Method for realizing STAR-RIS assisted dual-micro base station communication

Abstract

The invention discloses a method for realizing STAR-RIS assisted communication of double micro base stations. The method realizes bidirectional communication between the two micro base stations and simultaneous communication between the two micro base stations and respective destination nodes. On the basis of satisfying the lowest communication rate of the two micro base stations, the transmission phase shift matrix and the amplitude coefficient of the STAR-RIS are optimized by taking maximization of the reachable rate of the two destination nodes as the target. According to the simulation part, the achievable rate of the system and the influence of the capacity of the self-interference elimination technology on the system performance are analyzed. The method has the advantages that the advantages of STAR-RIS can be fully utilized, the signal coverage range can be remarkably expanded in a 5G / 6G indoor scene, and better network performance can be achieved.

Description

Technical Field

[0001] The invention relates to a method for realizing STAR-RIS assisted dual micro base station communication, belonging to the technical field of communication. Background Art

[0002] At present, mobile communications are developing rapidly, the fifth generation mobile communication system (5G) has entered the commercial stage, and the research on the sixth generation mobile communication system (6G) has also begun. The high frequency band used by 5G / 6G has the advantages of wide channel, fast transmission, and low latency. However, the high frequency band also has limitations. The ability to bypass objects and penetrate is weak, and it is easily interfered over long distances. Therefore, it is blocked by buildings, and the signal of the outdoor macro base station is severely attenuated indoors, forming an indoor 5G signal blind spot. Micro base stations with low power consumption and miniaturization can be fully deployed at the ends that macro base stations cannot reach, deeply covering population hotspots, and effectively solving the problem of indoor 5G / 6G signal blind spots. In practical applications, the use of micro base stations in conjunction with macro base stations can achieve a wider coverage range and better network performance.

[0003] At the same time, STAR-RIS with reflection and transmission functions provides new ideas for the construction of 5G / 6G. STAR-RIS is composed of a large number of low-cost passive reflection and transmission elements. Each unit can independently reflect and transmit the incident signal and adjust its amplitude and phase to achieve the reconstruction of the communication channel. Compared with the traditional RIS that can only provide semi-planar coverage, STAR-RIS can use the transmission and reflection functions to provide services to users on both sides of the RIS and achieve 360° full space coverage. Therefore, STAR-RIS can support and provide higher degrees of freedom in signal transmission. Summary of the invention

[0004] The purpose of the present invention is to address the defects and shortcomings of the above-mentioned prior art and propose a method for realizing STAR-RIS assisted dual micro base station communication. In this method, both micro base stations work in full-duplex mode, which can exchange information and send information to their respective terminals at the same time, thereby improving the system capacity. On this basis, a fast, reliable and low-complexity transmission method is provided.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a method for implementing STAR-RIS assisted dual micro base station communication, the method specifically comprising the following steps:

[0006] Step 1: Assume that there is no direct channel between the micro base station (i.e., S1, S2) and the terminal node (i.e., D1, D2) due to the long distance and obstacles, and communication can only be carried out with the help of STAR-RIS. S1 sends the superimposed signal s1 to S2 and D1, while S2 also sends the superimposed signal s2 to S1 and D2. For S1, S2 and D1 are a pair of NOMA users it serves, and for S2, S1 and D2 are a pair of NOMA users it serves.

[0007] Step 2: The signal of S1 is partially reflected by STAR-RIS to S2 and partially transmitted to D1. At the same time, the signal of S2 can also be partially reflected by STAR-RIS to S1 and partially transmitted to D2, thus realizing the bidirectional communication between S1 and S2 and the simultaneous communication between S1 and D1, S2 and D2. Assuming that the channel state information is completely known, the reflection phase shift matrix of STAR-RIS can be determined according to the phase shift matrix of S1-STAR-RIS link (i.e. H1) and STAR-RIS-S2 link (i.e. H2). Considering the far-field communication, H1 and H2 can be considered as reciprocal channels, so the forward channel and the backward channel between S1 and S2 are the same. Therefore, the determined reflection phase shift matrix can align H1 and H2 and obtain the maximum signal-to-noise ratio of the bidirectional communication between S1 and S2. The transmission phase shift matrix of STAR-RIS is used to simultaneously assist the communication between S1 and D1, S2 and D2, and is determined by maximizing the minimum rate in D1 and D2. These functions are achieved thanks to the fact that the reflection phase shift matrix and the transmission phase shift matrix of STAR-RIS can be adjusted independently.

[0008] Step 3: S1, S2, D1, and D2 decode their signals. S1 and D2 receive the superimposed signal from S2. Assuming that S1's channel state is better than D2's, and that it is a strong user in the NOMA user pair and is assigned a smaller power, according to the basic principle of NOMA, S1 first decodes D2's information and uses SIC to subtract it from the received signal, and then decodes its own information. The weak user D2, which is assigned a larger power, directly decodes its own information, and treats the received information from S1, the interference signal from S2, and the additive Gaussian noise as interference noise. Similarly, S2 and D1 receive the superimposed signal from S1. Assuming that S2 is a strong user and D1 is a weak user, S2 needs to use SIC to decode its own information, and D2 receives S2's information, the interference signal from S1, and the additive Gaussian noise as interference noise and directly decodes its own information.

[0009] Beneficial effects:

[0010] 1. The reconfigurable intelligent surface of the present invention can transmit and reflect at the same time, realizes dual micro base station communication, and can significantly improve signal coverage and achieve better network performance.

[0011] 2. The present invention realizes bidirectional communication between two micro base stations and simultaneous communication between them and their respective destination nodes. On the basis of satisfying the minimum communication rate of the two micro base stations, the transmission phase shift matrix and amplitude coefficient of STAR-RIS are optimized with the goal of maximizing the minimum reachable rate of the two destination nodes.

[0012] 3. The present invention can make full use of the advantages of STAR-RIS and can significantly improve signal coverage and achieve better network performance in 5G / 6G indoor scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the system model of the dual micro base station communication solution based on STAR-RIS of the present invention.

[0014] Figure 2 It is a curve diagram showing the variation of S1, S2, D1, D2 and the achievable rate of the system with the transmission signal-to-noise ratio of the present invention.

[0015] Figure 3 It is a curve diagram showing the total achievable rate changing with the transmission signal-to-noise ratio under different numbers of STAR-RIS units of the present invention.

[0016] Figure 4 It is a curve diagram showing the variation of the achievable rate of the system of the present invention with the self-interference elimination coefficient. DETAILED DESCRIPTION

[0017] The specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. This embodiment is implemented on the premise of the technical solution of the present invention, and provides a detailed implementation mode and specific operation process.

[0018] The system of the present invention includes a STAR-RIS, two micro base stations and two users. The system of the present invention adopts the NOMA solution and serves two groups of NOMA user pairs at the same time. For S1, S2 and D1 are a pair of NOMA users it serves, and for S2, S1 and D2 are a pair of NOMA users it serves, realizing two-way communication between S1 and S2 and simultaneous communication between S1 and D1, and S2 and D2.

[0019] 1. System Model

[0020] like Figure 1 As shown, two full-duplex micro base stations S1 and S2 of the present invention communicate bidirectionally with each other by means of STAR-RIS composed of N elements and communicate with their respective single-antenna destination nodes D1 and D2 at the same time, S1 and S2 are located on one side of STAR-RIS, and D1 and D2 are located on the other side of STAR-RIS. It is assumed that there is no direct channel between S1 and S2, between S1 and D1, and between S2 and D2 due to obstacles.

[0021] Let H i =(h i,1 ,h i,2 ,L,h i,N ) T ∈? N×1 and G i =(g i,1 , g i,2 ,L,g i,N ) T ∈? N×1 , i∈{1,2}, respectively, represent the Rayleigh fading channels from Si to STAR-RIS and STAR-RIS to Di. Assuming that all elements on STAR-RIS have the same amplitude coefficient, the reflection and transmission factor matrices are:

[0022]

[0023] The amplitude satisfies And β t +β r =1; the phase shift satisfies θ k,n ∈[0,2π), n∈N={1,2,LN}.

[0024] For NOMA-assisted communication systems, two micro base stations transmit superimposed signals of two users simultaneously. S1 sends a superimposed signal containing S2 and D1 information. At the same time, S2 sends a superimposed signal containing S1 and D2 information Among them, x1, x2, z1, z2 are the information of S1, S2, D1, D2 respectively, α1, β1 and α2, β2 are two pairs of NOMA allocation coefficients, satisfying α1+β1=1 and α2+β2=1. Therefore, the received signals of S1 and S2 can be expressed as:

[0025]

[0026] in, C0 represents the path loss at a reference distance of 1 meter, d H1 and d H2 Represent the distances from S1 and S2 to STAR-RIS, respectively, α is the path loss index; i∈{1,2}, if i=1 then P i represents the transmission power of Si, s i represents the transmission signal of Si; since Si works in full-duplex mode and there is self-interference, h I,i represents the self-interference channel of Si, k i Characterize the capability of Si self-interference cancellation technology, k i = 0 represents perfect self-interference elimination, ki =1 means no self-interference elimination is performed; n Si represents the additive white Gaussian noise at Si with power σ.

[0027] The received signal at D1 and D2 can be expressed as:

[0028]

[0029] in, d Gi represents the distance of Di from STAR-RIS; n Di Represents the additive white Gaussian noise at Di with power σ.

[0030] 2. Signal-to-Noise-Distortion Ratio

[0031] According to the NOMA decoding rules, the strong user needs to first decode the weak user's information and then use the SIC technology to delete it, and finally decode its own information, while the weak user directly decodes its own information. Assuming that S1 and S2 are the strong users in the two pairs of NOMA users in the present invention, they are allocated with smaller power, that is, β1>α1 and β2>α2, then their SINR for decoding the weak user information is:

[0032]

[0033] After decoding the weak user information, use SIC to subtract it from the received signal, and then decode the SINR of its own information:

[0034]

[0035] in is the transmission signal-to-noise ratio of Si. Therefore, the achievable rate of Si is R Si =log2(1+γ Si ).

[0036] The weak users D1 and D2 regard the strong user's information as interference and directly decode their own information. The SINR is:

[0037]

[0038] Therefore, the achievable rate of Di is R Di =log2(1+γ Di ).

[0039] 3. Reflection and transmission phase shift matrix design

[0040] By designing a reflection phase shift matrix to assist the two-way communication between S1 and S2, thanks to the fact that the forward channel and the backward channel of the communication between S1 and S2 are the same, we can easily get the optimal reflection phase shift as:

[0041]

[0042] This can maximize the signal-to-noise ratio at both S1 and S2.

[0043] The transmission phase shift matrix is ​​designed to assist the communication between S1 and D1 and between S2 and D2. Since the two pairs of users need to communicate simultaneously, the optimal transmission phase shift cannot be directly obtained. Therefore, we design the following optimization problem to obtain the transmission phase shift matrix. Under the premise of ensuring the minimum communication rate requirements of S1 and S2, the amplitude coefficient and transmission phase shift matrix of STAR-RIS are optimized with the goal of maximizing the minimum communication rate of D1 and D2. The specific optimization problem is as follows:

[0044]

[0045] Where R 1,min and R 2,min Represent the minimum communication rate requirements of S1 and S2 respectively. The first and second constraints ensure the QoS requirements of the communication rates of S1 and S2. The third constraint is the amplitude constraint of STAR-RIS. Note that the present invention uses a fixed power allocation coefficient. Optimizing the power allocation coefficient can further improve the performance of the system, but this is beyond the scope of the present invention.

[0046] A feasible solution to optimize the above optimization equations is given below to verify the performance of the STAR-RIS system.

[0047] The optimal phase shift for bidirectional communication between S1 and S2 is given by equation (7). Combining equations (5) and (7), the SINR of Si can be further expressed as

[0048]

[0049] By R Si =log2(1+γ Si ), constraint 1 can be expressed as Simplifying, we can get β r ≥Δ1, where Similarly, constraint 2 can be expressed as It can be simplified to β r ≥Δ2, where

[0050] In summary, in order to meet the minimum communication rate requirements of S1 and S2, β r ≥max{Δ1,Δ2}. At the same time, the minimum communication rate of D1 and D2 must be maximized. According to the constraints β r +β t =1, we can get

[0051]

[0052] Therefore, problem (8) can be further transformed into finding a transmission phase shift matrix that maximizes the minimum communication rate of D1 and D2, that is,

[0053]

[0054] 4. Performance Analysis

[0055] The present invention also analyzes the reachable rate of the system and the influence of the capability of the self-interference elimination technology on the system performance.

[0056] Superimposed signal containing S2 and D1 information The achievable rate at S1 is:

[0057] C S1 =E(log2(1+γ S1 )), (12)

[0058] The achievable rate at D1 is:

[0059] C D1 =E(log2(1+γ D1 )). (13)

[0060] A superimposed signal containing both S1 and D2 information The achievable rates from S2 to S1 and D2 are:

[0061] C S2 =E(log2(1+γ S2 )) (14)

[0062] and

[0063] C D2 =E(log2(1+γ D2 )). (15)

[0064] Therefore, the overall achievable rate C of the communication system is the sum of the achievable rates of each link, that is, C = C S1 +C D1 +C S2 +C D2 .

[0065] The simulation scenario of the dual micro base station communication solution based on STAR-RIS in the embodiment of the present invention is set as follows: Figure 1 As shown in Figure 1, the simulation area consists of two micro base stations, two single-antenna users and a STAR-RIS. Assume that all links in the communication system obey Rayleigh fading, and the distance between B1 and B2 and STAR-RIS is 2m, that is, d h1 =dh2 =2, the distances between D1 and D2 and STAR-RIS are 5m and 8m respectively, that is, d G1 =5,d G2 =8,

[0066] The path loss index α=2, and the path loss C0=0dB at a reference distance of 1 meter.

[0067] Figure 2 It is a curve showing the achievable rate of the system of the present invention and the achievable rates of S1, S2, D1 and D2 as the signal-to-noise ratio of S1 or S2 varies. The NOMA allocation coefficient is set to α1=α2=0.2, β1=β2=0.8, and the self-interference elimination coefficient is set to k1=k2=0.1. Assuming that the transmission power of S1 and S2 is equal, that is, P1=P2, the transmission power of the system is P=P1+P2. The number of STAR-RIS units is set to 6, and the minimum communication rates of S1 and S2 are R 1,min = 0.2 and R 2,min =0.1. Figure 2 As shown in the figure, as the SNR of S1 or S2 increases, the achievable rates of S1, S2, D1 and D2 and the achievable rate of the system all increase. At the same time, when the SNR of S1 or S2 increases to a certain value, the achievable rates of S1, S2, D1 and D2 and the achievable rate of the system tend to a constant value.

[0068] Figure 3 This is a curve showing the variation of the achievable rate of the system of the present invention with the S1 or S2 transmission signal-to-noise ratio under different numbers of STAR-RIS units. In addition to the number of STAR-RIS units, Figure 3 The simulation parameter settings and Figure 2 Consistent. Figure 3 It can be seen that with the increase of the number of STAR-RIS units in the low SNR area, the system achievable rate increases significantly, while in the high SNR area, the system achievable rate does not change significantly with the number of STAR-RIS units.

[0069] Figure 4 It is a curve of the achievable rate of the system of the present invention changing with the self-interference elimination coefficient. In this simulation scenario, the number of STAR-RIS units is set to 6. Through observation, we can see that when the transmission signal-to-noise ratio at the S1 and S2 ends is 20dB, 30dB, and 40dB, the achievable rate of the system decreases with the increase of the self-interference elimination coefficient. At the same time, we found that the larger the signal-to-noise ratio, the smaller the change trend of the achievable rate of the system with the self-interference elimination coefficient, and the difference between the achievable rate of the system at 30dB and the achievable rate of the system at 20dB is larger, and the difference with the achievable rate of the system at 40dB is smaller. This conclusion is consistent with Figure 3 The results are consistent.

Claims

1. A method for implementing STAR-RIS assisted dual micro base station communication, characterized in that: The method comprises the following steps: Step 1: There is no direct channel between the micro base station (i.e., S1, S2) and the terminal node (i.e., D1, D2) due to the long distance and obstacles, so they can only communicate with each other through STAR-RIS. S1 sends a superimposed signal s1 to S2 and D1, while S2 also sends a superimposed signal s2 to S1 and D2. For S1, S2 and D1 are a pair of NOMA users it serves, and for S2, S1 and D2 are a pair of NOMA users it serves; Step 2: The signal of S1 is partially reflected by STAR-RIS to S2 and partially transmitted to D1. Meanwhile, the signal of S2 can also be partially reflected by STAR-RIS to S1 and partially transmitted to D2. In this way, bidirectional communication between S1 and S2 and simultaneous communication between S1 and D1, S2 and D2 are realized. Assuming that the channel state information is completely known, the reflection phase shift matrix of STAR-RIS can be determined according to the phase shift matrices of the S1-STAR-RIS link (i.e., H1) and the STAR-RIS-S2 link (i.e., H2). Considering far-field communication, H1 and H2 can be considered as reciprocal channels, so the forward channel and the backward channel between S1 and S2 are the same. Therefore, the determined reflection phase shift matrix can align H1 and H2 to obtain the maximum signal-to-noise ratio of the bidirectional communication between S1 and S2. The transmission phase shift matrix of STAR-RIS is used to simultaneously assist the communication between S1 and D1, and S2 and D2, and is determined by maximizing the minimum rate in D1 and D2. Step 3: S1, S2, D1, and D2 decode their own signals. S1 and D2 receive the superimposed signal from S2. Assuming that S1's channel state is better than D2's, it is a strong user in the NOMA user pair and is allocated a smaller power. According to the basic principle of NOMA, S1 first decodes D2's information and subtracts it from the received signal using SIC, and then decodes its own information. The weak user D2, which is allocated a larger power, directly decodes its own information and treats the received information from S1, the interference signal from S2, and the additive Gaussian noise as interference noise. S2 and D1 receive the superimposed signal from S1. Assuming that S2 is a strong user and D1 is a weak user, S2 needs to use SIC to decode its own information. The information from S2, the interference signal from S1, and the additive Gaussian noise received by D2 are all treated as interference noise and directly decode their own information.

2. A method for implementing STAR-RIS assisted dual micro base station communication according to claim 1, characterized in that: The method includes: two full-duplex micro base stations S1 and S2 perform bidirectional communication with their respective single-antenna destination nodes D1 and D2 by means of a STAR-RIS composed of N elements, S1 and S2 are located on one side of the STAR-RIS, and D1 and D2 are located on the other side of the STAR-RIS, assuming that there is no direct channel between S1 and S2, between S1 and D1, and between S2 and D2 due to obstacles, specifically including: Let H i =(h i,1 ,h i,2 ,L,h i,N ) T ∈? N×1 and G i =(g i,1 , g i,2 ,L,g i,N ) T ∈? N×1 , i∈{1,2}, respectively represent the Rayleigh fading channels from Si to STAR-RIS and STAR-RIS to Di. Assuming that all elements on STAR-RIS have the same amplitude coefficient, the reflection and transmission factor matrices are: The amplitude satisfies And β t +β r =1; the phase shift satisfies θ k,n ∈[0,2π), n∈N={1,2,LN}, For the NOMA-assisted communication system, two micro base stations simultaneously transmit the superposition signal of two users, and S1 sends the superposition signal containing S2 and D1 information. At the same time, S2 sends a superimposed signal containing S1 and D2 information Among them, x1, x2, z1, z2 are the information of S1, S2, D1, D2 respectively, α1, β1 and α2, β2 are two pairs of NOMA allocation coefficients, satisfying α1+β1=1 and α2+β2=1, so the received signals of S1 and S2 can be expressed as: in, C0 represents the path loss at a reference distance of 1 meter, d H1 and d H2 Represents the distance from S1 and S2 to STAR-RIS, α is the path loss index; i∈{1,2}, if i=1 then P i represents the transmission power of Si, s i represents the transmission signal of Si; since Si works in full-duplex mode and there is self-interference, h I,i represents the self-interference channel of Si, k i Characterize the capability of Si self-interference cancellation technology, k i = 0 represents perfect self-interference elimination, κ i =1 means no self-interference elimination is performed; n Si represents the additive white Gaussian noise at Si, with power σ; The received signal at D1 and D2 can be expressed as: in, d Gi represents the distance of Di from STAR-RIS; n Di Represents the additive white Gaussian noise at Di with power σ.

3. The method for implementing STAR-RIS assisted dual micro base station communication according to claim 1, characterized in that: The method includes: according to the NOMA decoding rule, the strong user needs to first decode the information of the weak user and then delete it using the SIC technology, and finally decode its own information, while the weak user directly decodes its own information. Assuming that S1 and S2 are strong users in the two pairs of NOMA users in the present invention, they are allocated with smaller power, that is, β1>α1 and β2>α2, then their SINR for decoding the weak user information is: After decoding the weak user information, use SIC to subtract it from the received signal, and then decode the SINR of its own information: in is the transmission signal-to-noise ratio of Si, so the achievable rate of Si is R Si =log2(1+γ Si ); The weak users D1 and D2 regard the strong user's information as interference and directly decode their own information. The SINR is: Therefore, the achievable rate of Di is R Di =log2(1+γ Di ).

4. The method for implementing STAR-RIS assisted dual micro base station communication according to claim 1, characterized in that: The method includes: assisting the bidirectional communication between S1 and S2 by designing a reflection phase shift matrix. Since the forward channel and the backward channel of the communication between S1 and S2 are the same, we can easily obtain the optimal reflection phase shift as: The signal-to-noise ratio can be maximized at both S1 and S2. The transmission phase shift matrix is ​​designed to assist the communication between S1 and D1 and between S2 and D2. Since the two pairs of users need to communicate simultaneously, the optimal transmission phase shift cannot be directly obtained. Therefore, we design the following optimization problem to obtain the transmission phase shift matrix. Under the premise of ensuring the minimum communication rate requirements of S1 and S2, the amplitude coefficient and transmission phase shift matrix of STAR-RIS are optimized with the goal of maximizing the minimum communication rate of D1 and D2. The specific optimization problem is as follows: Where R 1,min and R 2,min They represent the minimum communication rate requirements of S1 and S2 respectively. The first and second constraints guarantee the QoS requirements of the communication rates of S1 and S2. The third constraint is the amplitude constraint of STAR-RIS. The optimal phase shift for bidirectional communication between S1 and S2 is given by equation (7). Combining equations (5) and (7), the SINR of Si can be further expressed as: By R Si =log2(1+γ Si ), constraint 1 can be expressed as Simplifying, we can get β r ≥Δ1, where Similarly, constraint 2 can be expressed as It can be simplified to β r ≥Δ2, where In order to meet the minimum communication rate requirements of S1 and S2, β r ≥max{Δ1,Δ2}, and at the same time maximize the minimum communication rate of D1 and D2, according to the constraints of three β r +β t =1, we can get: Therefore, problem (8) can be further transformed into finding a transmission phase shift matrix that maximizes the minimum communication rate of D1 and D2, that is,

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