| Integrated satellite-terrestrial network(ISTN)combines the advantages of satellite network and terrestrial wireless network,which is considered as a key technology of the sixth generation(6G)mobile communication system.However,in order to meet the urgent needs of ubiquitous access and the Internet of Things,future wireless communication networks will face rigorous challenges,which are brought by technical standard such as more user access,higher spectral efficiency,and higher transmission rates.High efficient and flexible multiple access technique is one of the effective measures to solve these problems.Recently,non-orthogonal multiple access(NOMA)technology can support massive access with limited radio resources and significantly improve spectral efficiency,and is considered as the potential key technologies in wireless communication systems.Meanwhile,multi-antenna technology can make full use of space resources.The combination of multi-antenna technology and NOMA technology will significantly improve wireless commnunication system performance,which attracts lots of attention from both academic and industrial circles.Due to the characteristics of diverse scenarios and complex environments of wireless communication,for multi-user transmission scenarios in ISTN,this thesis researches about transmission strategy and evaluates the system performance,which provides technical support for improving the spectral efficiency and transmission reliability of satellite communication systems.The main work of this thesis is summarized as:(1)In the multi-user downlink ISTN with the amplify-and-forward(AF)relay protocol,multi-user scheduling algorithm with the threshold-based feedback scheme is proposed.Then,by assuming that the satellite link experiences Shadowing-Rician fading while the terrestrial links undergo Rayleigh fading,the closed-form expressions for outage probability,ergodic capacity and the average number of feedback users are derived based on the proposed scheme.Finally,computer simulation not only verifies the correctness of the analytical results,but also proves that multi-user scheduling algorithm with the threshold-based feedback scheme can reduce the amount of feedback greatly while the system performance is almost undiminished.(2)In the multi-user downlink unmanned aerial vehicle(UAV)-assisted ISTN in the presence of interference,multi-user transmission strategy based on statistical channel state information(SCSI)is proposed,and the system ergodic sum rate(ESR)is analyzed.In the ISTN,the UAV achieves the multi-user access with space division multiple access(SDMA)technology,which is also corrupted by co-channel interference.To maximize the ESR of the considered system,based on the signal-to-leakage-and-noise ratio(SLNR)and generalized Rayleigh quotient,a beamforming(BF)scheme based on SCSI is proposed.Then,by assuming that the satellite-UAV link and the UAV-terrestrial links undergo correlated Shadowed-Rician fading and correlated Rayleigh fading,respectively,the analytical expression of ESR is derived with the proposed transmission strategy.Finally,the numerical results are provided to verify the correctness of the theoretical analysis and demonstrate the superiority of our proposed scheme.(3)In the multi-user uplink mixed radio frequency/free space optical(RF/FSO)ISTN,the multi-user transmission strategy combining the NOMA technology with BF scheme is proposed.Furthermore,the system sum rate is investigated.Based on the SCSI of users,in the case of the uplink NOMA with imperfect successive interference cancellation(SIC),the BF scheme is proposed to obtain maximum sum rate.Then,by assuming that the terrestrial-UAV links and the UAV-satellite link undergoes correlated Rayleigh fading and Gamma-Gamma fading,respectively,the analytical expression of ESR is derived with the proposed multi-user transmission strategy.Finally,numerical simulation experiments are conducted.The results demonstrate that the proposed scheme improves the system performance remarkably with comparison to orthogonal multiple access(OMA)transmission strategy.(4)In the uplink multi-user ISTN,the system security performance is studied with the semi-grant free(SGF)NOMA transmission strategy.Firstly,after multiple users are divided into multiple NOMA groups with grouping scheme,there is a grant based user(GBU)and some grant free users(GFUs)in one group.Secondly,as there is an eavesdropper in the system,combining the uplink NOMA technology and SDMA technology,the SNR for users is obtained.Then,to maximize the achievable rate of the GBU,the suboptimal BF scheme based on SCSI is proposed.Furthermore,the closed-form expressions of ergodic secrecy sum rate for GBUs and GFUs are derived.Finally,the numerical results not only verify the correctness of the theoretical derivation,but also show that the proposed transmission strategy enhances the security sum rate for GBU and GFUs. |
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