Cascade RIS auxiliary wireless communication system transmission method

Abstract

The invention discloses a cascaded RIS auxiliary wireless communication system transmission method, the cascaded RIS auxiliary wireless communication system comprises a base station, N RIS cascades and a user, the N IRSs are respectively named as IRS1 to IRSn according to a transmission sequence, the nth RIS is provided with Ln reflection units, and the base station and the user can only communicate through the cascaded RIS. The system can flexibly adjust the signal reflection angle and the transmission power according to the channel state information so as to realize efficient bidirectional data transmission between the base station and the user. According to the invention, through accurate transmission power regulation and control or simpler RIS deployment, the requirements of transmission interruption probability or system traversal capacity required by a user are met, efficient utilization of resources is realized, and the system efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the field of wireless communication technologies, and particularly relates to a transmission method for a cascaded RIS (Reconfigurable Intelligent Surfaces) - assisted wireless communication system, especially considering the power setting of the system and the RIS deployment method under loop interference. Background Art

[0002] With the commercial deployment of 5G systems, researchers have begun to explore energy - efficiency and spectrum - efficiency solutions for the sixth - generation (6G) communication systems. As a promising technology, the intelligent reflecting surface (RIS) is expected to play an important role in expanding signal coverage, enhancing system capacity, and improving energy efficiency due to its advantages such as high array gain, low cost, and low power consumption. The RIS - assisted communication system has evolved from a single RIS to collaborative applications of multiple RISs, which can effectively improve the diversity gain and reliability of the system. The existing technologies do not consider the problems of limited connection of a single RIS and proximal RISs in a dense urban environment, such as insufficient coverage of a single RIS, blockage of the base - station - RIS link, unstable control link of proximal RISs, and user resource competition. The cascaded RIS network proposed in the present invention can bypass obstacles and further expand network coverage. In addition, with the increasing demand for spectrum efficiency in 6G communication, RIS - assisted two - way transmission is an important means to improve spectrum efficiency and channel capacity. Summary of the Invention

[0003] The purpose of the present invention is to provide a transmission method for a cascaded RIS - assisted wireless communication system, which solves the problems that the existing single - RIS network has a small coverage range, and the system needs to increase the transmission power or deploy more RIS units to meet the outage probability and ergodic capacity required for transmission quality.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] A cascaded RIS - assisted wireless communication system, which is a two - way transmission system, includes a base station, N cascaded RISs, and a user, where N≥2. The N IRSs are respectively named IRS1 to IRS N , and the nth RIS has L n reflecting units, where n∈{1,2,...,N}. Communication between the base station and the user can only be carried out through the cascaded RISs.

[0006] As an implementation, the transmission method of the cascaded RIS - assisted wireless communication system includes the following steps:

[0007] Step 1: Channel estimation is performed among the base station, at least two cascaded RISs, and the user to estimate the channel conditions between the base station and the first RIS in the cascaded RISs, and between the last RIS in the cascaded RISs and the user.

[0008] Step 2: According to the channel state information in Step 1 and the requirements of the user's transmission quality for outage probability or ergodic capacity, set the transmit power P of the base station S and the transmit power P of the user U ;

[0009] Step 3: Adjust the reflection angles of the first RIS and the last RIS in the cascaded RISs respectively, so that the angle of each unit of the RIS is exactly equal to the opposite of the angle of the complex plane of the channel at both ends.

[0010] Step 4: The base station sends a signal to the cascaded RIS. Each unit of the RIS reflects the signal according to the set angle. The user receives the signal reflected from the cascaded RIS. At the same time, the user sends a signal to the cascaded RIS, and each unit of the RIS reflects the signal according to the set angle, and the base station receives the signal reflected from the cascaded RIS.

[0011] In Step 2, when the user's transmission quality requirement index is that the outage probability needs to reach , the transmit power P of the base station S and the transmit power P of the user U need to satisfy

[0012] where,

[0013]

[0014] In Step 2, when the user's transmission quality requirement index is that the system ergodic capacity needs to reach C u , the transmit power P of the base station S and the transmit power P of the user U need to satisfy

[0015]

[0016] where, α and β are the parameters corresponding to approximating the received signal-to-interference-plus-noise ratio of the cascaded RIS link as a Gamma distribution using the moment matching method, γ th is the threshold of the given outage probability, L n is the number of reflection units for deploying the RIS, is the user's desired outage probability, m F n (·) is the hypergeometric function, and respectively represent the loop interference power of the user and the base station side, and Γ(·) represents the Gamma function, represents the Meijer - G function.

[0017] As another implementation, the cascaded RIS - assisted wireless communication system transmission method includes the following steps:

[0018] Step 1, the base station, at least two cascaded RISs, and the user perform channel estimation to estimate the channel conditions between the base station and the first RIS in the cascaded RISs, and between the last RIS in the cascaded RISs and the user;

[0019] Step 2, according to the channel state information in Step 1 and the requirements of the user's transmission quality for the outage probability or the ergodic capacity, deploy the number of reflecting elements L of the RIS n ;

[0020] Step 3, adjust the reflection angles of the first RIS and the last RIS in the cascaded RISs respectively, so that the angle of each unit of the RIS is exactly equal to the opposite of the angle of the complex plane of the channels at both ends;

[0021] Step 4, the base station sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, the user receives the signal reflected from the cascaded RIS, and at the same time the user sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, and the base station receives the signal reflected from the cascaded RIS;

[0022] In Step 2, when the user's transmission quality requirement index is that the outage probability needs to reach the number of reflecting elements of the deployed RIS satisfies

[0023] where,

[0024]

[0025] In Step 2, when the user's transmission quality requirement index is that the system ergodic capacity needs to reach C u the number of reflecting elements L of the deployed RIS n satisfies

[0026]

[0027] where α and β are the parameters corresponding to approximating the received signal - to - interference - plus - noise ratio of the cascaded RIS link as a Gamma distribution using the moment - matching method, γ th is the threshold of the given outage probability, P S and P U respectively represent the transmission powers of the base station and the user, is the user-expected outage probability, m F n (·) is the hypergeometric function, and represent the loop interference power at the user side and the base station side respectively, Γ(·) represents the Gamma function, represents the Meijer-G function.

[0028] Compared with the prior art, the power setting of the cascaded RIS-assisted two-way transmission and the RIS deployment method given by the present invention can significantly reduce the energy consumption, improve the system efficiency, and achieve efficient utilization of resources through a more streamlined RIS deployment when meeting the same transmission performance requirements of users compared with the traditional single RIS-assisted transmission method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic structural diagram of the cascaded RIS-assisted wireless communication system of the present invention.

[0030] Figure 2 is a schematic flow diagram of the transmission method (power setting) of the cascaded RIS-assisted wireless communication system of the present invention.

[0031] Figure 3 is a schematic flow diagram of the transmission method (RIS deployment) of the cascaded RIS-assisted wireless communication system of the present invention.

[0032] Figure 4 and Figure 5 are the outage probability curves and the ergodic capacity curves of the two systems at different transmit powers respectively.

[0033] Figure 6 and Figure 7 are the outage probability curves and the ergodic capacity curves of the two systems at different numbers of reflection units respectively. DETAILED DESCRIPTION OF THE INVENTION

[0034] The following is further described in conjunction with the drawings and specific embodiments.

[0035] As Figure 1 shown, a cascaded RIS-assisted wireless communication system includes a base station, N cascaded RISs (N≥2), and a user. The N IRSs are named IRS1 to IRS in the transmission order N , and the nth RIS has L n reflection units, where n ∈ {1, 2,..., N}. The cascaded RIS is located between the user and the base station and is used to adjust the phase and optimize the signal transmission. And due to the complexity of the environment, only the base station and the user cannot communicate directly and communicate through the cascaded RIS.

[0036] In addition to receiving noise, there are some interference sources around the user, which are simplified as loop interference (LI).

[0037] The instantaneous received signals at the user and the base station can be expressed as

[0038]

[0039] where the right side of the equation represents the desired signal, self-interference, and Gaussian noise in turn. Among them, x S and x U are the transmitted signals of the base station and the user respectively, P S and P U are the average transmitted powers of the base station and the user respectively, ω U and ω S are the Gaussian white noises generated at D and BS respectively. H C represents the cascaded RIS channel. and are the loop interference channels at the user and the base station respectively, following a complex Gaussian distribution with zero mean and variances of and respectively. Among them, h i and g k represent the channel amplitude from the base station to the i-th reflection element of the first RIS and the channel amplitude from the k-th reflection element of the last RIS to the user respectively, both following independent Nakagami-m fading. θ i and represent the corresponding fading channel phases. represents the channel phase between cascaded RISs, representing the channel phase between the p-th reflection element on the n-th RIS and the q-th reflection element on the n + 1-th RIS. For the convenience of analysis, it is assumed that the communication channels between RISs are not affected by small-scale fading, the channel amplitude is 1, and their phases are a deterministic variable r is the Euclidean distance between each RIS, and λ is the wavelength. Ideally, the adjustable phase of the RIS reflection element is set to cancel the influence of the channel phase to obtain the optimal RIS transmission effect, that is, the incident phase and the reflection phase of the cascaded RIS cancellation system are adjusted to maximize the received signal. Then, the optimal phase shift of the l-th element in the n-th RIS is

[0040]

[0041] For the convenience of analysis, let Therefore, the equivalent channel can be approximated as a Gamma distribution according to the moment matching theory for V1 and V k That is

[0042] V1 to Gamma(L n α h , β h ), V k to Gamma(L n α g , β g )

[0043] Among them, α h , β h and α g , β g are expressed as follows:

[0044]

[0045] Among them, h i to Nakagami - m(m h , Ω h ), g k to Nakagami - m(m g , Ω g ). E[h i represents the expectation of h i , Var[h i represents the variance of h i , μ h (1) is the first - order moment of h i , μ h (2) is the second - order moment of h i . E[g k represents the expectation of g k , Var[g k represents the variance of g k , μ g (1) is the first - order moment of g k , μ g (2) is the second - order moment of g k , m h and m g respectively represent the shape parameters of the channels h i and g k following the Nakagami - m distribution, Ω h and Ω g respectively represent the shape parameters of the channels h i and g k following the Nakagami - m distribution.

[0046] According to the additivity of the gamma distribution, V1 to Gamma(L n α h , β h ), V k to Gamma(L nα g , β g ).

[0047] Based on the instantaneous received signal formulas at the user and the base station, the instantaneous signal-to-interference-plus-noise ratio (SINR) at the user and the base station can be obtained as

[0048]

[0049]

[0050] Let It can be approximated as a Gamma distribution with parameters α and β according to the moment matching theory, i.e., Z ~ Gamma(α, β)

[0051] where the values of α and β are expressed as follows:

[0052]

[0053] E[Z] represents the expectation of Z = |V1 V k | 2 and Var[Z] represents the variance of Z = |V1 V k | 2 μ Z (1) is the first moment of Z = |V1V k | 2 and μ Z (2) is the second moment of Z = |V1V k | 2 .

[0054] Since the energy of the received noise (Gaussian noise) is relatively small compared to the loop interference, the noise part can be ignored. At this time, the probability density functions at the user and the base station are

[0055]

[0056]

[0057] According to the relationship between the probability density function and the cumulative density function (CDF), the cumulative density functions of the SINR at the user and the base station are obtained as

[0058]

[0059] where m F n (·) is the hypergeometric function.

[0060] The outage probability in a wireless communication system is defined as the probability that the instantaneous signal-to-interference-plus-noise ratio γ of the system is less than a given outage threshold γ th , when γ < γ thThe wireless communication system can be considered to be in an outage state, so a given outage threshold is γ th Then the mathematical expression of the outage probability can be written as

[0061]

[0062] The ergodic capacity refers to the maximum amount of information transmitted by a disturbed channel per unit time. According to Shannon's formula, the system capacity that follows the channel distribution can be expressed as

[0063] EC = C1 + C2

[0064] = E[log2(1 + γ U )] + E[log2(1 + γ S )]

[0065] where E represents the average expectation, C1 is the ergodic capacity of the base station-RIS-user link, and C2 is the ergodic capacity of the user-RIS-base station link. According to the probability density function of SINR, the ergodic capacity expression of the system can be obtained as

[0066]

[0067] where Γ(·) represents the Gamma function, represents the Meijer-G function.

[0068] Such as Figure 2 shown in Scheme 1: The cascaded RIS-assisted wireless communication system transmission method specifically includes the following steps:

[0069] Step 1, the base station, at least two cascaded RISs, and the user perform channel estimation to estimate the channel conditions between the base station and the first RIS in the cascaded RISs, and between the last RIS in the cascaded RISs and the user;

[0070] Step 2, according to the channel state information in Step 1 and the requirements for the outage probability or ergodic capacity of the user transmission quality, set the transmit power of the base station and the user transmit power;

[0071] Step 3, adjust the reflection angles of the first RIS and the last RIS in the cascaded RISs respectively, so that the angle of each unit of the RIS is exactly equal to the opposite of the angle of the complex plane of the channel at both ends of it, and cancel the incident phase and the reflection phase through the RIS to maximize the received signal;

[0072] Step 4, the base station sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, the user receives the signal reflected from the cascaded RIS, and at the same time the user sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, and the base station receives the signal reflected from the cascaded RIS.

[0073] In step 2, when the user transmission quality requirement index is that the outage probability needs to reach The method for setting the transmit power of the base station and the user transmit power is as follows:

[0074] The outage probability needs to satisfy That is Then the transmit power of the base station and the user transmit power need to satisfy

[0075]

[0076] In step 2, when the user transmission quality requirement index is that the system ergodic capacity needs to reach C u The method for setting the transmit power of the base station is as follows:

[0077] The ergodic capacity needs to satisfy

[0078]

[0079] , then the transmit power of the base station satisfies

[0080]

[0081] Such as Figure 3 Scheme 2 shown: The cascaded RIS-assisted wireless communication system transmission method specifically includes the following steps:

[0082] Step 1, the base station, at least two cascaded RISs, and the user perform channel estimation to estimate the channel conditions between the base station and the first RIS in the cascaded RIS and between the last RIS in the cascaded RIS and the user;

[0083] Step 2, according to the channel state information in step 1 and the requirements for outage probability or ergodic capacity of the user transmission quality requirement, deploy the number of reflecting elements of the RIS;

[0084] Step 3, adjust the reflection angles of the first RIS and the last RIS in the cascaded RIS respectively so that the angle of each unit of the RIS is exactly equal to the opposite of the angle of the complex plane of the channel at both ends;

[0085] Step 4, the base station sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, the user receives the signal reflected from the cascaded RIS, and at the same time the user sends a signal to the cascaded RIS, each unit of the RIS reflects the signal according to the set angle, and the base station receives the signal reflected from the cascaded RIS.

[0086] In step 2, when the user transmission quality requirement index is that the outage probability needs to reach The method for deploying the number of reflecting elements of the RIS is as follows:

[0087] The outage probability needs to satisfy That is Then the number of reflecting elements of the RIS satisfies Where

[0088]

[0089] In step 2, when the user transmission quality requirement index is that the system ergodic capacity needs to reach C u The method for the number of reflecting elements of the deployed RIS is as follows:

[0090] The ergodic capacity needs to satisfy

[0091]

[0092] , then the number of reflecting elements of the RIS satisfies

[0093]

[0094] Figure 4 and Figure 5 respectively give the comparison diagrams of the outage probability and ergodic capacity of Scheme 1 with respect to the traditional single-RIS assisted transmission method. It can be seen from the figures that implementing Scheme 1 can effectively meet the communication requirements of users with an outage probability of 10 -1 , and the transmit power of the base station set by the single-RIS scheme is lower; at the same time, this scheme can effectively meet the communication requirements of users with an ergodic capacity of 25 (bit / s / Hz). It can be seen that the method given in the present invention can reduce energy consumption, enhance the signal coverage range, and thus improve the user transmission satisfaction.

[0095] Figure 6 and Figure 7 respectively give the comparison diagrams of the outage probability and ergodic capacity of Scheme 2 with respect to the traditional single-RIS assisted transmission method. It can be seen from the figures that implementing Scheme 2 can effectively meet the communication requirements of users with an outage probability of 10 -1 , and the number of RIS units deployed by the single-RIS scheme is less; at the same time, this scheme can effectively meet the communication requirements of users with an ergodic capacity of 35 (bit / s / Hz), and the number of RIS units deployed by the single-RIS scheme is less. It can be seen that the method given in the present invention can achieve efficient utilization of resources through a more streamlined RIS deployment.

Claims

1. A cascaded RIS-assisted wireless communication system transmission method, characterized in that: The cascaded RIS assisted wireless communication system includes a base station, N RIS cascades and a user, N≥2, and the N IRSs are named IRS1 to IRS2 in the order of transmission. N , the nth RIS has L n reflection units, where n∈{1,2,...,N}, the base station and the user can only communicate through cascaded RIS, Specifically including the following steps: Step 1: The base station, at least two RIS cascades, and the user perform channel estimation, estimating the channel conditions between the base station and the first RIS in the cascaded RIS, and between the last RIS in the cascaded RIS and the user; Step 2: According to the channel state information in step 1 and the user's transmission quality requirements for interruption probability or traversal capacity, set the base station's transmission power P S and the user's transmission power P U ; Step 3, adjusting the reflection angles of the first RIS and the last RIS in the cascaded RIS, respectively, so that the angle of each unit of the RIS is exactly equal to the opposite of the complex plane angle of the channels at both ends thereof; Step 4: The base station sends a signal to the cascaded RIS, and each unit of the RIS reflects the signal according to a set angle, and the user receives the signal reflected from the cascaded RIS. At the same time, the user sends a signal to the cascaded RIS, and each unit of the RIS reflects the signal according to a set angle, and the base station receives the signal reflected from the cascaded RIS.

2. The cascaded RIS-assisted wireless communication system transmission method according to claim 1, characterized in that: In step 2, when the user transmission quality requirement indicator is that the interruption probability must reach When the base station transmits power P S and the user's transmission power P U Needs to be satisfied in, In step 2, when the user transmission quality requirement indicator is that the system traversal capacity needs to reach C u When the base station transmits power P S and the user's transmission power P U Needs to be satisfied Among them, α and β are the parameters corresponding to the Gamma distribution of the received signal-to-interference-to-noise ratio of the cascaded RIS link using the moment matching method, and γ th is a given interruption probability threshold, L n is the number of reflector units deployed in RIS, is the user's expected outage probability, m F n (·) is the hypergeometric function, and denote the loop interference power at the user and base station ends respectively, Γ(·) denotes the Gamma function, represents the Meijer-G function.

3. A cascaded RIS-assisted wireless communication system transmission method, characterized in that: The cascaded RIS assisted wireless communication system includes a base station, N RIS cascades and a user, N≥2, and the N IRSs are named IRS1 to IRS2 in the order of transmission. n , the nth RIS has L n reflection units, where n∈{1,2,…,N}, the base station and the user can only communicate through cascaded RIS, The specific steps include: Step 1: The base station, at least two RIS cascades, and the user perform channel estimation, estimating the channel conditions between the base station and the first RIS in the cascaded RIS, and between the last RIS in the cascaded RIS and the user; Step 2: According to the channel state information in step 1 and the user's transmission quality requirements for interruption probability or traversal capacity, the number of reflection units L deployed in RIS is n ; Step 3, adjusting the reflection angles of the first RIS and the last RIS in the cascaded RIS, respectively, so that the angle of each unit of the RIS is exactly equal to the opposite of the complex plane angle of the channels at both ends thereof; Step 4: The base station sends a signal to the cascaded RIS, and each unit of the RIS reflects the signal according to a set angle, and the user receives the signal reflected from the cascaded RIS. At the same time, the user sends a signal to the cascaded RIS, and each unit of the RIS reflects the signal according to a set angle, and the base station receives the signal reflected from the cascaded RIS.

4. The cascaded RIS-assisted wireless communication system transmission method according to claim 1, characterized in that: In step 2, when the user transmission quality requirement indicator is that the interruption probability must reach P o u ut When the number of reflector units deployed in RIS satisfies in, In step 2, when the user transmission quality requirement indicator is that the system traversal capacity needs to reach C u When the number of reflector units deployed in the RIS is L n satisfy Among them, α and β are the parameters corresponding to the Gamma distribution of the received signal-to-interference-to-noise ratio of the cascaded RIS link using the moment matching method, and γ th is a given interruption probability threshold, P S and P U Represent the transmission power of the base station and the user respectively, is the user's expected outage probability, m F n (·) is the hypergeometric function, and denote the loop interference power at the user and base station ends respectively, Γ(·) denotes the Gamma function, represents the Meijer-G function.

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