| With the development of communication technology,ground communication cannot meet the requirement of global seamless coverage.Satellite communication has become a global communication network in the future due to its superiority such as easy to setup,flexibility,widely covered,unconstrained by environment and so on.Among different types of satellite network,Low Earth Orbit(LEO)satellites have caused extensive concern due to the small transmission loss,short communication delay,low dissipation and so on.With the gradual development of 5G technology,3GPP has listed satellite access as one of 5G multiple access technologies.The convergence of satellite communication and terrestrial network has become the trend of future mobile communication development,so it is of great significance to construct a satellite mobile communication system for 5G.This thesis fouces on the uplink transmission technology based on 5G for LEO satellite mobile communication systems.First,the research on LEO satellite mobile communication channel is presented.Combined with the transmission scenario of LEO satellite mobile communication system,the effects of large-scale fading,small-scale fading and large Doppler frequency shift are investigated,and the relevant channel model is presented.Then,the low PAPR transmission method for LEO satellite mobile communication systems uplink is investigated.Firstly,The commonly used types of PAPR suppression methods are introduced,which are signal distortion method,signal coding method and probability class method.Then,for the problem that the PAPR of the uplink transmission signal is too large,using the low PAPR characteristic of ?/2-BPSK modulation in 5G NR,a group modulation method based on ?/2-BPSK modulation is proposed.In this method,the idea is to split the high-order modulation signal from the transmitter into a linear combination of multiple ?/2-BPSK modulated signals,and separately transmit and modulate each group of signals and combine them after passing through the High Power Amplifier(HPA),so as to avoid the high PAPR signal passing through the HPA.Finally,the advantages and disadvantages of the above methods are analyzed and compared.The simulation results show that the group modulation method can effectively reduce the PAPR of the transmitted signal without affecting the system error and rate.Finally,the low complexity signal detection algorithm for the uplink of massive MIMO systems in LEO satellites is studied.Firstly,the system model of the uplink of the LEO satellite massive MIMO system is given,and three commonly used linear receivers include MF,ZF and MMSE receiver are introduced.For the problem of high computational complexity in the process of MMSE equalization of in massive MIMO systems,two simplified calculation methods are proposed respectively.The first method is to transform the high-dimensional matrix inversion problem into the solution of linear equations.Using the characteristics of channel approximate orthogonality in massive MIMO systems,the matrix iterative method is used to approximate the linear equations,and linear detections based on Richardson Iteration and SSOR iteration are introduced.The second method is to perform QR decomposition on the inversion matrix,and the QR decomposition method based on Gram-Schmidt orthogonalization and its improved algorithm are introduced.The complexity analysis and performance simulation results of the above methods are given.The results show that the first method replaces the computational complexity with the acceptable error performance loss;and the second method reduces the computational complexity of iterative algorithm under the premise that the bit error rate is not affected.In addition,an iterative receiver based on MMSE equalization is introduced.The steps of applying QR decomposition method to the MMSE iterative receiver are given.The complexity analysis and performance simulation results of the above methods are also given. |
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