RIS units, h SU is the direct channel from the satellite user, H SR is the RIS channel from the satellite, h RU For direct connection channels from users, is the channel from the satellite to the i-th EVE, is the channel from RIS to the ith EVE, h u is the overall channel from the satellite to the user, is the ideal channel from satellite to user, Δh u is the channel error from satellite to user, is the total channel from the satellite to the i-th EVE, is the ideal channel from the satellite to the ith EVE, is the channel error from the satellite to the i-th EVE, H VR is the interference channel from the drone to the user, h v It is the communication channel between the drone and RIS.

[0084] Step 2: The satellite transmitter transmits a signal through wireless communication and modulates the baseband signal onto a carrier signal; the power of the satellite transmitter satisfies constraint condition CR1, and the power constraint condition CR1 means that the total power of the satellite transmitter at the current moment is less than the fixed constraint power.

[0085] The transmission signal is represented as

[0086] s[n]=w[n](x[n]+n s [n])

[0087] Where: the signal is represented by x, and w is the beamforming data that meets the transmit power constraint requirements.

[0088] Step 3: The user receiver simultaneously receives the signal sent by the satellite transmitter, the interference signal transmitted by the UAV, and the signal reflected by the RIS.

[0089] The signal received by the receiver is expressed as

[0090] y U [n] = h u s[n]+h ν s in [n]+z l [n]

[0091] in: It is the noise received by EVE.

[0092] Step 4: The EVE receiver receives the signal sent by the satellite and the signal reflected by the RIS;

[0093] The signal received by the i-th EVE is expressed as shown in formula (3);

[0094]

[0095] in: is the noise received by the user.

[0096] Step 5: For the RIS-assisted secure communication system, with the goal of maximizing the secure rate, the constraints include the satellite transmit power constraint CR1 and the RIS constant modulus constraint CR2, and construct a secure communication system optimization problem; by taking the maximization of the secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication; the satellite transmit power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal.

[0097] The objective function of the secure communication system optimization problem is as follows:

[0098]

[0099]

[0100]

[0101] max R s

[0102] w[n],Φ[n]

[0103] st||w[n]|| 2 ≤P u

[0104]

[0105] Wherein: CR1 is the maximum transmit power constraint, Pu is the maximum transmit power of the transmitter; CR2 is the RIS amplitude constant modulus constraint.

[0106] Step 6: According to Jensen's inequality, when considering the constraints, the RIS constant modulus constraint CR2 and the objective function are both non-convex problems that are difficult to solve directly. For the single-user passive RIS security communication system, the single-user active RIS security communication system, and the multi-user non-ideal security communication system, non-convex methods are constructed to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function, and the non-convex security communication system optimization problem is obtained. The non-convex security communication system optimization problem is jointly optimized to obtain the maximum security rate of the security communication system, ensuring that the multi-user security communication system can still communicate reliably and securely under noise uncertainty.

[0107] ① For the single-user passive RIS secure communication system, the precoding parameter w and RIS phase parameter are optimized using an iterative optimization algorithm.

[0108] Step ①.1, fix the RIS phase parameter to optimize the precoding parameter w, and use the LogSumExp function to transform the objective function of the secure communication system optimization problem into

[0109]

[0110] The KKT method is used to solve the optimization problem of the precoding parameter w and obtain the optimal solution of the precoding parameter w.

[0111] In step ①.2, based on step ①.1, the optimal solution of the precoding parameter w is obtained, the optimal transmit beamformer is generated according to the parameter, and the receive beamformer is fixed, and the projected gradient ascent method is used to process the RIS constant modulus constraint CR2 problem.

[0112] Step ①.3: Based on step ①.1 and step ①.2, the optimization problem of the secure communication system is repeatedly iterated to jointly optimize the base station transmit beamformer and the RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, the iteration is terminated, and the jointly optimized base station transmit beamformer and RIS reflection signal matrix are output, that is, the optimal base station transmit beam and RIS reflection parameters are obtained to maximize the secure rate. Otherwise, continue to iterate.

[0113] Example 2

[0114] For a single-user active RIS safety communication system, such as Figure 6 As shown, the present embodiment discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, and the specific implementation steps are as follows:

[0115] Step 1: The secure communication system includes a satellite transmitter, RIS, UAV, receiver, and multiple EVEs. UAV is used to transmit harmful interference signals and build a secure communication system model. RIS is used to reflect signals; users and EVEs jointly receive signals transmitted by Alice and reflected by RIS; among them, Willie is used to detect whether Alice is sending useful signals. The secure communication system model includes the channel model of the satellite-to-ground communication system, the channel model from satellite to user, the channel model from satellite to EVE, the channel model from satellite to user and EVE, and the channel model from UAV to user.

[0116] Considering rain attenuation and free path loss, the channel model of the satellite-to-ground communication system is expressed as

[0117]

[0118] The channel model from satellite to user in the satellite-to-ground secure communication system based on RIS is expressed as

[0119]

[0120] The satellite-to-EVE channel model of the RIS-based satellite-to-ground secure communication system is expressed as

[0121]

[0122] Under non-ideal conditions, the channel model from satellite to user and EVE in the RIS-based satellite-to-ground secure communication system is expressed as

[0123]

[0124] The channel model from UAV to user in RIS-based satellite-to-ground secure communication system is expressed as

[0125]

[0126] Where: C L represents the free space loss, λ s is the carrier wavelength satellite ground channel, f is the overall satellite channel ξ rain attenuation, Φ[n] is the phase matrix of RIS, N R is the number of RIS units, h SU is the direct channel from the satellite user, H SR is the RIS channel from the satellite, h RU For direct connection channels from users, is the channel from the satellite to the i-th EVE, is the channel from RIS to the ith EVE, h u is the overall channel from the satellite to the user, is the ideal channel from satellite to user, Δh u is the channel error from satellite to user, is the total channel from the satellite to the i-th EVE, is the ideal channel from the satellite to the ith EVE, is the channel error from the satellite to the i-th EVE, H VR is the interference channel from the drone to the user, h v It is the communication channel between the drone and RIS.

[0127] Step 2: The satellite transmitter transmits a signal through wireless communication and modulates the baseband signal onto a carrier signal; the power of the satellite transmitter satisfies constraint condition CR1, and the power constraint condition CR1 means that the total power of the satellite transmitter at the current moment is less than the fixed constraint power.

[0128] The transmission signal is represented as

[0129] s[n]=w[n](x[n]+n s [n])

[0130] Where: the signal is represented by x, and w is the beamforming data that meets the transmit power constraint requirements.

[0131] Step 3: The user receiver simultaneously receives the signal sent by the satellite transmitter, the interference signal transmitted by the UAV, and the signal reflected by the RIS.

[0132] The signal received by the receiver is expressed as

[0133] y U [n] = h u s[n]+h ν s in [n]+z l [n]

[0134] in: It is the noise received by EVE.

[0135] Step 4: The EVE receiver receives the signal sent by the satellite and the signal reflected by the RIS;

[0136] The signal received by the i-th EVE is expressed as shown in formula (3);

[0137]

[0138] in: is the noise received by the user.

[0139] Step 5: For the RIS-assisted secure communication system, with the goal of maximizing the secure rate, the constraints include the satellite transmit power constraint CR1 and the RIS constant modulus constraint CR2, and construct a secure communication system optimization problem; by taking the maximization of the secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication; the satellite transmit power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal.

[0140] The objective function of the secure communication system optimization problem is as follows:

[0141]

[0142]

[0143]

[0144] max R s

[0145] w[n],Φ[n]

[0146] st||w[n]|| 2 ≤P u

[0147]

[0148] Wherein: CR1 is the maximum transmit power constraint, Pu is the maximum transmit power of the transmitter; CR2 is the RIS amplitude constant modulus constraint.

[0149] Step 6: According to Jensen's inequality, when considering the constraints, the RIS constant modulus constraint CR2 and the objective function are both non-convex problems that are difficult to solve directly. For the single-user passive RIS security communication system, the single-user active RIS security communication system, and the multi-user non-ideal security communication system, non-convex methods are constructed to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function, and the non-convex security communication system optimization problem is obtained. The non-convex security communication system optimization problem is jointly optimized to obtain the maximum security rate of the security communication system, ensuring that the multi-user security communication system can still communicate reliably and securely under noise uncertainty.

[0150] ② For the multi-user active RIS secure communication system, the precoding parameter w and RIS phase and amplitude parameters are optimized using an iterative optimization algorithm.

[0151] Step ②.1, fix the RIS phase parameter to optimize the precoding parameter w, use the LogSumExp function, and transform the objective function of the secure communication system optimization problem into

[0152]

[0153] The KKT method is used to solve the optimization problem of the precoding parameter w and obtain the optimal solution of the precoding parameter w.

[0154] Use

[0155] Step ②.2: Based on step ②.1, the optimal solution of the precoding parameter w is obtained, and the optimal transmit beamformer is generated according to the parameter. The receive beamformer is fixed, and the reflection amplitude is fixed to use the projected gradient ascent iterative solution method to process the RIS constant modulus constraint CR2 problem.

[0156] Step ②.3: Based on the optimal transmit beamformer obtained in step ②.1, fix the receive beamformer and the angle and use the derivative iterative solution method to deal with the amplification parameter constraint problem in the RIS reflection coefficient optimization.

[0157] Step ②.4: Based on step ②.1, step ②.2, and step ②.3, iterate repeatedly to jointly optimize the base station transmit beamformer and RIS parameters; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS parameters to maximize the safety rate. Otherwise, continue to iterate.

[0158] Example 3

[0159] For multi-user non-ideal secure communication systems, such as Figure 6 As shown, the present embodiment discloses a RIS-assisted satellite-to-ground secure communication method in the presence of interference, and the specific implementation steps are as follows:

[0160] Step 1: The secure communication system includes a satellite transmitter, RIS, UAV, receiver, and multiple EVEs. UAV is used to transmit harmful interference signals and build a secure communication system model. RIS is used to reflect signals; users and EVEs jointly receive signals transmitted by Alice and reflected by RIS; among them, Willie is used to detect whether Alice is sending useful signals. The secure communication system model includes the channel model of the satellite-to-ground communication system, the channel model from satellite to user, the channel model from satellite to EVE, the channel model from satellite to user and EVE, and the channel model from UAV to user.

[0161] Considering rain attenuation and free path loss, the channel model of the satellite-to-ground communication system is expressed as

[0162]

[0163] The channel model from satellite to user in the satellite-to-ground secure communication system based on RIS is expressed as

[0164]

[0165] The satellite-to-EVE channel model of the RIS-based satellite-to-ground secure communication system is expressed as

[0166]

[0167] Under non-ideal conditions, the channel model from satellite to user and EVE in the RIS-based satellite-to-ground secure communication system is expressed as

[0168]

[0169] The channel model from UAV to user in RIS-based satellite-to-ground secure communication system is expressed as

[0170]

[0171] Where: C L represents the free space loss, λ s is the carrier wavelength satellite ground channel, f is the overall satellite channel ξ rain attenuation, Φ[n] is the phase matrix of RIS, N R is the number of RIS units, h SU is the direct channel from the satellite user, H SR is the RIS channel from the satellite, h RU For direct connection channels from users, is the channel from the satellite to the i-th EVE, is the channel from RIS to the ith EVE, h u is the overall channel from the satellite to the user, is the ideal channel from satellite to user, Δh u is the channel error from satellite to user, is the total channel from the satellite to the i-th EVE, is the ideal channel from the satellite to the ith EVE, is the channel error from the satellite to the i-th EVE, H VR is the interference channel from the drone to the user, h v It is the communication channel between the drone and RIS.

[0172] Step 2: The satellite transmitter transmits a signal through wireless communication and modulates the baseband signal onto a carrier signal; the power of the satellite transmitter satisfies constraint condition CR1, and the power constraint condition CR1 means that the total power of the satellite transmitter at the current moment is less than the fixed constraint power.

[0173] The transmission signal is represented as

[0174] s[n]=w[n](x[n]+n s [n])

[0175] Where: the signal is represented by x, and w is the beamforming data that meets the transmit power constraint requirements.

[0176] Step 3: The user receiver simultaneously receives the signal sent by the satellite transmitter, the interference signal transmitted by the UAV, and the signal reflected by the RIS.

[0177] The signal received by the receiver is expressed as

[0178] y U [n] = h u s[n]+h ν s in [n]+z l [n]

[0179] in: It is the noise received by EVE.

[0180] Step 4: The EVE receiver receives the signal sent by the satellite and the signal reflected by the RIS;

[0181] The signal received by the i-th EVE is expressed as shown in formula (3);

[0182]

[0183] in: is the noise received by the user.

[0184] Step 5: For the RIS-assisted secure communication system, with the goal of maximizing the secure rate, the constraints include the satellite transmit power constraint CR1 and the RIS constant modulus constraint CR2, and construct a secure communication system optimization problem; by taking the maximization of the secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication; the satellite transmit power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal.

[0185] The objective function of the secure communication system optimization problem is as follows:

[0186]

[0187]

[0188]

[0189] max R s

[0190] w[n],Φ[n]

[0191] st||w[n]|| 2 ≤P u

[0192]

[0193] Wherein: CR1 is the maximum transmit power constraint, Pu is the maximum transmit power of the transmitter; CR2 is the RIS amplitude constant modulus constraint.

[0194] Step 6: According to Jensen's inequality, when considering the constraints, the RIS constant modulus constraint CR2 and the objective function are both non-convex problems that are difficult to solve directly. For the single-user passive RIS security communication system, the single-user active RIS security communication system, and the multi-user non-ideal security communication system, non-convex methods are constructed to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function, and the non-convex security communication system optimization problem is obtained. The non-convex security communication system optimization problem is jointly optimized to obtain the maximum security rate of the security communication system, ensuring that the multi-user security communication system can still communicate reliably and securely under noise uncertainty.

[0195] ③ For multi-user non-ideal secure communication systems, an iterative optimization algorithm is used to optimize the precoding parameter w and RIS phase and amplitude parameters.

[0196] Step ③.1. Optimize the precoding parameter w by fixing the RIS phase parameter, and use the genetic algorithm GA method to solve the optimal solution of the precoding parameter w. In the genetic algorithm GA method, each entity is regarded as a potential solution to the optimization problem. In each iteration, the fitness score of the entity is calculated, and then the entities are sorted. The entity with the highest score is selected as the parent to generate a new individual. The strategy adopted is roulette wheel selection or elite selection. A one-point crossover operator is used to exchange value segments between two parent entities of the same generation, thereby generating two new offspring. A bit flip mutation operator is also used to introduce mutations. Repeat the iterative genetic algorithm GA until the genetic algorithm GA reaches convergence. After reaching convergence, the optimal individual within the population is determined through continuous iteration and population evolution.

[0197] Step ③.2: Based on the optimal solution of the precoding parameter w obtained in step ③.1, the optimal transmit beamformer is generated according to the parameter, and the receive beamformer is fixed, and the branch and bound method is used to deal with the RIS constant modulus constrained CR2 problem. By dividing the RIS constant modulus constrained CR2 problem into smaller sub-problems (branches) and solving sub-problems that do not meet the predetermined criteria (boundaries), the RIS constant modulus constrained CR2 problem is deconvexified. The branch and bound method is applicable to both passive and active RIS scenarios.

[0198] Step ③.3, based on the repeated iterations of step ③.1 and step ③.2, jointly optimize the base station transmit beamformer and RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS reflection signal matrix, that is, obtain the optimal base station transmit beam and RIS reflection parameters to maximize the safety rate. Otherwise, continue to iterate.

[0199] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A RIS-assisted satellite-to-ground secure communication method in the presence of interference, characterized in that: The following steps are included: Step 1: The secure communication system includes a satellite transmitter, RIS, UAV, receiver, and multiple EVEs; UAV is used to transmit harmful interference signals and build a secure communication system model; RIS is used to reflect signals; users and EVEs jointly receive signals transmitted by Alice and reflected by RIS; Willie is used to detect whether Alice is sending useful signals; the secure communication system model includes the channel model of the satellite-to-ground communication system, the channel model from satellite to user, the channel model from satellite to EVE, the channel model from satellite to user and EVE, and the channel model from UAV to user; Step 2: The satellite transmitter transmits a signal through wireless communication and modulates the baseband signal onto a carrier signal; the power of the satellite transmitter satisfies a constraint condition CR1, and the power constraint condition CR1 means that the total power of the satellite transmitter at the current moment is less than a fixed constraint power; Step 3: The user receiver simultaneously receives the signal sent by the satellite transmitter, the interference signal transmitted by the UAV, and the signal reflected by the RIS; Step 4: The EVE receiver receives the signal sent by the satellite and the signal reflected by the RIS; Step 5: For the RIS-assisted secure communication system, with the goal of maximizing the secure rate, the constraints include the satellite transmit power constraint CR1 and the RIS constant modulus constraint CR2, and the secure communication system optimization problem is constructed; by taking the maximization of the secure rate as the optimization goal, it is ensured that the worst receiver in the single-user and multi-user communication systems can also achieve secure communication, so that each receiver user in the secure communication system can achieve secure reception while ensuring secure communication; the satellite transmit power constraint CR1 is used to ensure the effective transmission of the signal transmitter, and the RIS constant modulus constraint CR2 is used to ensure the reflection of the signal; Step 6: For the single-user passive RIS security communication system, the single-user active RIS security communication system, and the multi-user non-ideal security communication system, respectively construct non-convex methods to perform non-convex processing on the RIS constant modulus constraint CR2 and the objective function to obtain the non-convex security communication system optimization problem, and jointly optimize the non-convex security communication system optimization problem to obtain the maximized security rate of the security communication system, ensuring that the multi-user security communication system can still communicate reliably and securely under noise uncertainty.

2. The RIS-assisted satellite-to-ground secure communication method in the presence of interference as claimed in claim 1, characterized in that: In step one, Considering rain attenuation and free path loss, the channel model of the satellite-to-ground communication system is expressed as The channel model from satellite to user in the satellite-to-ground secure communication system based on RIS is expressed as The satellite-to-EVE channel model of the RIS-based satellite-to-ground secure communication system is expressed as Under non-ideal conditions, the channel model from satellite to user and EVE in the RIS-based satellite-to-ground secure communication system is expressed as The channel model from UAV to user in RIS-based satellite-to-ground secure communication system is expressed as Where: C L represents the free space loss, λ s is the carrier wavelength satellite ground channel, f is the overall satellite channel ξ rain attenuation, Φ[n] is the phase matrix of RIS, N R is the number of RIS units, h SU For the direct channel from the satellite user, H SR is the RIS channel from the satellite, h RU For direct connection channels from users, is the channel from the satellite to the i-th EVE, is the channel from RIS to the ith EVE, h u is the overall channel from the satellite to the user, is the ideal channel from satellite to user, Δh u is the channel error from satellite to user, is the total channel from the satellite to the i-th EVE, is the ideal channel from the satellite to the ith EVE, is the channel error from the satellite to the i-th EVE, H VR is the interference channel from the drone to the user, h v It is the communication channel between the drone and RIS.

3. A RIS-assisted satellite-to-ground secure communication method as claimed in claim 2, characterized in that: In step 2, the sending signal is represented by s[n]=w[n](x[n]+n s [n]) Where: the signal is represented by x, and w is the beamforming data that meets the transmit power constraint requirements.

4. A RIS-assisted satellite-to-ground secure communication method as claimed in claim 3, characterized in that: In step 3, the signal received by the receiver is expressed as y U [n]=h u s[n]+h ν s in [n]+z l [n] in: It is the noise received by EVE.

5. A RIS-assisted satellite-to-ground secure communication method as claimed in claim 4, characterized in that: In step 4, the signal received by the i-th EVE is expressed as shown in formula (3); in: is the noise received by the user.

6. A RIS-assisted satellite-to-ground secure communication method as claimed in claim 5, characterized in that: In step 5, the objective function of the secure communication system optimization problem is as follows: s.t.||w[n]|| 2 ≤P u Where: CR2 is the maximum transmit power constraint, Pu is the maximum transmit power of the transmitter; CR1 is the RIS amplitude constant modulus constraint.

7. A RIS-assisted satellite-to-ground secure communication method as claimed in claim 6, characterized in that: In step six, ① For the single-user passive RIS secure communication system, the precoding parameter w and RIS phase parameter are optimized using an iterative optimization algorithm; Step ①.1, fix the RIS phase parameter to optimize the precoding parameter w, and use the LogSumExp function to transform the objective function of the secure communication system optimization problem into s.t.‖w[n]‖ 2 ≤P u The KKT method is used to solve the optimal solution of the precoding parameter w, thereby obtaining the maximum safe rate. Step ①.2: Based on the precoding parameters obtained in step ①.1, construct the optimal base station transmit beamformer, fix the receive beamformer, use the projected gradient ascent method to process the RIS constant modulus constraint CR2 problem, and solve the optimal RIS phase parameters. Step ①.3, based on step ①.1 and step ①.2, repeatedly iterate the optimization problem of the secure communication system, and jointly optimize the base station transmit beamformer and the RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS reflection signal matrix, that is, obtain the optimal base station transmit beam and RIS reflection parameters to maximize the security rate; if not, continue to iterate; ② For the single-user active RIS secure communication system, the precoding parameter w and RIS phase and amplitude parameters are optimized using an iterative optimization algorithm; Step ②.1, fix the RIS phase parameter to optimize the precoding parameter w, use the LogSumExp function, and transform the objective function of the secure communication system optimization problem into The KKT method is used to solve the optimal solution of the precoding parameters to obtain the maximum safe rate. Step ②.2: Based on the precoding parameters obtained in step ②.1, construct the optimal base station transmit beamformer, fix the receive beamformer, and fix the reflection amplitude to use the projected gradient ascent iterative solution method to solve the RIS constant modulus constraint CR2 problem. Step ②.3: Based on the RIS phase parameters obtained in step ②.2, the Lagrange multiplier method is used to further optimize the amplitude parameters of the RIS, and the derivative iterative solution method is used at a fixed angle to deal with the amplification parameter constraint problem in the RIS reflection coefficient optimization. Step ②.4, based on step ②.1, step ②.2, and step ②.3, iterate repeatedly to jointly optimize the base station transmit beamformer and RIS parameters; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS parameters to maximize the safety rate; if not, continue to iterate; ③ For multi-user non-ideal secure communication systems, an iterative optimization algorithm is used to optimize the precoding parameter w and the RIS phase and amplitude parameters; Step ③.

1. Optimize the precoding parameter w by fixing the RIS phase parameter, and use the genetic algorithm GA method to solve the coding parameter w; in the genetic algorithm GA method, each entity is regarded as a potential solution to the optimization problem; in each iteration, the fitness score of the entity is calculated, and then the entities are sorted; the entity with the highest score is selected as the parent to generate new individuals; The strategy adopted is roulette wheel selection or elite selection; a one-point crossover operator is used to exchange value segments between two parent entities of the same generation, thereby generating two new offspring; a bit flip mutation operator is also used to introduce mutations; the genetic algorithm GA is repeatedly iterated until the genetic algorithm GA reaches convergence; after reaching convergence, the optimal individual in the population is determined through continuous iteration and population evolution; Step ③.2: Based on the optimal transmit beamformer obtained in step ③.1, fix the receive beamformer and use the branch and bound method to solve the RIS constant modulus constrained CR2 problem; by dividing the RIS constant modulus constrained CR2 problem into smaller sub-problems and solving sub-problems that do not meet the predetermined criteria, the RIS constant modulus constrained CR2 problem is non-convex; the branch and bound method is applicable to both passive and active RIS scenarios; Step ③.3, based on the repeated iterations of step ③.1 and step ③.2, jointly optimize the base station transmit beamformer and the RIS reflection matrix; determine whether the result is less than the convergence accuracy threshold, or whether the maximum number of iterations has been reached. If so, terminate the iteration and output the jointly optimized base station transmit beamformer and RIS reflection signal matrix, that is, obtain the optimal base station transmit beam and RIS reflection parameters to maximize the safety rate; otherwise, continue to iterate.

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