Method for constructing RIS-assisted FSO and THz hybrid relay system

Abstract

The invention relates to the technical field of communication, in particular to a method for constructing an RIS-assisted FSO and THz hybrid relay system. The method comprises the following steps: forming a relay communication system through a source node S, a relay node R, an RIS surface with N reflection units and a destination node D; according to the method for constructing the RIS-assisted hybrid FSO and THz relay system, THz communication is introduced, the problem that RF frequency band resources are scarce in a traditional hybrid FSO / RF communication system is solved, meanwhile, an intelligent metasurface (RIS) is adopted to assist an FSO link to solve the problem of FSO link line-of-sight transmission, the RIS-assisted FSO link obeys a D-GG fading distribution model, and therefore the RIS-assisted FSO and THz relay system can be obtained. A THz communication link obeys an alpha-mu fading distribution model, and a probability density function and an accumulative distribution function of an end-to-end instantaneous signal-to-noise ratio of a system are derived, so that an accurate expression of an outage probability and an average bit error rate of the system is obtained; therefore, parameters such as the number, the deployment positions and the pointing error coefficients of the RIS reflection units all have obvious influences on the system performance.

Description

Technical Field

[0001] The present invention relates to the field of communication technology, and in particular to a method for constructing a RIS-assisted hybrid FSO and THz relay system. Background Art

[0002] Terahertz (THz) communication has attracted much attention as one of the potential technologies for 6G wireless networks. It can not only provide ultra-high available bandwidth and high-speed wireless data transmission rate, but also alleviate the problem of insufficient radio frequency (RF) communication band resources. However, THz communication has serious path loss, resulting in a short transmission distance, which limits its further development. On the other hand, free space optical (FSO) communication has a longer transmission distance than THz communication, up to several kilometers, and has the characteristics of transmission security, anti-interference and low spatial transmission loss. By combining the advantages of FSO communication and THz communication, a hybrid FSO / THz relay system is established using relay technology [Zedini E, Sourly H, Alouini M-S. On the Performance Analysis of Dual-Hop Mixed FSO / RF Systems [J]. IEEE Transactions on Wireless Communications, 2016, 15 (5): 3679-3689] to provide data links with wider coverage, higher rate and higher robustness.

[0003] In fact, relevant research has been exploring the advantages of FSO / THz hybrid networks. Singya et al. [Singya PK, Makki B, D'errico A, et al. Hybrid FSO / THz-Based Backhaul Network for millimeter wave terrestrial communication [J]. IEEE Transactions on Wireless Communications, 2023, 22 (7): 4342-4359] proposed a millimeter wave terrestrial communication system based on an FSO / THz hybrid backhaul network. The backhaul link in this system is a parallel THz and FSO link with different frequencies. Soft switching is used to reduce frequent link switching in practical applications. Simulation results show that the hybrid FSO / THz parallel link can provide high-speed reliable backhaul communication.

[0004] In addition, Li et al. [Li S, Yang L, Zhang J, et al. Mixed THz / FSO Relaying Systems: Statistical Analysis and Performance Evaluation [J]. IEEE Transactions on Wireless Communications, 2022, 21 (12): 10996-11010] studied the performance of a dual-hop FSO / THz hybrid network based on the amplification and forwarding protocol (AF), in which one hop works in the FSO link and the other hop works in the THz link. The article derived precise expressions for system performance such as outage probability and average bit error rate. Finally, through simulation, it was observed that the diversity order of the hybrid FSO / THz system depends on the fading parameters and pointing errors of the two links. However, the strict line-of-sight link (LOS) requirements make the FSO link extremely susceptible to obstacles and communication interruptions. The intelligent metasurface (RIS) technology can solve this problem. RIS is a surface composed of multiple low-cost, passive reflective units that can be flexibly deployed in complex environments [Aboagye S, Ndjiongue AR, Ngatched TMN, et al. RIS-Assisted Visible Light Communication Systems: A Tutorial [J]. IEEE Communications Surveys & Tutorials, 2023, 25 (1): 251-288]. The reflective units on its surface can also control parameters such as the amplitude, phase, frequency, and polarization direction of the incident signal.

[0005] These characteristics of RIS enable it to be applied to many complex scenarios. Nguyen et al. [Nguyen TV, LeH D, Pham A T. On the Design of RIS-UAV Relay-Assisted Hybrid FSO / RF Satellite-Aerial-Ground Integrated Network[J]. IEEE Transactions on Aerospace and Electronic Systems, 2022, 59(2): 757-771] proposed a RIS-UAV relay-assisted hybrid FSO / RF satellite-aerial-ground integrated network system. The UAV carrying RIS acts as a temporary relay node to ensure the continuous operation of the communication network when the link is blocked. The article derives the link interruption probability, average transmission rate and spectrum efficiency of the system, and the results show that the performance of the system on turbulent fading channels is greatly improved.

[0006] In the ground communication network, Agheli et al. [Agheli P, Beyranvand H, Emadi M J. High-Speed ​​Trains Access Connectivity Through RIS-Assisted FSO Communications [J]. Journal of Lightwave Technology, 2022, 40(21): 7084-7094] used RIS on high-speed trains and proposed a RIS-assisted high-speed train FSO communication system. The results showed that the RIS-assisted FSO system is superior to the traditional relay network due to its programmable structure, which increases its data transmission rate by 63% and expands its coverage by 446%.

[0007] In the multi-hop communication system, Chapala et al. [Chapala VK, Zafaruddin S M. Multiple RIS-Assisted Mixed FSO-RF Transmission Over Generalized Fading Channels[J]. IEEE Systems Journal, 2023, 17(3): 3515-3526] proposed a multi-hop FSO-RF system assisted by multiple RIS units. The simulation results prove the effectiveness of multiple RIS units in achieving reliable connection of FSO LOS transmission and vehicle communication, and multiple RIS units can reduce the impact of the overall pointing error of the system.

[0008] In addition, Kumar et al. [Kumar LB, Naik RP, Krishnan P, et al. RIS Assisted Triple-Hop RF-FSO Convergent With UWOC System [J]. IEEE Access, 2022, 10: 66564-66575] also proposed a RIS-assisted RF-FSO-UWOC three-hop communication system, using RIS for both RF links and FSO links to achieve longer-distance non-line-of-sight (NLOS) communication. The results show that in the case of weak turbulence and weak pointing errors, increasing the number of RIS can significantly improve the system's bit error rate and interruption performance.

[0009] In the literature [Sikri A, Mathur A, Verma G, et al. Distributed RIS-based Dual-hop Mixed FSO-RF Systems With RIS-Aided Jammer[C] / / 2021IEEE 94th Vehicular Technology Conference (VTC2021-Fall), September 27-30, 2021, Norman, OK, USA: IEEE, 2021, 9625385], Sikri et al. considered a RIS-assisted distributed dual-hop FSO-RF system, which places multiple RIS surfaces at different locations in the RF link and considers the impact of RIS-assisted jammers on the system. The results show that the performance of the system will be seriously affected by the RIS-assisted jammer, while increasing the number of RIS in the RF link will significantly improve the system performance. In addition, direct detection (HD) is more suitable for this system than IM / DD schemes. In the literature [Bao Chaoyuan, Cao Yang, Peng Xiaofeng, Xing Wenjun. Performance analysis of RIS-assisted MUD-RF / FSO hybrid system under co-channel interference[J]. Acta Optica Sinica, 2023, 43(22): 2206003.

[0010] Bao CY,Cao Y,Peng XF,et al.Performance Analysis of RIS-AssistedMUD-RF / FSO Hybrid System Under Co-Channel Interference[J].Acta Optica Sinica,2023,43(22):2206003], Bao et al. proposed a RIS-assisted MUD-RF / FSO system. The system considers that the RF link with multiple CCI signals obeys the Rayleigh fading distribution model, and the RIS-assisted FSO link obeys the Gamma-Gamma fading distribution model. The simulation results show that the increase in the number of users and the number of RIS reflection units will improve the system performance to varying degrees.

[0011] The above studies have proved that applying RIS to different links and scenarios can effectively improve the end-to-end performance of the system. However, there has been no research on applying RIS to FSO / THz hybrid systems.

[0012] To this end, a method for constructing a RIS-assisted hybrid FSO and THz relay system is designed to provide another technical solution to the above technical problem. Summary of the invention

[0013] Based on this, it is necessary to provide a method for constructing a RIS-assisted hybrid FSO and THz relay system to solve the technical problems raised in the above background technology.

[0014] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0015] A method for constructing a RIS-assisted hybrid FSO and THz relay system, the steps are as follows:

[0016] The relay communication system is composed of a source node S, a relay node R, a RIS surface with N reflection units and a destination node D.

[0017] As a preferred implementation of the method for constructing a RIS-assisted hybrid FSO and THz relay system provided by the present invention, a source node S sends an optical signal to achieve LOS communication through a RIS surface having N reflection units;

[0018] The optical signal reflected from the RIS is converted into an electrical signal through the relay node R, and communicates with the destination node D through the THz link.

[0019] As a preferred implementation of the method for constructing a RIS-assisted hybrid FSO and THz relay system provided by the present invention, in the FSO link stage, the transmission signal at the source node S is reflected to the relay node R through the RIS surface having N reflection units, and the received signal can be expressed as:

[0020]

[0021] Where P s represents the average power of the transmitted signal, x k represents the transmission symbol of the kth path, I k =I a I p I l is the channel gain of the kth path, I a ,I p and I l are respectively atmospheric turbulence effect, pointing error and atmospheric path loss, and the atmospheric path loss is normalized to 1; n 1 The mean is 0 and the variance is N 1 Additive white Gaussian noise.

[0022] As a preferred implementation of the method for constructing a RIS-assisted hybrid FSO and THz relay system provided by the present invention, the distance between the source node S and the RIS surface is L 1 , the distance from the RIS surface to the relay node R is L 2 , L=L1 +L 2 ;

[0023] The overall instantaneous signal-to-noise ratio of the signal at R can be obtained by maximum ratio combining (MRC):

[0024]

[0025] In the formula, γ FSO,k Represents the instantaneous signal-to-noise ratio of the kth channel.

[0026] As a preferred implementation of the method for constructing a RIS-assisted hybrid FSO and THz relay system provided by the present invention, in the THz link stage, the relay node R transmits the signal to the destination node D through the THz link, and the received signal can be expressed as:

[0027]

[0028] Among them, P R is the average power of the received signal at R, h = h l h pf is the THz channel fading gain, h l is the path loss, and the expression is:

[0029]

[0030] Among them, G t , G r represents the transmission and receiving antenna gains, d represents the THz link propagation distance, C and f represent the speed of light and the operating frequency respectively, and k α (f) is the temperature-dependent absorption coefficient.

[0031] As a preferred implementation of the method for constructing a RIS-assisted hybrid FSO and THz relay system provided by the present invention, h pf is the channel coefficient of the THz channel, h pf The PDF of is:

[0032]

[0033] in, A and ξ represent the pointing error parameters of the THz link, A o1 For the constant term, ξ represents the ratio between the equivalent beam radius and the standard deviation of the pointing error displacement at the receiver, and α and μ refer to the attenuation parameters of the α-μ distribution, is the α-rms value of the fading channel envelope;

[0034] THz link instantaneous signal-to-noise ratio γ THz The PDF is:

[0035]

[0036] In the formula, γ THz The CDF of is derived as:

[0037]

[0038] It can be seen without a doubt that the above-mentioned technical solution of the present application can definitely solve the technical problem to be solved by the present application.

[0039] At the same time, through the above technical solutions, the present invention has at least the following beneficial effects:

[0040] The present invention provides a method for constructing a RIS-assisted hybrid FSO and THz relay system, introduces THz communication, solves the problem of scarce RF frequency band resources in traditional hybrid FSO / RF communication systems, and adopts an intelligent metasurface (RIS)-assisted FSO link to solve the problem of FSO link line-of-sight transmission. The RIS-assisted FSO link obeys a double generalized Gamma (D-GG) fading distribution, while the THz link obeys an α-μ fading distribution. Both links consider the influence of pointing errors, and a decode-and-forward (DF) protocol is adopted at a relay node R. Then, by deriving the RIS-assisted FSO link and the THz link, the RIS-assisted FSO link and the THz link are obtained. The instantaneous signal-to-noise ratio is calculated, and the probability density function (PDF) and cumulative distribution function (CDF) of the system are obtained. Then, the precise expressions of the end-to-end outage probability (OP) and average bit error rate (BER) of the system are derived. Through numerical simulation experiments, the influence of parameters such as the number of RIS reflection units, the distance of RIS-assisted FSO link and the pointing error coefficient on the system performance are analyzed. Therefore, increasing the number of reflection units on the RIS surface and reducing the pointing error coefficient will improve the performance of the system. In addition, deploying the RIS surface close to the receiving end can effectively reduce the pointing error of the FSO link, thereby improving all aspects of the system performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0042] Figure 1 Schematic diagram of the RIS-assisted hybrid FSO / THz relay system of the present invention;

[0043] Figure 2It is a schematic diagram of the interruption probability under different turbulence intensities and threshold values ​​of the present invention;

[0044] Figure 3 Schematic diagram of the interruption probability under the same RIS number and FSO link distance of the present invention;

[0045] Figure 4 Schematic diagram of the interruption probability under different pointing error coefficients and THz link distances of the present invention;

[0046] Figure 5 Schematic diagram of the interruption probability under different aperture radii and beam widths of the present invention;

[0047] Figure 6 It is a schematic diagram of average bit error rate under different modulation modes of the present invention;

[0048] Figure 7 It is a schematic diagram of average bit error rate under different pointing error angles and ICRN deflection error angles of the present invention;

[0049] Figure 8 It is a schematic diagram of the average bit error rate under the same aperture radius, beam width and FSO link distance of the present invention. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0051] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0052] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features and technical solutions in the embodiments may be combined with each other.

[0053] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0054] Reference Figure 1-Figure 8 , a method for constructing a RIS-assisted hybrid FSO and THz relay system.

[0055] like Figure 1As shown in the figure, a RIS-assisted hybrid FSO / THz relay communication system is proposed, which consists of a source node S, a relay node R, a RIS surface with N reflection units, and a destination node D. The source node S sends an optical signal. Assuming that there is no LOS link between the source node S and the relay node R, RIS is introduced to adjust the channel path to achieve LOS communication. The relay node R converts the optical signal reflected from the RIS into an electrical signal and communicates with the destination node D through a THz link. There are N transmitting and receiving apertures at both S and R, and the DF protocol is considered at the relay node R. In addition, it is assumed that each link in the system can obtain complete channel state information (CSI).

[0056] 1.1FSO Link Model

[0057] In the FSO link stage, the transmitted signal at the source node S is reflected to the relay node R through the RIS surface with N reflection units, and the received signal can be expressed as:

[0058]

[0059] Where P s represents the average power of the transmitted signal, x k represents the transmission symbol of the kth path, I k =I a I p I l is the channel gain of the kth path, I a ,I p and I l are respectively atmospheric turbulence effect, pointing error and atmospheric path loss, and the atmospheric path loss is normalized to 1; n 1 The mean is 0 and the variance is N 1 The rest of the meanings are the same as those in formula (1).

[0060] 1.1.1 Atmospheric turbulence

[0061] Atmospheric turbulence obeying D-GG distribution I a The corresponding PDF is

[0062]

[0063] In the formula, k 0 =Δ(σ:β 1 ),Δ(λ:β 2 ), Γ(·) represents the Gamma function, represents Meijer's G function. λ and σ are positive integers satisfying relationship, in addition And β 1 and β2 is the shaping parameter that defines the turbulence-induced fading, α 1 ,α 2 ,Ω 1 ,Ω 2 is the distribution parameter of the D-GG turbulence model.

[0064] 1.1.2 Pointing Error

[0065] The channel fading caused by pointing errors, which considers the jitter of transmitter, receiver and intelligent channel reconfigurable node (ICRN), can be expressed as Approximately, A 0 =[erf(v)] 2 , erf(·) represents the error function, represents the ratio of aperture radius to beam width, ω z represents the beam width, which can be expressed as ω z =φL, φ represents the divergence angle of the light beam. Therefore, I p The PDF can be expressed as:

[0066]

[0067] In the formula, is the RIS-assisted FSO link pointing error coefficient, σ θ is the pointing error angle, describing the beam jitter, σ β is the deflection error angle of ICRN. For the kth RIS-assisted FSO link, I k The PDF of can be given by:

[0068]

[0069] In the formula,

[0070] Using random variable transformation, The PDF can be expressed as:

[0071]

[0072] The mean and variance of B are:

[0073]

[0074] According to formula (6), γ FSO , the PDF of k is expressed as:

[0075]

[0076] When N is large enough, γ FSO Approximately The variance is A Gaussian random variable, so γ FSO The PDF of is:

[0077]

[0078] According to formula (10), we can get γ FSO The CDF of is:

[0079]

[0080] 1.2THz link model

[0081] In the THz link stage, the relay node R transmits the signal to the destination node D through the THz link, and the received signal can be expressed as:

[0082]

[0083] Among them, P R is the average power of the received signal at R, h = h l h pf is the THz channel fading gain, where h l is the path loss, and h l for:

[0084]

[0085] And h pf is the channel coefficient of the THz channel, considering the combined effects of the THz link pointing error and α-μ fading, h pf The PDF of is:

[0086]

[0087] in, A and ξ represent the pointing error parameters of the THz link, A o1 For the constant term, ξ represents the ratio between the equivalent beam radius and the standard deviation of the pointing error displacement at the receiver. α and μ refer to the attenuation parameters of the α-μ distribution, is the α root mean square value of the fading channel envelope. The instantaneous signal-to-noise ratio γ of the THz link is obtained TH The PDF of z is:

[0088]

[0089] In the formula, γ TH The CDF of z is derived as:

[0090]

[0091]

[0092] 2 System Performance Analysis

[0093] 2.1 Interruption Probability Analysis

[0094] The outage probability is defined as the instantaneous received SNR falling below a given SNR threshold γ th , since the decode-and-forward protocol is used at the relay node R, the end-to-end OP can be expressed as:

[0095] P out =P(min(γ FSO ,γ THz )<γ th )

[0096] =F γFSO (γ th )+F γTHz (γ th )-F γFSO (γ th )F γTHz (γ th ) (16)

[0097] Substituting the above formula into formula (10) and (14), we get the closed-form expression of OP:

[0098]

[0099] 2.2 Average Bit Error Rate Analysis

[0100] For different binary modulation methods, the system BER can be expressed as:

[0101]

[0102] Different parameters p and q represent different modulation schemes. When p = 1, q = 1, differential binary phase shift keying (DBPSK) is used, and when p = 0.5, q = 1, binary phase shift keying (BPSK) is used. The average BER of a dual-hop communication system with a DF protocol can be expressed as

[0103] P BER =P FSO +P THz -2P FSO P THz (19)

[0104] Where P FSO represents the average BER of the RIS-assisted FSO link, P THzrepresents the average BER of the THz link. The average BER of the FSO link can be calculated by equation (20):

[0105]

[0106] The Q function can be approximately expressed as Substituting this approximate value, the average BER of the FSO link is:

[0107]

[0108] P THz represents the average bit error rate of the THz link. By combining equation (18) with [21, Eq. (07.34.21.0012.01)], we can get the average BERP of the THz link: THz for:

[0109]

[0110] According to equations (21) and (22), the average BER of the system is derived as:

[0111]

[0112] 3 Simulation Analysis

[0113] Through numerical simulation, the influence of parameters such as the number of RIS reflection units N, the aperture radius of the transmitting beam, the beam width, the pointing error coefficient, and the RIS-assisted FSO link distance on the system performance are analyzed. Table 1 shows some parameters in the simulation process.

[0114] Table 1 Simulation parameters

[0115]

[0116]

[0117] Figure 2 for weak, moderate, and strong turbulence conditions and different interruption thresholds γ th The system outage probability under , considering that the FSO link and the THz link have the same signal-to-noise ratio, that is Number of RIS N = 64, L 1 =L 2 =150m, d=30m. The results show that when the signal-to-noise ratio is lower than 55dB, the system outage probability remains basically the same under the three turbulence conditions, but when it is higher than 55dB, as the turbulence conditions deteriorate, the system outage probability decreases significantly, and under strong turbulence, the system outage probability no longer decreases with the increase of the signal-to-noise ratio, and it is difficult to reach 10 -4In addition, under the same turbulence intensity, when the threshold increases from 1dB to 3dB, the system can obtain a gain of about 2dB under the same level of bit error rate.

[0118] Signal-to-noise ratio of a fixed THz link under moderate turbulence conditions Parameter settings: a = 0.2, ω z =1.2,γ th =1dB, the impact of RIS number and link distance L on system performance is as follows Figure 3 As shown in the figure. When the number of RIS N increases from 32 to 64, the system has a signal-to-noise ratio gain of about 2dB, which shows that increasing the number of RIS reflector units N can effectively improve the interruption performance of the system. The simulation parameters of the distance are set to L 1 =100m,L 2 =400m, L 1 =250m,L 2 =250m and L 1 =400m,L 2 = 100m three groups, from the simulation results we can see that when L 2 The smaller the value is, that is, the closer the RIS is placed to the receiving end, the smaller the probability of system interruption is.

[0119] Under moderate turbulence conditions, the system outage probability under different c, ξ values ​​and different THz link distances d is shown in Figure 4 As shown, the parameters are set to a = 0.2, ω z =1.2,L 1 =L 2 =150m. The smaller the values ​​of c and ξ, the larger the pointing error. The results show that the increase of pointing error and distance will increase the probability of system interruption. In addition, the change of distance d has a greater impact on the system than the pointing error. This is because THz signals will experience severe channel attenuation, resulting in rapid performance degradation, and are not suitable for long-distance transmission. Therefore, the combination with RIS-assisted FSO can effectively solve this problem.

[0120] Under moderate turbulence conditions, the distance parameter is set to L 1 =L 2 = 150m, considering the same signal-to-noise ratio for FSO and THz links, the number of RIS N = 64, the aperture radius a and the beam width ω z The impact on the system outage probability is as follows: Figure 5 From the simulation diagram, it can be seen that when the aperture radius a increases from 0.1 to 0.2, the interruption performance of the system is greatly improved. Under the interruption probability of 10-3, the system has a signal-to-noise ratio gain of about 12dB. When the aperture radius is kept constant, increasing the beam width ω zThe value of will reduce the performance of the system, and the impact of the aperture radius a on the system interruption performance is greater than the beam width ω z Impact on interrupt performance.

[0121] Under strong turbulence conditions, the signal-to-noise ratio of the FSO link and the THz link is considered to be the same, and the parameters are set to N = 128, a = 0.1, ω z =1.2, L 1 =L 2 =150m. Figure 6 The average bit error rate performance under four modulation modes, namely differential binary phase shift keying (DBPSK), coherent binary phase shift keying (CBPSK), coherent binary frequency shift keying (CBFSK) and on-off keying (OOK), is demonstrated. Among them, the system average bit error rate performance under DBPSK modulation is significantly better than that of the other three modulation modes, CBPSK is second, and OOK is the worst.

[0122] Figure 7 The average BER of the system is described under different pointing error angles and ICRN deflection error angles. The simulation conditions are set to weak turbulence conditions and a fixed THz link signal-to-noise ratio.

[0123] N = 128, a = 0.15, ω z =1.2,L 1 =L 2 =150m. θ and σ β The values ​​of represent very weak, weak, medium and strong pointing errors of the FSO link from small to large. Under very weak pointing error, the system reaches 10 at 35.5dB. -2 BER performance,As the pointing error increases, the system requires a higher signal-to-noise ratio to achieve the same performance.,Therefore, with the increase of pointing error angle and ICRN deflection error angle, the system average BER performance,deteriorates and the end-to-end system performance decreases.

[0124] Figure 8 For different aperture radius a, beam width ω z The average bit error rate performance of the system at the FSO link distance L. The simulation conditions are a weak turbulence environment, N = 128, and a fixed THz link signal-to-noise ratio σ θ =1mrad,σ β =0.5mrad. With the same interruption probability, increasing the value of the aperture radius a can significantly improve the bit error rate performance of the system, while reducing the beam width ω z The improvement on system performance is small. When a=0.2 and the FSO link distance L is reduced from 500m to 250m, the link has a gain of 4dB.

[0125] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the present invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for constructing a RIS-assisted hybrid FSO and THz relay system, characterized in that: Here are the steps: The relay communication system is composed of a source node S, a relay node R, a RIS surface with N reflection units and a destination node D.

2. A method for constructing a RIS-assisted hybrid FSO and THz relay system according to claim 1, characterized in that: The source node S sends an optical signal to achieve LOS communication through the RIS surface with N reflection units; The optical signal reflected from the RIS is converted into an electrical signal through the relay node R, and communicates with the destination node D through the THz link.

3. A method for constructing a RIS-assisted hybrid FSO and THz relay system according to claim 2, characterized in that: In the FSO link stage, the transmitted signal at the source node S is reflected to the relay node R through the RIS surface with N reflection units, and the received signal can be expressed as: Where P s represents the average power of the transmitted signal, x k represents the transmission symbol of the kth path, I k =I a I p I l is the channel gain of the kth path, I a ,I p and I l are the atmospheric turbulence effect, pointing error and atmospheric path loss respectively. The atmospheric path loss is normalized to 1. n1 is the additive Gaussian white noise with a mean of 0 and a variance of N1.

4. A method for constructing a RIS-assisted hybrid FSO and THz relay system according to claim 3, characterized in that: The distance from the source node S to the RIS surface is L1, and the distance from the RIS surface to the relay node R is L2, L = L1 + L2; The overall instantaneous signal-to-noise ratio of the signal at R can be obtained by maximum ratio combining (MRC): In the formula, γ FSO ,k represents the instantaneous signal-to-noise ratio of the kth channel.

5. The method for constructing a RIS-assisted hybrid FSO and THz relay system according to claim 2, characterized in that: In the THz link stage, the relay node R transmits the signal to the destination node D through the THz link, and the received signal can be expressed as: Among them, P R is the average power of the received signal at R, h = h l h pf is the THz channel fading gain, h l is the path loss, and the expression is: Among them, G t , G r represents the transmission and receiving antenna gains, d represents the THz link propagation distance, C and f represent the speed of light and the operating frequency respectively, and k α (f) is the temperature-dependent absorption coefficient.

6. A method for constructing a RIS-assisted hybrid FSO and THz relay system according to claim 5, characterized in that: h pf is the channel coefficient of the THz channel, h pf The PDF of is: in, A and ξ represent the pointing error parameters of the THz link, A o1 For the constant term, ξ represents the ratio between the equivalent beam radius and the standard deviation of the pointing error displacement at the receiver, and α and μ refer to the attenuation parameters of the α-μ distribution, is the α-rms value of the fading channel envelope; THz link instantaneous signal-to-noise ratio γ TH The PDF of z is: In the formula, γ TH The CDF of z is derived as:

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