Mobile anchor point based smart reflector system channel estimation method

Abstract

The application relates to a mobile-anchor-based reconfigurable intelligent surface (RIS) system channel estimation method. The method comprises the following steps: 1, a mobile anchor node is deployed near the RIS, and a random user in the system is selected to participate in channel estimation; 2, pilot signals are transmitted among a base station (BS), the user and the anchor node through the RIS, corresponding channel estimation is completed, channel state information (CSI) obtained is fed back to the BS, and the anchor node hides from the attack of an eavesdropper by using the mobile capability during the period; 3, the BS calculates the CSI of a common channel BS-RIS by using the CSI fed back by each node; and 4, the CSI of the common channel BS-RIS is used by the user in the system to complete the estimation of a cascaded channel BS-RIS-user, so that the purpose of reducing channel estimation cost is achieved. The mobile-anchor-based RIS system channel estimation method provided by the application can reduce the channel estimation cost of a communication system under a multi-user condition, and further provides better eavesdropping prevention capability for the communication system.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology and relates to a channel estimation method for an intelligent reflective surface system based on a moving anchor point. It is a method to reduce the training overhead of communication systems and improve the system's security. Background Technology

[0002] Unlike traditional active repeaters, intelligent reflectors (RIS) composed of passive components have lower hardware costs. By controlling the reflection coefficient of each RIS element, the reflected signal can be attenuated or added to the signal in other paths to enhance or suppress the signal at a designated receiving point, thereby enhancing physical layer security, optimizing the transmission environment of the wireless communication system, and improving the spectrum and energy efficiency of the wireless communication system. Therefore, RIS is considered an important technology in sixth-generation (6G) wireless networks.

[0003] To achieve the aforementioned advantages, the RIS (Reflection System) needs to obtain the Channel State Information (CSI) of the communication system through channel estimation, thereby modifying the reflection coefficients on each RIS element and changing the phase or amplitude of the incident signal. Undoubtedly, channel estimation is a crucial part of a RIS-assisted communication system and typically involves multiple wireless channels: the direct channel between the base station (BS) and each user, the channel between the RIS and the BS, and the channel between the RIS and each user. However, channel estimation in a RIS-assisted communication system is also extremely challenging because wireless channels are not as fixed and predictable as wired channels; they possess a high degree of randomness, making it difficult to obtain accurate CSI within the wireless channel. Furthermore, the more passive reflective elements and users the RIS is equipped with, the more complex the channel estimation becomes, and the more pilot signals are consumed.

[0004] To address the issue of excessive pilot overhead in channel estimation, current methods include leveraging the shared BS-RIS channel among all users in a communication system to design new channel estimation schemes, thereby reducing training overhead in multi-user systems. However, this approach heavily relies on the estimation of the common BS-RIS channel, which in turn introduces security vulnerabilities. If the CSI of the common channel is obtained by unauthorized nodes, information leakage is highly likely. Furthermore, with increasing acceptance of RIS technology and the growing number of users supported by these systems, the transmitted signals may contain highly sensitive personal information. Neglecting security could have serious consequences for RIS-assisted communication systems. Therefore, reducing channel estimation overhead cannot be achieved at the expense of security. A method that simultaneously reduces channel estimation overhead and ensures security is needed. Designing a scheme that balances security and reduced training overhead is therefore a crucial task. Summary of the Invention

[0005] This invention aims to solve the problems in the prior art mentioned above and proposes a channel estimation method for intelligent reflector systems based on moving anchor points.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] Step 1: Deploy an anchor node A with UAV flight capability near the RIS, and select a random user (user k) within the BS service range to participate in channel estimation;

[0008] Step 2: Each node transmits and receives pilot signals in the order and process of BS-RIS-user, user-RIS-anchor node, anchor node-RIS-BS. Each node performs channel estimation based on the received signals and feeds back the estimated CSI to the BS.

[0009] Step 3: The BS uses the CSI fed back by each node to calculate the common CSI between the BS and the RIS;

[0010] Step 4: The remaining users in the system take turns sending signals to the BS, and use the previously obtained common CSI to complete the estimation of the cascaded channel BS-RIS-user.

[0011] Furthermore, the specific process of step one includes:

[0012] The system contains a total of K users. The number of antennas of the BS is M, and the number of reflector arrays of the RIS is N. The BS randomly selects a user k within its service range. User k can be replaced by other users after completing a full solution process. An anchor node A with flight capabilities is deployed near the RIS. The anchor node can be considered as a drone with signal transmission and reception capabilities and a certain amount of computing power. The channels from the base station to the RIS, anchor node A, and user k are respectively represented as H. bs ∈C M×N , The channels from RIS to anchor node A and user k are h, respectively. sa ∈C N×1 , The channel between anchor node A and the user is The cascaded channels BS-RIS-user k, user k-RIS-A, and A-RIS-BS can be represented as follows: H bsa =H bs diag(h sa In addition, Φ i =diag(v 1,i ,v 2,i ,....,v N,i ) represents the phase shift matrix of RIS in time slot i.

[0013] Furthermore, the specific process of step two includes:

[0014] In time slot i1, the BS sends N+1 pilot signals to a user k within its monitoring range. Signal received by user k The expression is:

[0015]

[0016] This is the CSCG (circularly symmetric complex Gaussian) noise at the receiving node. Let... Let p represent the phase transition matrix on RIS at time slot i1, where p is the transmitted power of the signal. but It can be represented as:

[0017]

[0018] make The total received signal, consisting of all N+1 pilot signals received by user k, is:

[0019]

[0020] In the above formula, It can be constructed using the Discrete Fourier Transform (DFT) method, and must satisfy the following conditions: That is, i1 ≥ N+1. Let i1 = N+1 to minimize the number of pilots used in the channel estimation stage, such that... This can be obtained using the least squares (LS) estimation method.

[0021]

[0022] The above formula constructs for:

[0023]

[0024] After receiving the pilot signal from the base station, user k will immediately send N+1 pilot signals to anchor node A in time slot i2, and simultaneously transmit the acquired channel information. The CSI is fed back to the BS to prevent changes in the channel information subsequently acquired due to the movement of user k. The signal expression received by anchor node A is:

[0025]

[0026] Same as above, order make The above formula can be expressed as:

[0027]

[0028] make I1 represents the time period containing all time slots i1. The total received signal at anchor node A is:

[0029]

[0030] Based on the above formula, the channel estimation task in this step can be completed using the LS estimation method:

[0031]

[0032] In the above formula The construction method is different from the previous one. same.

[0033] After receiving the pilot signal from user k, anchor node A will immediately send N+1 pilot signals to BS in time slot i3, and simultaneously transmit the acquired channel information. The CSI signal is fed back to the BS. The signal expression received by the BS is:

[0034]

[0035] in make The above formula can be expressed as:

[0036]

[0037] make I2 represents the time period that includes all slots i1 and i2. The total received signal of the BS is:

[0038]

[0039] Based on the above formula, the LS estimation method is used again to complete the channel estimation task in this step:

[0040]

[0041] Similarly, in the above formula The construction method is different from the previous one. Similarly, BS estimates and obtains the channel. CSI.

[0042] Furthermore, the specific process of step three includes:

[0043] The base station receives the channel feedback from user k and the anchor node. and After CSI, combined with its own channel estimation, the result is... It can be obtained through the LS estimation method That is, H bs With H bs The Hadamard product, the operation details are as follows:

[0044]

[0045] Based on the above formula and the definition of the Hadamard product, we can obtain... LS estimation.

[0046] Furthermore, step four specifically includes the following process:

[0047] In step three, the random user k participating in the estimation has obtained the channel state information it needs. The next channel estimation goal is to complete the channel estimation for the remaining K-1 users to the RIS. The remaining K-1 users only need to send a pilot signal to the BS individually in time slot i4, so that The identity matrix is ​​defined as follows: RIS does not change the phase of the transmitted signal in time slot i4. The transmitted signals of any of the remaining K-1 users are... The expression for the BS received signal can then be rewritten as:

[0048]

[0049] in, Based on what has been obtained It can be obtained using the LS estimation method.

[0050]

[0051] in, If the signal is received by the BS in time slot i4, then the channel estimate for each remaining user is:

[0052]

[0053] Based on the above formula, the cascaded channel information for any of the remaining K-1 users can be obtained. Furthermore, a total of K-1 pilot signals are required for i4.

[0054] The beneficial effects of this invention are as follows: This invention proposes a channel estimation technique based on a RIS-assisted communication system, including: 1) deploying an anchor node near the RIS and selecting a random user within the BS service range, obtaining the CSI of the common channel BS-RIS with the help of the anchor node and the user's assisted estimation; 2) utilizing the correlation between the cascaded channels of users in the system and the common channel to reduce the overhead of each user estimating the complete cascaded channel, thus alleviating the training overhead pressure on multi-user communication systems; 3) the selected random user can be replaced by other users after a complete scheme process, avoiding long-term exposure to the attack range of eavesdroppers, and the anchor node can also use its own flight capability to select the best workplace to avoid estimation from eavesdroppers, thus improving the security performance of the communication system while taking into account the advantage of reducing training overhead.

[0055] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0056] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0057] Figure 1 This describes the process of a channel estimation method for a smart reflector system based on moving anchor points.

[0058] Figure 2 The application scenario of this invention: a model diagram of a RIS-assisted multi-user communication system;

[0059] Figure 3 This is a diagram of the working modules of an intelligent reflective surface system based on moving anchor points. Detailed Implementation

[0060] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0061] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0062] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0063] Figure 1 This is a flowchart of the channel estimation method for an intelligent reflector system based on moving anchor points proposed in this invention.

[0064] like Figure 2 As shown, the application scenario of this invention is first introduced: Consider a communication system consisting of a base station, a RIS, K users, and an anchor node A deployed near the RIS. The RIS is deployed to assist data transmission between the base station and the users. In this model, the base station is a uniform linear array (ULA) with M antennas configured, and the RIS is a uniform square array (UPA) with N reflector elements configured.

[0065] Figure 3 The following is a working block diagram of the intelligent reflective surface system based on moving anchor points proposed in this invention. The system includes the following operational modules, described below with reference to the accompanying drawings:

[0066] Module 1: A mobile anchor node is located near the RIS. In this scenario, the anchor node can be viewed as a drone with signal transmission and reception capabilities and certain computing power, flying at a fixed altitude. Simultaneously, a random user is selected within the BS's service area to participate in the common channel estimation. This user can be replaced by another user after the complete channel estimation process.

[0067] Module 2: When participating in channel estimation, the anchor node can freely choose the optimal workplace to avoid attacks from eavesdroppers. At the same time, it participates in the channel estimation process with the BS and random user k. The pilot signal is transmitted and received in the order of BS-RIS-user, user-RIS-anchor node, anchor node-RIS-BS. Then, each node performs channel estimation according to the received signal and feeds back the estimated CSI to the BS.

[0068] Module 3: The BS uses the CSI fed back from each node to calculate the common CSI between the BS and the RIS;

[0069] Module 4: The remaining users in the system take turns sending signals to the BS. The corresponding transmission scheme is selected according to the number of BS antennas and the number of RIS reflector arrays. The previously obtained common CSI is used to complete the estimation of the cascaded channel BS-RIS-user.

[0070] Optionally, module one specifically includes:

[0071] Deployment module. Assume all communication nodes are placed in a 3D Cartesian coordinate system, with the horizontal coordinates of the ground user and the eavesdropper being W... G =[x G ,y G ] T With W E =[x E ,y E ] T The anchor node is at a constant height Z A The flight path, its horizontal trajectory in time slot i, is: q A,i =[x A,i ,y A,i ] T The eavesdropper's coordinates are fixed on the ground.

[0072] Optionally, Module Two specifically includes:

[0073] Evasion module. The path loss between nodes is: At this time, the cascaded channel anchor node-RIS-ground user and anchor node-RIS-eavesdropper can be represented as:

[0074]

[0075]

[0076] in:

[0077]

[0078]

[0079] ρ is the path loss at a reference distance of 1m, α is the path loss exponent for the two links: anchor node-RIS-ground user and anchor node-RIS-eavesdropper. For the direct connection channel, anchor node-ground user and anchor node-eavesdropper, the corresponding distance path loss model is as follows:

[0080]

[0081]

[0082] In the above formula, t represents the path loss parameters between the anchor node and the ground user, and between the anchor node and the eavesdropper in the direct channel connection. Using the above channel model, the received signal-to-noise ratio (SNR) of the ground user and the eavesdropper in time slot i can be expressed as follows:

[0083]

[0084]

[0085] In the above formula, With σ 2 Here, are the phase shift matrix and noise variance at time slot i, respectively, and p is the signal power transmitted by the anchor node. Therefore, the information rate achievable by the ground user and the eavesdropper in time slot i is:

[0086] R AG,i =log2(1+γ) AG,i (49)

[0087] R AE,i =log2(1+γ) AE,i (50)

[0088] Based on the above formula, the system's security rate during the operational phase of the movable anchor node can be obtained as follows:

[0089]

[0090] Anchor nodes select the most secure work location based on the real-time confidentiality rate within their work area to avoid attacks from eavesdroppers.

[0091] Optionally, Module 3 specifically includes:

[0092] Estimation Module. The anchor node participates in the common channel estimation process among all nodes from its optimal location. Together with the BS and a randomly selected user k, it sequentially completes the channel estimation for the three channels: BS-RIS-user, user-RIS-anchor node, and anchor node-RIS-BS, obtaining the channel estimates. as well as Using the channel estimation results above, CSI and BS obtain:

[0093]

[0094] and obtain LS estimation.

[0095] Optionally, Module 4 specifically includes:

[0096] Comparison module: When the number of BS antennas M ≥ the number of RIS reflector arrays N: each user only needs to send a pilot signal to the BS individually in time slot i4. Let For the identity matrix, send signals The expression for the BS received signal in the above formula can then be rewritten as:

[0097]

[0098] in, Based on what has been obtained It can be obtained using the LS estimation method.

[0099]

[0100] in, If the signal is received by the BS in time slot i4, then the channel estimate for each remaining user is:

[0101]

[0102] When the number of BS antennas M < the number of RIS reflector arrays N: at this time The number of unknowns N that need to be estimated exceeds the number of antennas M. If the method for M≥N is used, in order to completely estimate... Each of the remaining users needs to take turns sending to the BS. Each pilot signal requires a total of [amount] [units]. A pilot signal. In the... When rounding up, each user incurs some overhead because each user sends a signal to the BS independently, and the values ​​of N and M are unknown. To reduce the training overhead in this step, the remaining users can be grouped, ensuring that the number of pilot signals consumed by each group is exactly an integer, avoiding unnecessary overhead and reducing the total pilot signal consumption to a minimum.

[0103] Divide the remaining K-1 users into L groups. Each of the first L-1 groups has M users, and the Lth group has M1 users (M1 ≤ M). K-1 = (L-1)M + M1. Taking the first group as an example, let... Representing users in the first group in time slot i 4,1 Pilot signals sent to BS, k = 1,...,M, i 4,1=i3+1,...,i3+N represents the time slots for transmitting the first group of pilot signals. The pilot signals transmitted by the user can be represented in vector form as follows: Therefore, we can obtain BS in i 4,1 The received signal vector is:

[0104]

[0105] This represents the reflection coefficient matrix of the RIS. The pilot signal matrix transmitted by the first group of users is designed appropriately. and RIS in i 4,1 reflection parameter matrix If the rank of B1 is MN, i.e., rank(B1) = MN, then h1 can be obtained using the LS estimation method:

[0106]

[0107] We can set the first group of M users in i 4,1 The transmitted pilot signal matrix is:

[0108]

[0109] in Suppose that the N reflective elements of the RIS are in i 4,1 The reflection parameters at that time are:

[0110]

[0111] Then we can obtain the estimation results of the cascaded channel for the first group of users:

[0112]

[0113] The signal transmission and reception procedures and channel estimation methods for the other groups are similar to those for the first group, and will not be repeated here.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A channel estimation method for a smart reflector system based on a mobile anchor point, applied to an uplink communication system including a base station (BS), a smart reflector (RIS), multiple users, and a mobile anchor point (A), wherein the base station (BS) is configured with multiple antennas, characterized in that... Includes the following steps: Step 1: Deploy anchor node A with mobility, signal transmission and reception capabilities and computing capabilities near the RIS, and randomly select user k from the service range of the BS as the current user participating in channel estimation. After user k completes a complete channel estimation process, it is replaced by other users. Step 2: Control the BS, user k, and anchor node A to send and receive three pilot signals in the following order: BS to user k via RIS, user k to anchor node A via RIS, and anchor node A to BS via RIS. Each receiving node performs channel estimation on the corresponding BS-RIS-user k concatenated channel, user k-RIS-A concatenated channel, and A-RIS-BS concatenated channel based on the received pilot signals to obtain the corresponding CSI. The CSI obtained by user k and anchor node A is then fed back to the BS. During the transmission and reception of the pilot signals, anchor node A moves and adjusts its working position to avoid attacks from eavesdroppers. Step 3: The BS performs joint processing on the CSIs of the BS-RIS-user k concatenated channel, the user k-RIS-A concatenated channel, and the A-RIS-BS concatenated channel. The CSI of the BS-RIS common channel is obtained by performing the Hadamard product operation between the CSIs of the three concatenated channels and by using least squares estimation. Step 4: Each remaining user except user k sends a pilot signal to the BS in sequence. The BS uses the CSI of the BS-RIS common channel to perform channel estimation on the pilot signals sent by each remaining user, and obtains the CSI of the BS-RIS-user concatenated channel corresponding to each remaining user. Together with the CSI of the BS-RIS-user k concatenated channel corresponding to user k, they form the concatenated channel CSI of all users in the system.

2. The channel estimation method for a smart reflector system based on a moving anchor point according to claim 1, characterized in that: Step 1: Anchor node A with mobility is a drone with signal transmission and reception capabilities and computing power. The randomly selected user k participating in the channel estimation will be replaced by another user after completing a complete channel estimation process.

3. The channel estimation method for a smart reflector system based on a moving anchor point according to claim 1, characterized in that: The pilot signal sequence between nodes in step two is as follows: the BS first sends a pilot signal, which is reflected by the RIS to user k. Then user k also sends a pilot signal, which is reflected by the RIS to the anchor node. Finally, the anchor node sends a pilot signal, which is reflected by the RIS to the BS. The transmission and reception of the three pilot signals must be strictly performed in this order. Afterward, each node performs channel estimation based on the received pilot signal and feeds back the obtained CSI to the BS. During this process, the anchor node uses the drone's flight capability to select a work site to avoid attacks from eavesdroppers and improve the system's security.

4. The channel estimation method for a smart reflector system based on a moving anchor point according to claim 1, characterized in that: Step 3, the CSI of the BS-RIS common channel, i.e., the CSI of the BS using feedback, is calculated according to the formula. , , These represent the cascaded channels BS-RIS-user k, user k-RIS-A, and A-RIS-BS, respectively, followed by... Extract the BS-RIS common channel required by the scheme. CSI.

5. The channel estimation method for a smart reflector system based on a moving anchor point according to claim 1, characterized in that: Step four describes the cascaded channel BS-RIS-user, where the remaining users send signals to the BS. The BS then uses the received signals to calculate the RIS and the user's information. Channel vectors between ,in Each element represents a reflection unit of the RIS and a user. The channel coefficients between them; finally, according to the formula Obtain the cascaded channels of all users within the system. The CSI is used to complete the cascaded channel estimation.

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