A RIS-assisted sky-ground integrated network relay transmission method

Abstract

The present invention discloses a RIS-assisted sky-ground integrated network relay transmission method, which is characterized in that the method uses a ground base station node antenna to send an encoded radio frequency signal to the RIS, and transmits the encoded radio frequency signal to the RIS through the RIS. N After the amplitude and phase are adjusted by different reconfigurable surfaces, the high-altitude platform relay node receives the signal reflected by RIS, converts the radio frequency signal into an optical signal through the photoelectric converter, and sends it to the target satellite node through the FSO channel. The target satellite node receives the transmission signal, and realizes the RIS-assisted sky-ground integrated network relay transmission. Compared with the prior art, the present invention has the advantages of increasing the channel capacity of the system, reducing the interruption probability and bit error rate of the system, improving the communication performance of the system, alleviating the problem of tight radio frequency communication spectrum resources, and improving the link performance of the RIS-assisted sky-ground integrated network by reconstructing the channel using RIS.

Description

Technical Field

[0001] The present invention relates to the field of satellite communication technology, and in particular to a RIS-assisted sky-ground integrated network relay transmission method. Background Art

[0002] The Sky-Ground Integrated Network (SAGIN) consisting of satellites, high-altitude platforms and ground base stations can provide ubiquitous high-speed network services and is considered an indispensable component of the sixth-generation communication network. The existing SAGIN works in the radio frequency (RF) band and has bottlenecks in communication capacity and spectrum resources, and cannot meet the requirements of high transmission bandwidth and large transmission capacity. Free space optical communication (FSO) has the advantages of high transmission bandwidth, strong anti-electromagnetic interference ability, no need for spectrum licenses, and large transmission capacity. In recent years, the application of FSO in SAGIN has attracted the attention of academia and industry. However, FSO links are easily affected by channel fading factors such as pointing errors and atmospheric turbulence.

[0003] In the prior art, the RF / FSO relay communication in SAGIN usually uses Nakagami-m distribution and Rayleigh distribution to characterize the multipath and shadow effects on the RF link, and the atmospheric turbulence and pointing error damage on the FSO link are usually characterized by Gamma-Gamma distribution and generalized K distribution. In SAGIN, the channel model for the FSO link has high accuracy only for specific atmospheric turbulence conditions, but loses its applicability under a wider range of weather conditions. In addition, the channel model for the RF link fails to consider the existence of shadow and multipath effects at the same time, which greatly reduces the scope of use and practicality of the RF / FSO relay system in SAGIN. Summary of the invention

[0004] The purpose of the present invention is to provide a RIS-assisted sky-ground integrated network relay transmission method for the deficiencies of the prior art. The modulated RF transmission signal is reflected after adjusting the amplitude and phase using multiple different reconfigurable surfaces of RIS, and the RF transmission signal is sent to the high-altitude platform relay through the RF channel when the channel state of the high-altitude platform relay node meets the preset conditions. The high-altitude platform relay node demodulates the RF signal, converts the demodulated RF signal into an optical signal through a photoelectric converter, and sends the optical signal to the target satellite node through the FSO channel when the channel state of the satellite node meets the preset conditions. The satellite node eliminates the interference signal from the received optical signal to obtain a transmission signal corresponding to the satellite node. The method is applicable to the RIS-assisted sky-ground integrated network relay communication system, and can study the fading loss caused by atmospheric turbulence and pointing errors between the high-altitude platform and the satellite to the system, effectively improve the system communication performance, and solve the problem of tight spectrum resources in traditional RF communication. At the same time, the link performance of the RIS-assisted sky-ground integrated network is improved by reconstructing the channel using RIS.

[0005] The object of the present invention is achieved as follows: a RIS-assisted sky-ground integrated network relay transmission method, characterized in that the relay transmission method uses the base station antenna of the ground base station node to send the encoded radio frequency signal to the RIS, and the N different reconfigurable surfaces of the RIS adjust the amplitude and phase and then reflect, the high-altitude platform relay node receives the signal reflected by the RIS, converts the radio frequency signal into an optical signal through an optoelectronic converter, and sends it to the target satellite node through an FSO channel, the satellite node eliminates the interference signal from the received optical signal, and the target satellite node receives the transmission signal, thereby realizing RIS-assisted sky-ground integrated network relay transmission.

[0006] The communication system constructed by the present invention is composed of a ground base station S assisted by RIS, a high-altitude platform relay node R and a land mobile satellite D. The RIS-assisted ground base station S transmits the RF signal to the high-altitude platform relay node R through a shadowed-Rician fading channel. The instantaneous signal-to-noise ratio γ1 of the radio frequency link can be expressed by the following formula (a):

[0007]

[0008] in, is the average signal-to-noise ratio of the RF link, N is the number of RIS reflective elements, Indicates the transmission signal. In, δ 1,k and φ 1,k are the amplitude and phase of the signal, ψ 1,k is the phase of RIS change. Therefore, the instantaneous signal-to-noise ratio of the RF link can be rewritten from the following formula (b-1):

[0009]

[0010] In order to obtain the maximum value of the instantaneous signal-to-noise ratio of the radio frequency link, the present invention adjusts the reflection phase to the transmission signal phase (i.e. ). Using the small signal approximation (SAA) method, the sum of N shadow Rice variables can be approximated as the sum of N square shadow Rice variables. Therefore, the instantaneous signal-to-noise ratio γ from the ground base station D to the high-altitude platform relay node R of the RIS is assisted RF The probability density function of is given by the following formula (d):

[0011]

[0012] where 1F1(·,·,·) is defined as the first kind of confluent hypergeometric function, m1 is a parameter representing the severity of fading, 2b1 represents the average power of the scattered component, and Ω1 represents the average power of the direct path component.

[0013] By integrating the probability density function, we can obtain the corresponding cumulative distribution function represented by the following formula (e):

[0014]

[0015] in, is an incomplete Gamma function, and Γ(x) is a Gamma function.

[0016] Considering the variable turbulence environment in the link from the high-altitude platform relay R to the satellite node D, this link adopts the Malaga distribution, which is more applicable in atmospheric turbulence. This model is applicable to three conditions: weak, moderate and strong turbulence. In addition, considering that the FSO link will also produce pointing error loss due to the vibration of the transmission beam. Therefore, under the influence of atmospheric turbulence and pointing error, the probability density function and cumulative distribution function of the FSO link can be expressed by the following (f) to (g) formulas:

[0017]

[0018]

[0019] in,

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026]

[0027] α is a parameter positively correlated with the large-scale unit effective number of the scattering process, and β is a natural number representing the fading size. h = 2b2 (1-ρ) is the average power of the off-axis eddy current scattering component, 2b2 represents the average power of the total scattering component, and ρ represents the scattering power coupled to the line-of-sight (LOS) component, satisfying the condition 0 < ρ < 1. represents the average power of the coherent term, Ω'2 is the average power of the LOS component, Φ A and Φ B are the phases of the LOS and the scattering term coupled to the LOS, respectively. In addition, ξ represents the ratio of the equivalent beamwidth to the standard deviation of the receiver pointing error. In addition, r is the detection technology used by the FSO link, where r = 1 is heterodyne detection and r = 2 is intensity modulation / direct detection technology. Represents the MeijerG function.

[0028] The present invention simultaneously considers the amplify-and-forward and decode-and-forward relay protocols to calculate the end-to-end outage probability, average bit error rate and ergodic capacity of the system. Based on the probability density function and cumulative distribution function expressions of the RF and FSO links obtained above, the end-to-end probability density function and cumulative distribution function between the RIS-assisted ground base station and the satellite under the amplify-and-forward relay protocol can be further calculated, which are respectively expressed by the following formulas (h) to (i):

[0029]

[0030]

[0031] According to the end-to-end probability density function and cumulative distribution function expressions under the above-mentioned amplify-and-forward relay protocol, the end-to-end outage probability, average bit error rate and ergodic capacity between the RIS-assisted ground base station and the satellite can be further calculated, which are expressed by the following formulas (j) to (l):

[0032]

[0033]

[0034]

[0035] The present invention also calculates that under the decode-forward relay protocol, the end-to-end interruption probability, average bit error rate and ergodic capacity between the RIS-assisted ground base station and the satellite are respectively expressed by the following (m) to (o) formulas:

[0036]

[0037]

[0038]

[0039] in,

[0040]

[0041]

[0042]

[0043]

[0044]

[0045] The present invention is applied to RIS-assisted ground base station nodes, high-altitude platform relay nodes and satellite nodes. Among them, the RIS-assisted ground base station transmits RF signals to the high-altitude platform relay through an RF link, and the high-altitude platform relay receives the signal and performs photoelectric conversion, and then transmits the FSO signal to the satellite. The Málaga and shadow Rice channel models are used to describe the channel fading of the RIS-assisted ground base station-high-altitude platform relay and high-altitude platform relay-satellite links, respectively. This method is based on the RIS-assisted RF / FSO relay transmission method of the SAGIN system. RIS is a new disruptive technology emerging in the field of wireless communications, which improves the spectrum efficiency and energy efficiency of the communication system in a low-cost manner. RIS is composed of a large number of reconfigurable passive reflective elements, and the amplitude and phase of the reflected signal can be changed by controlling the reflection coefficient. The RIS-assisted method is that the RIS is closely connected to the signal source, and the energy loss between the two can be ignored. In view of the damage caused by channel fading on the RF and FSO links, the advantages of RIS such as low energy consumption, reconfigurability, easy deployment and low energy consumption can be used to improve the communication performance of the RF / FSO relay system in SAGIN.

[0046] Compared with the prior art, the present invention has the advantages of increasing the channel capacity of the system, reducing the interruption probability and bit error rate of the system, effectively improving the communication performance of the system, and alleviating the problem of tight radio frequency communication spectrum resources. At the same time, by reconstructing the channel using RIS, the link performance of the RIS-assisted sky-ground integrated network is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a flow chart of the present invention;

[0048] Figure 2A schematic diagram of the system structure constructed for the present invention. DETAILED DESCRIPTION

[0049] The core of the present invention is to provide a RIS-assisted sky-ground integrated network relay transmission method, which uses RIS technology to improve the interruption performance, bit error rate performance and traversal capacity performance of the sky-ground integrated network relay system. In order to enable people in the field of technology to better understand the present invention, the present invention is further described and explained in detail below based on the specific implementation of RIS-assisted ground base stations, high-altitude platform relay nodes and satellite nodes:

[0050] Example 1

[0051] See also Figure 1 The present invention is applied to RIS-assisted ground base stations, high-altitude platform relay nodes and satellite nodes, and specifically includes:

[0052] Step S101: The ground base station sends a radio frequency signal

[0053] The base station used in the present invention is a 5G base station, the core device of the 5G network, which can provide wireless coverage, realize wireless signal transmission between the wired communication network and the wireless terminal, and transmit modulated radio frequency signals through the base station.

[0054] Step S102: N different reconfigurable surfaces of RIS independently optimize the phase and amplitude of the reflected signal

[0055] The encoded RF signal is sent to the RIS using the base station antenna and reflected by the N different reconfigurable surfaces of the RIS. The RIS is an artificial electromagnetic material surface that can effectively control the reflected signal, such as phase, amplitude, and frequency, without the need for complex decoding and encoding processing operations. The RIS independently optimizes the phase and amplitude of the reflected signal, enabling each RF chain to achieve an N-stream virtual MIMO system. The base station is closely connected to the RIS, and the signal attenuation between the two is negligible.

[0056] Step S103: The RIS reflected signal is converted into an optical signal and sent to the satellite node

[0057] The high-altitude platform relay node receives the signal reflected by the RIS, converts the RF signal into an optical signal through an optoelectronic converter and sends it to the satellite node.

[0058] In the present invention, since the channel links between the RIS-assisted base station node-high-altitude platform relay node and the high-altitude platform relay node-satellite node are independent of each other, the channel state information of the RIS-assisted base station node-high-altitude platform relay node is obtained at the high-altitude platform relay node, and when the channel state meets the preset conditions, the RIS-assisted base station node transmits the radio frequency signal to the high-altitude platform node. The high-altitude platform relay node demodulates the radio frequency signal and converts the demodulated radio frequency signal into an optical signal through an optoelectronic converter. The relay node of the high-altitude platform is equipped with a device having two relay forwarding strategies of amplification and forwarding and decoding and forwarding. By comparing the effects of the two relay forwarding strategies on the interruption performance, bit error rate performance and traversal capacity performance of the system, a relay forwarding strategy with better performance is selected.

[0059] Step S104: The satellite node receives the optical signal sent by the high-altitude platform

[0060] The satellite node obtains the channel status information between the high-altitude platform relay node and the satellite node, and when the channel status meets the preset conditions, the high-altitude platform relay node transmits the optical signal to the satellite node. The satellite node is equipped with a receiver with an optical signal detection method with intensity modulation-direct detection and heterodyne detection. Based on the impact of the receiver with the optical signal detection method on the interruption performance, bit error rate performance and traversal capacity performance of the system, the optical signal detection receiver with better performance is selected to receive the optical signal forwarded by the high-altitude platform node.

[0061] The above is a detailed introduction to a RIS-assisted sky-ground integrated network relay transmission method and system provided by the present invention. This article uses specific examples to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method and core ideas of the present invention, and is not used to limit the patent of the present invention. All equivalent implementations of the present invention should be included in the scope of the claims of the patent of the present invention.

Claims

1. A RIS-assisted sky-ground integrated network relay transmission method, characterized in that: The relay transmission method uses the RIS-assisted ground base station node antenna to send the encoded radio frequency signal to the RIS, and the N different reconfigurable surfaces of the RIS adjust the amplitude and phase of the radio frequency signal and then reflect it. The high-altitude platform relay node receives the signal reflected by the RIS, converts the radio frequency signal into an optical signal through a photoelectric converter, and sends it to the target satellite node through an FSO channel. The target satellite node eliminates the interference signal from the received optical signal. The target satellite node receives the optical signal after the interference is eliminated, thereby realizing RIS-assisted sky-ground integrated network relay transmission.

2. The RIS-assisted sky-ground integrated network relay transmission method according to claim 1, characterized in that: The RIS-assisted ground base station node specifically includes: using RIS as a MIMO transmitter with low complexity and high energy saving, using the ground base station node feeder to send the encoded radio frequency signal to RIS, and reflecting through N different reconfigurable surfaces of RIS. The N different reconfigurable surfaces of RIS independently optimize the phase and amplitude of the reflected signal, so that a single radio frequency chain realizes an N-stream virtual MIMO system and the ground base station node is closely connected with RIS.

3. The RIS-assisted sky-ground integrated network relay transmission method according to claim 1, characterized in that: The channel state information of the high-altitude platform relay node and the target satellite node specifically includes: obtaining the channel state information from the RIS-assisted ground base station node to the high-altitude platform relay node at the high-altitude platform relay node, and transmitting the radio frequency signal when the channel state meets the preset conditions; obtaining the channel state information from the high-altitude platform relay node to the target satellite node at the target satellite node, and transmitting the optical signal when the channel state meets the preset conditions.

4. The RIS-assisted sky-ground integrated network relay transmission method according to claim 1 or 3, characterized in that: The transmission device of the high-altitude platform relay node specifically includes: the relay node of the high-altitude platform is equipped with a device with two relay forwarding strategies of amplification and forwarding and decoding and forwarding. Based on the comparison of the impact of the two relay forwarding strategies on the system's interruption performance, bit error rate performance and traversal capacity performance, a relay forwarding strategy with better performance is selected.

5. The RIS-assisted sky-ground integrated network relay transmission method according to claim 1 or 3, characterized in that: The detection method of the target satellite node optical signal specifically includes: the target satellite node is equipped with a receiver with an optical signal detection method having intensity modulation-direct detection and heterodyne detection, and based on the influence of the receiver with the optical signal detection method on the interruption performance, bit error rate performance and ergodic capacity performance of the system, a better optical signal detection receiver is selected to receive the optical signal forwarded by the high-altitude platform relay node.

6. The RIS-assisted sky-ground integrated network relay transmission method according to claim 1 or 3, characterized in that: The link from the high-altitude platform relay node to the satellite node obeys the Malaga distribution, and the link from the RIS-assisted ground base station node to the high-altitude platform relay node adopts the shadow Rice distribution model to accurately characterize the impact of shadow conditions on signal transmission in the air-to-ground channel.

7. The RIS-assisted sky-ground integrated network relay transmission method according to claim 6, characterized in that: The RIS-assisted ground base station node to high-altitude platform relay node link is solved by a small signal parameter approximation method to construct a channel model of the RIS-assisted RF link. The small signal parameter approximation method specifically includes: the instantaneous signal-to-noise ratio γ1 of the RF link is expressed by the following formula: in, is the average signal-to-noise ratio of the RF link; N is the number of RIS reflective elements; ε 1,k is a shadow Ricean random variable; δ 1,k and φ 1,k are the amplitude and phase of the signal respectively; ψ 1,k is the phase changed by RIS; after RIS adjusts the phase, let ψ 1,k =φ 1,k , the instantaneous signal-to-noise ratio of the RF link is simplified to the following formula: The sum of squares of shadowed Rice random variables is approximately distributed as the sum of squares of shadowed Rice variables. Then the instantaneous signal-to-noise ratio of the RF link is expressed by the following formula:

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