Research On Performance Enhancement Technologies Of Space-Air-Ground Integrated Network Based On Reconfigurable Intelligent Surface

With the rapid development of communication technologies today,wireless communication has become a basic demand of modern society.It is predicted that by 2030,the global Internet data traffic will reach 5016 Ebytes and the data rate will expand to 1 Tbps.While the exploration of the 5th generation communications has led to many significant achievements in terrestrial communications in recent years,providing users with higher data rates and lower latency networks,this service is still not available to users located in remote or complex areas due to limited network capacity and coverage areas.For this reason,the industry has proposed to extend the terrestrial network to the air and even space to form the space-air-ground integrated network(SAGIN),in which the communication nodes include satellites,unmanned aerial vehicles(UAVs),ground base stations,and other communication devices from space across the atmosphere to the ground,and the links therein contain both links between nodes in the same layer and cross-layer links between nodes in different layers.Moreover,the design of the previous five generations of wireless networks followed the principle that channel was not artificially controllable and could only be compensated by designing complex transmission and reception schemes.However,improvements only on the transmitting and receiving sides do not meet the ultra-high demands placed on wireless networks by present and future society,in addition to greatly increasing system complexity.In the study of the 6th generation communications,researchers have proposed a new technology that can regulate the communication environment,namely the reconfigurable intelligent surface(RIS).RIS is usually a panel consisting of several reflecting elements arranged in a regular pattern,and is popular because reflecting elements can reconfigure the wireless environment.Specifically,by adjusting the amplitude and phase of the coefficient,each element in RIS can change the intensity and direction of the corresponding incident wave.Depending on whether active devices are used or not,reflecting elements are classified into active or passive ones.The passive elements can only weaken or maintain the intensity of the incident signal,while the active elements can amplify the signal at the cost of increasing energy consumption and introducing noise.In this thesis,we focus on two key technologies,SAGIN and RIS,and investigate the performance enhancement technologies of SAGIN based on RIS.Firstly,inspired by the concept of index modulation,a new general RIS-aided receive quadrature reflecting modulation(RISRQRM)for improving system reliability and effectiveness is proposed.In RIS-RQRM,the sent information bits are divided into two equal parts,and each equal bit part is composed of two parts: one part is called index bit,which is mapped to the receiving antenna index,and the other is called spatial bit,which is mapped to the in-phase or inverted state of the received signal.At the same time,the RIS elements are divided into two equal parts,which are controlled by two equal transmitting bits,respectively.Specifically,based on the channel state information(CSI)between the transmitter and the receiving antenna selected by the index bits,the RIS coefficients are adjusted so that the reflected signals are directed to the specific receiving antenna and then the received signal is kept in phase or becomes inverted based on the spatial bit.Based on the beamforming rules of RIS-RQRM,a low-complexity noncoherent detector that does not require CSI is also proposed in this thesis.The inclusion of RIS enhances the effectiveness(spectral efficiency)of the system compared to conventional communication systems.In addition,the theoretical bit error rate(BER)using the RIS-RQRM modulation principle and the proposed detector is analyzed.Simulation results verify the theoretical analysis and demonstrate that the reliability(BER)of this network is better than that of its existing counterparts at low signal-tonoise ratios.Secondly,wireless communication is essential in SAGIN due to the long distance,but its broadcast characteristics may lead to the secret signal being captured and decoded by untrusted receiving devices.In SAGIN,wireless communication is often used to transmit critical information,such as flight plans,flight status,etc.,the security of which is directly related to the safety of air traffic.Therefore,it is critical for SAGIN to improve security.At the same time,communication between satellites and UAVs is important in many application scenarios,such as military reconnaissance,disaster monitoring,and resource exploitation.In this thesis,the communication scenario between satellite and UAV is taken as an example to discuss the security of RIS-assisted space-air network.Specifically,because of the periodic movement of the satellite,direct communication between it and the UAV is extremely rare.Therefore,a scheme that uses a high-altitude platform-carried RIS to relay between them is proposed in this thesis.Considering that high-frequency waves can bring high data rate to the system and maintain signal stability and reliability in SAGIN,this thesis relies on terahertz(THz)waves for communication.The approximate expression of ergodic secrecy rate is obtained when taking the attenuation of the THz wave caused by atmosphere turbulence,the pointing error caused by the mutual jitter between the narrow THz beam and the limited receive area of receiver,the phase error caused by the limited adjustment precision of RIS elements,and the phase error from the imperfect channel estimation into consideration,in the presence of an eavesdropping UAV near the legitimate UAV.Besides,under the conditions that only statistical CSI of the eavesdropping UAV can be obtained and its perfect CSI can be obtained,the lower bound of secrecy rate(SR)and instantaneous SR are maximized by optimizing the coefficients of RIS,respectively.The simulation results confirm the accuracy and feasibility of the theoretical analysis and optimization algorithm,and demonstrate that even when the jitter standard deviation of the legitimate UAV is larger than that of the eavesdropping UAV,the lower bound of SR and the instantaneous SR can still be guaranteed to be positive by optimizing the reflect coefficients.Finally,for communication networks,the methods to improve system reliability include increasing transmit power and using more efficient detection algorithms.However,when untrusted receivers exist,the above methods will also increase the channel quality of untrusted links,resulting in increased security threats to the system.In other words,system reliability and security are coupled to each other,and it is important to discuss the relationship between them.Compared with the terrestrial network,SAGIN has the advantage that the addition of aircraft can effectively expand the communication range and improve the communication quality.Therefor,this thesis takes air-ground cooperative network as an example to discuss system security and reliability.In the system,several half-duplex decode-and-forward relays located on the ground are used to assist terrestrial transmitters and receivers who cannot communicate directly due to environmental factors,while an active RIS is mounted on the hovering UAV to augment the freedom of the system.Each symbol transmission takes up two time slots,and if at least one relay succeeds in the detection in the first time slot,one of them is selected for signal forwarding in the second time slot; otherwise,all of them remain stationary in the second time slot.Similarly,the reflecting elements can be adjusted in each time slot,and the receiver can use the received signals of both time slots to make a selection combining before detection.In this thesis,the outage probability and intercept probability of the system are analyzed when considering the case of a legitimate user and an eavesdropping user at the same time.The simulation experiments verify the theoretical analysis and show that the active RIS can better promote the system to attain a good security and reliability tradeoff(SRT).Moreover,the optimal location of the RIS is obtained based on the simulation results of SRT.