RIS-Assisted Low-Complexity Terahertz Beamforming Communication System and Method

Abstract

The present invention provides a RIS-assisted low-complexity terahertz beamforming communication system and method, including a base station, a RIS system, and a user equipment; the RIS system aggregates terahertz near-field beam splitting to achieve beam focusing in a preset user direction. The present invention uses the RIS system to assist the information transmission of the terahertz system, uses a double-layer delay network to implement RIS beamforming, alleviates the near-field beam splitting phenomenon, increases the beam array gain in the user direction, and approaches the optimal performance.

Description

Technical Field

[0001] The present invention relates to the technical field of wireless communication, and in particular, to a RIS-assisted low-complexity terahertz beamforming communication system and method. Background Art

[0002] Terahertz (THz) communication has become a promising candidate technology for future sixth-generation (6G) wireless communication due to its ultra-wide bandwidth, and theoretically can achieve data rates up to several trillion bits per second (Tb / s). However, THz signals face significant challenges, including severe transmission attenuation and poor scattering, which limit the transmission range. Fortunately, reconfigurable intelligent surface (RIS) provides a promising solution to address these challenges. By using a large number of passive reflecting elements, RIS can direct the signal to any desired direction, thereby enhancing the signal strength and making it a practical solution for future THz communication.

[0003] Despite a large amount of research work having been carried out, most of the existing RIS beamforming schemes perform poorly when the signal bandwidth is large, especially in practical THz communication systems. This is mainly because practical RIS is usually equipped with frequency-independent phase-shift circuits, resulting in frequency-independent beamforming for RIS-assisted communication. Therefore, this leads to the beam splitting problem in RIS-assisted THz systems. To address this issue, a tunable-delay metasurface has been proposed, where each RIS element is connected to a time-delay module. Due to the applied time delay, frequency-dependent RIS beamforming can be achieved, thus solving the beam splitting effect. However, since the number of RIS elements is relatively large in the THz scenario, equipping each element with a time-delay module becomes impractical, resulting in excessive hardware cost and power consumption.

[0004] In the existing research work, there are also time-delay-based RIS schemes designed by block connection methods. These works only solve the far-field beam splitting effect problem. Although some research has proposed time-delay-based solutions to overcome the near-field beam splitting effect of RIS, the number of required time-delay modules is still large, and the practical limitations of THz-band time-delay technology are ignored. In fact, the research on implementing time delay in the THz band is still in its infancy, and the achievable time-delay range is limited. Specifically, the time-delay module under discussion can only achieve a maximum time-delay range of 14.272 picoseconds (ps). Given this limitation, it is crucial to consider the practical THz time-delay capability when designing time-delay-based RIS. Summary of the Invention

[0005] Aiming at the deficiencies in the prior art, the purpose of the present invention is to provide a RIS-assisted low-complexity terahertz beamforming communication system and method.

[0006] The RIS-assisted low-complexity terahertz beamforming communication system provided by the present invention includes a base station, a RIS system, and a user equipment;

[0007] The base station adopts a terahertz single-antenna system;

[0008] The base station and the user equipment are located within the preset near-field range of the RIS system; the phasedifference is decomposed into two independent components, i.e., the phase difference between sub-panels and the phase difference within sub-panels, by using the block far-field model;

[0009] The RIS system eliminates the phase difference through a double-layer time-delay network, performs beam gain in a preset user direction, obtains the desired aggregated user-direction beamforming, realizes near-field beam focusing, and alleviates the near-field beam splitting effect.

[0010] Preferably, it further includes a RIS panel and an FPGA intelligent controller;

[0011] The RIS panel includes a plurality of reflection units;

[0012] The RIS panel includes a plurality of sub-panels, and each sub-panel is connected to a time-delay module; each sub-panel is located within the preset far-field range of the RIS system;

[0013] The RIS panel performs passive beamforming through the FPGA intelligent controller to control the phase of each reflection unit.

[0014] Preferably, the double-layer time-delay network eliminates the phase difference between sub-panels through the time-delay component and eliminates the phase difference within sub-panels through the reflection phase;

[0015] The double-layer time-delay network realizes the required delay amount of the system through the series connection of a plurality of time-delay modules; the first-layer time-delay network processes the phase difference of the RIS along one axis, and the second-layer time-delay network processes the phase difference of the RIS along another axis, thereby reducing the phase inconsistency between sub-panels.

[0016] Preferably, the RIS system adopts a multi-carrier information transmission scheme, and the center frequency is denoted as f c ; the coordinates of the base station are denoted as (x b , y b , z b ); the RIS panel is a uniform planar array composed of N = N y N z reflection units, its center is located at the origin, there are N y units along the y-axis, and there are N z units along the z-axis; the user equipment is a single-antenna system, and its coordinates are denoted as (x u , y u , zu );

[0017] Denote the near - field channel between the base station and the RIS on the \(m\) - th sub - carrier, and its elements are expressed as:

[0018]

[0019] where, denotes the free - space path loss between the base station and the RIS, \(e\) represents the natural constant, \(c\) represents the speed of light; \(j\) represents the imaginary unit, denotes a vector of dimension \(N\times1\), \(f\) m represents the \(m\) - th sub - carrier frequency; denotes the distance between the base station and the \((n\) y , \(n\) z ) - th RIS element, expressed as:

[0020]

[0021] where, \(d\) represents the RIS element spacing;

[0022] Denote the near - field channel between the RIS and the user on the \(m\) - th sub - carrier, and its elements are expressed as:

[0023]

[0024] where, denotes the free - space path loss between the RIS and the user, denotes the distance between the \((n\) y , \(n\) z ) - th RIS element and the user, expressed as:

[0025]

[0026] The RIS reflection coefficient matrix is expressed as:

[0027]

[0028] where, \(\theta\) n \(\in[0,2\pi)\) is the reflection phase of the \(n\) - th RIS element.

[0029] Preferably, the entire RIS panel is divided into \(K\) y \(K\) z sub - panels, each sub - panel contains \(S\times S\) reflection elements, satisfying \(S = N\) y / \(K\) y = \(N\) z / \(K\) z ; The distance from the base station to the \((k\) y , \(k\)z ) distance from the center of the panel is expressed as:

[0030]

[0031] wherein, represents the distance from the base station to the center of the RIS panel, represents the elevation angle from the base station to the center of the RIS panel, represents the azimuth angle from the base station to the center of the RIS panel; then the distance from the base station to the (k y , k z )th panel and the (s y , s z )th unit is expressed as:

[0032]

[0033] wherein,

[0034]

[0035] the near - field channel between the base station and the (k y , k z )th sub - panel and the (s y , s z )th unit is:

[0036]

[0037] the near - field channel between the user and the (k y , k z )th sub - panel and the (s y , s z )th unit is:

[0038]

[0039] Preferably, the block far - field cascaded channel model is expressed as:

[0040]

[0041] wherein, its phase difference is decomposed into two components: the phase difference between sub - panels and the phase difference within the sub - panel wherein:

[0042]

[0043] To eliminate the phase difference between sub - panels, the (k y , k z) The required delay for each panel is:

[0044]

[0045] Preferably, for the th delay module in the first-layer delay network, the delay where the superscript f represents the first layer, to eliminate the phase difference between the (k y , 1)th and the (k y + 1, 1)th, which is expressed as:

[0046]

[0047] For the y th group in the second-layer delay network, the delay of the th delay module where the superscript s represents the second layer, to eliminate the phase difference between the th and the th, which is expressed as:

[0048]

[0049] Preferably, to eliminate the phase difference within the sub-panel, the phase of the y , k z )th RIS reflection unit on the (s y , s z )th sub-panel is expressed as:

[0050]

[0051] According to the RIS-assisted low-complexity terahertz beamforming communication method provided by the present invention, RIS reflection beamforming with incremental delay is performed through the parameters of a two-layer delay network to achieve beam gain in a preset user direction, alleviating the terahertz near-field beam splitting effect. Specifically, it includes the following steps:

[0052] Block far-field model approximation step: By dividing the entire RIS panel into multiple sub-panels, approximating the near-field models of the base station - RIS and RIS - user equipment as far-field models;

[0053] Two-layer delay network design step: Calculate the phase difference between sub-panels and set the parameters of the two-layer delay network;

[0054] Passive beamforming step: The RIS panel performs passive beamforming through an FPGA intelligent controller to control the phase of each reflection unit.

[0055] Preferably, in the step of approximating the far-field model of the divided blocks, the entire RIS panel is divided into multiple sub-panels to obtain the expression of the far-field channel approximation model;

[0056] In the step of designing the delay components, a first-layer delay network is designed to eliminate the phase difference along one axis between the sub-panels, and a second-layer delay network is designed to eliminate the phase difference along the other axis between the sub-panels;

[0057] In the step of passive beamforming, the RIS system generates a passive beamforming control signal, and by establishing the phases of all elements in the RIS coefficient matrix Θ, passive beamforming is performed on the incident signal of the RIS panel to eliminate the phase difference inside the sub-panels.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] (1) The present invention uses RIS-assisted low-complexity terahertz beamforming communication system for information transmission, realizes low-power passive beamforming, alleviates the near-field beam dispersion effect, and increases the beam array gain in the user direction;

[0060] (2) The present invention uses the method of incremental delay to avoid the delay range beyond the hardware capabilities and make it practical;

[0061] (3) The proposed architecture of the present invention has low hardware deployment cost and low additional power consumption, and is a new type of green terahertz system. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, objects and advantages of the present invention will become more apparent:

[0063] Figure 1 is the architecture diagram of the RIS-assisted low-complexity terahertz beamforming communication system;

[0064] Figure 2 is the terahertz user array gain diagram of the traditional non-delay RIS-assisted with N = 100×100;

[0065] Figure 3 is the terahertz user array gain diagram of the double-layer incremental delay RIS-assisted with N = 100×100;

[0066] Figure 4 is the comparison diagram of the terahertz user array gain with or without near-field beam dispersion phenomenon of the double-layer incremental delay RIS-assisted with N = 100×100. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0067] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made. These all belong to the protection scope of the present invention.

[0068] Embodiment 1

[0069] The present invention provides a RIS-assisted low-complexity terahertz beamforming communication system, as Figure 1 shown, which includes a base station, a RIS panel, an FPGA intelligent controller, a two-layer time-delay network, and a user equipment.

[0070] The RIS-assisted low-complexity terahertz beamforming communication system eliminates the phase difference through a two-layer time-delay network, performs beam gain in the preset user direction, obtains the desired aggregated user-direction beamforming, realizes near-field beam focusing, and alleviates the near-field beam splitting effect.

[0071] In the RIS system, the RIS performs passive beamforming through the FPGA intelligent controller to control the phase of each reflection unit.

[0072] RIS beamforming scheme: The base station and the user equipment are located within the near-field range of the RIS, while each sub-panel is located within the far-field range; the block far-field model is used to decompose the phase difference into two independent components, namely the phase difference between sub-panels and the phase difference within sub-panels; the phase difference between sub-panels is eliminated through the time-delay component, and the phase difference within sub-panels is eliminated through the reflection phase; the first-layer time-delay network processes the phase difference of the RIS along one axis, while the second-layer time-delay network processes the phase difference of the RIS along the other axis to reduce the phase inconsistency between sub-panels.

[0073] To meet the requirements of the next-generation wireless network for beam gain performance and cost reduction, the present invention provides a design of a novel two-layer incremental delay RIS beamforming system.

[0074] Considering the multi-carrier information transmission scheme, the center frequency is denoted as f c ; the base station is a terahertz single-antenna system, and its coordinates are denoted as (x b , y b , z b ); the RIS panel is a uniform planar array composed of N = N y N z reflection units, its center is located at the origin, there are N y units along the y-axis, and N z units along the z-axis; the user is a single-antenna system, and its coordinates are denoted as (x u , yu , z u ).

[0075] Denote the near - field channel between the base station and the RIS on the \(m\) - th sub - carrier, and its elements are expressed as:

[0076]

[0077] where, denotes the free - space path loss between the base station and the RIS, \(e\) denotes the natural constant, \(c\) denotes the speed of light; \(j\) denotes the imaginary unit, denotes a vector with dimension \(N\times1\), \(f\) m denotes the \(m\) - th sub - carrier frequency; denotes the distance between the base station and the \((n\) y , \(n\) z ) - th RIS element, and is expressed as:

[0078]

[0079] where, \(d\) denotes the RIS element spacing.

[0080] Denote the near - field channel between the RIS and the user on the \(m\) - th sub - carrier, and its elements are expressed as:

[0081]

[0082] where, denotes the free - space path loss between the RIS and the user, denotes the distance between the \((n\) y , \(n\) z ) - th RIS element and the user, and is expressed as:

[0083]

[0084] The RIS reflection coefficient matrix is expressed as:

[0085]

[0086] where, \(\theta\) n \(\in[0,2\pi)\) is the reflection phase of the \(n\) - th RIS element.

[0087] Figure 1 Describes the basic structural composition of the invention, Figure 2 and Figure 3 Describes the array gain of the user in the terahertz system with and without the invention, Figure 4 Verifies that the hardware cost required for the invention is low.

[0088] As a revolutionary technology, RIS (Reflecting Intelligence Surface) is expected to address the challenges of performance improvement and deployment cost in the development of terahertz communication systems. RIS can achieve channel reconstruction, providing sufficient multipath components for precise beamforming. In addition, RIS can also solve the blind coverage problem caused by the lack of line-of-sight channels. The passive characteristics of RIS greatly reduce the deployment cost and can be well integrated with existing terahertz communication systems. Compared with traditional RIS beamforming technology, the double-layer time-delay network in the present invention can effectively solve the near-field beam dispersion phenomenon in terahertz broadband systems, reduce the hardware cost and power consumption, and has important implementation value in next-generation wireless networks.

[0089] Embodiment 2

[0090] The present invention presents a block far-field approximation model, decomposing the phase difference into two independent components, namely the phase difference between sub-panels and the phase difference within sub-panels; and then eliminating the phase difference between sub-panels through the time-delay component, and eliminating the phase difference within sub-panels through the reflection phase.

[0091] (1) Design the block far-field approximation model and

[0092] The entire RIS panel is divided into K y K z sub-panels, each sub-panel containing S×S reflection units, satisfying: S = N y / K y = N z / K z ; the distance from the base station to the center of the (k y , k z )-th panel is expressed as:

[0093]

[0094] where represents the distance from the base station to the center of the RIS panel, represents the elevation angle from the base station to the center of the RIS panel, represents the azimuth angle from the base station to the center of the RIS panel; then the distance from the base station to the (s y , s z )-th unit on the (k y , s z )-th panel is expressed as:

[0095]

[0096] Among them,

[0097]

[0098] Therefore, the near-field channel between the (k y , k z )-th sub-panel of the base station and the RIS and the (s y , s z )-th unit can be approximated as:

[0099]

[0100] Similarly, the near-field channel between the (k y , k z )-th sub-panel of the user and the RIS and the (s y , s z )-th unit can be approximated as:

[0101]

[0102] Embodiment 3

[0103] The present invention provides a two-layer time-delay network design scheme. By the time-delay component, the phase difference between sub-panels is eliminated. The first-layer time-delay network processes the phase difference of the RIS along one axis, while the second-layer time-delay network processes the phase difference of the RIS along the other axis.

[0104] (2) Design the two-layer time-delay network parameter τ

[0105] Express the block far-field cascaded channel model as:

[0106]

[0107] Among them, its phase difference can be decomposed into two components: the phase difference between sub-panels and the phase difference within the sub-panel where:

[0108]

[0109] To eliminate the phase difference between sub-panels, the required delay of the (k y , k z )-th panel is:

[0110]

[0111] The actual delay is realized by connecting multiple time-delay modules in series to reduce the required time-delay range. Design the delay of the -th time-delay module in the first-layer time-delay network. Among them, the superscript f represents the first layer to eliminate the (ky , the phase difference between the k y +1, 1)th ones, expressed as:

[0112]

[0113] Design the delay of the k y th group in the second-layer delay network of the delay module where the superscript s represents the second layer to eliminate the phase difference between the th and the

[0114]

[0115] Example 4

[0116] The present invention provides a RIS passive beam design to eliminate the phase difference inside the sub-panel.

[0117] (3) Design the RIS passive beam θ

[0118] The k y , k z phase of the (s y , s z )th RIS reflection unit on the k sub-panel is expressed as:

[0119]

[0120] Example 5

[0121] The present invention also provides a RIS-assisted low-complexity terahertz beamforming communication method, which eliminates the phase difference between sub-panels through the delay component and eliminates the phase difference inside the sub-panel through the reflection phase; wherein the first-layer delay network processes the phase difference of the RIS along one axis, and the second-layer delay network processes the phase difference of the RIS along the other axis to reduce the phase inconsistency between sub-panels.

[0122] The method includes the following steps:

[0123] Block far-field model approximation step: By dividing the entire RIS panel into multiple sub-panels, approximate the near-field models of the base station-RIS and RIS-user equipment as far-field models;

[0124] Double-layer delay network design step: Calculate the phase difference between sub-panels and design the parameters of the double-layer delay network;

[0125] Passive beamforming step: The RIS panel performs passive beamforming through the FPGA intelligent controller to control the phase of each reflection unit.

[0126] In the step of approximating the block far-field model, the entire RIS panel is divided into multiple sub-panels to obtain the expression of the far-field channel approximation model;

[0127] In the step of designing the double-layer time-delay network, the first-layer time-delay network is designed to eliminate the phase difference along one axis between sub-panels, and the second-layer time-delay network is designed to eliminate the phase difference along the other axis between sub-panels;

[0128] In the step of passive beamforming, the RIS system generates a passive beamforming control signal, and by establishing the phases of all elements in the RIS coefficient matrix Θ, passive beamforming is performed on the incident signal of the RIS panel to eliminate the phase difference inside the sub-panels.

[0129] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present application.

[0130] Those skilled in the art know that in addition to implementing the systems, devices and their respective modules provided by the present invention in the form of pure computer-readable program codes, the method steps can be logically programmed to enable the systems, devices and their respective modules provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, embedded microcontrollers, etc. to implement the same program. Therefore, the systems, devices and their respective modules provided by the present invention can be regarded as a kind of hardware component, and the modules included therein for implementing various programs can also be regarded as the structure within the hardware component; the modules for implementing various functions can also be regarded as either software programs for implementing the methods or the structure within the hardware component.

[0131] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Without conflict, the embodiments of the present application and the features in the embodiments can be combined arbitrarily with each other.

Claims

1. A RIS-assisted low-complexity terahertz beamforming communication system, characterized in that: Including base stations, RIS systems and user equipment; The base station adopts a terahertz single antenna system; The base station and the user equipment are located within a preset near field range of the RIS system; a block far field model is used to decompose the phase difference into two independent components, namely, the phase difference between sub-panels and the phase difference within the sub-panel; The RIS system eliminates phase differences through a double-layer time delay network, performs beam gain in a preset user direction, obtains desired aggregated user direction beamforming, realizes near-field beam focusing, and alleviates near-field beam splitting effects; The double-layer time delay network eliminates the phase difference between sub-panels through the time delay component and eliminates the phase difference within the sub-panel through the reflection phase; The double-layer delay network realizes the required delay of the system by connecting multiple delay modules in series; the first layer of delay network processes the phase difference of RIS along one axis, and the second layer of delay network processes the phase difference of RIS along another axis, thereby reducing the phase inconsistency between sub-panels.

2. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 1, characterized in that: Also includes RIS panel and FPGA smart controller; The RIS panel includes a plurality of reflection units; The RIS panel includes a plurality of sub-panels, each of which contains a plurality of reflection units, and each of which is connected to a delay module; each of which is located within a preset far-field range of the RIS system; The RIS panel performs passive beamforming through an FPGA intelligent controller to control the phase of each reflective unit.

3. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 2, characterized in that: The RIS system adopts a multi-carrier information transmission scheme, with the center frequency denoted as f c ; The base station coordinates are marked as (x b ,y b ,z b ); the RIS panel is composed of N=N y N z A uniform planar array consisting of reflection units, with its center at the origin and N reflection units along the y-axis. y units, along the z-axis there are N z The user equipment is a single antenna system, and its coordinates are marked as (x u ,y u ,z u ); represents the near-field channel between the base station and the RIS on the mth subcarrier, and its elements are expressed as: Among them, e represents a natural constant, c represents the speed of light, and j represents an imaginary unit. represents a vector of dimension N×1, f m represents the mth subcarrier frequency; Indicates base station-(n y ,n z ) RIS units, expressed as: in, a={0,…,b-1}, d represents the RIS unit interval; represents the near-field channel between the RIS and the user on the mth subcarrier, and its elements are expressed as: in, Indicates the first (n y ,n z ) The distance between each RIS unit and the user is expressed as: The RIS reflection coefficient matrix is ​​expressed as: Among them, θ n ∈[0,2π) is the reflection phase of the nth RIS unit.

4. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 3, characterized in that: The entire RIS panel is divided into K y K z sub-panels, each sub-panel contains S×S reflection units, satisfying S=N y / K y =N z / K z ; Base station to (kth y ,k z ) The distance between the center of each panel It is expressed as: in, Indicates the distance from the base station to the center of the RIS panel. Indicates the elevation angle from the base station to the center of the RIS panel. represents the azimuth from the base station to the center of the RIS panel; then the base station to the (k y ,k z ) on the panel (s y ,s z ) units It is expressed as: in, Base station and RIS (k y ,k z ) on the sub-panel (s) y ,s z The near-field channel between the units is: The kth (kth) of the user and RIS y ,k z ) on the sub-panel (s) y ,s z The near-field channel between the units is:

5. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 4, characterized in that: Block far-field cascade channel model It is expressed as: Among them, its phase difference is decomposed into two components: the phase difference between the sub-panels Phase difference within the sub-panel in: To eliminate the phase difference between sub-panels, the (k y ,k z ) panels required delay is:

6. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 1, characterized in that: The first layer of the delay network The delay of each delay module The superscript f represents the first layer, to eliminate the (k y ,1) and (k) y +1,1) phase difference, expressed as: The kth y Group The delay of each delay module The superscript s indicates the second layer to eliminate the and The phase difference between them is expressed as:

7. The RIS-assisted low-complexity terahertz beamforming communication system according to claim 1, characterized in that: To eliminate the phase difference within the sub-panel, the (k y ,k z ) on the sub-panel (s) y ,s z ) phase of each RIS reflection unit It is expressed as:

8. A RIS-assisted low-complexity terahertz beamforming communication method, characterized in that: The RIS-assisted low-complexity terahertz beamforming communication system described in any one of claims 1 to 7 is applied to perform RIS reflection beamforming with incremental delay through double-layer delay network parameters, perform beam gain in a preset user direction, and alleviate the terahertz near-field beam splitting effect phenomenon, specifically comprising the following steps: Block far-field model approximation step: by dividing the entire RIS panel into multiple sub-panels, the near-field models of the base station-RIS and RIS-UE are approximated as far-field models; Double-layer time-delay network design steps: Calculate the phase difference between sub-panels and set the double-layer time-delay network parameters; Passive beamforming steps: The RIS panel performs passive beamforming through an FPGA intelligent controller to control the phase of each reflector unit.

9. The RIS-assisted low-complexity terahertz beamforming communication method according to claim 8, characterized in that: In the block far-field model approximation step, the entire RIS panel is divided into a plurality of sub-panels to obtain an expression of a far-field channel approximation model; In the delay component design step, a first layer of delay network is designed to eliminate the phase difference between the sub-panels along one axis, and a second layer of delay network is designed to eliminate the phase difference between the sub-panels along another axis; In the passive beamforming step, the RIS system generates a passive beamforming control signal, and passively beamforms the incident signal of the RIS panel by establishing the phases of all elements in the RIS coefficient matrix Θ, thereby eliminating the phase difference inside the sub-panel.

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