Research On High Energy Efficiency Waveform Design And Robust Beamforming Technology For Satellite Mobile Communication System

The satellite mobile communication systems can provide stable and reliable communication services for the areas that cannot be covered by the terrestrial communication network(such as oceans,deserts,etc.)and are not easily affected by natural disasters,complex terrain,and other factors on the ground.Therefore,the satellite mobile communication network can be an important supplement and extension of the terrestrial communication network.Furthermore,the integration of the satellite mobile communication system and the ground fifth-generation(5G)mobile communication system can achieve complementary advantages to improve the quality of communication services on a global scale.This paper focuses on the problems of the power-efficiency reduction caused by the limited on-board power and the large envelope fluctuation of the transmitting waveforms in the multi-beam satellite communication systems,as well as the transmission performance degradation due to the long satellite-to-ground communication link,positioning angle error and attitude jitter of the satellites,and studies the design of high-power-efficient transmission waveforms and robust downlink beamforming for the satellite mobile communications.Firstly,the transmission waveforms and the downlink beamforming technology of the multi-beam satellite communication system are analyzed.The service use cases of satellite communication systems in 5G application scenarios and the multi-beam satellite network access architecture are reviewed.On this basis,the power efficiency problems faced by satellite communication systems using cyclic-prefixed orthogonal frequency division multiple access(CP-OFDMA)waveforms are analyzed,and the signal models and modulation structures in the CP-OFDMA framework are given.Besides,the difficult problems to be solved in the multi-beam satellite downlink beamforming technology and the commonly used optimization design methods of satellite downlink beamformers are analyzed,and the fully-digital beamforming architecture,the analog beamforming architecture,and the hybrid beamforming architecture are analyzed and compared.Secondly,to meet the new requirements of high power efficiency applications in future wireless communications while maintaining a compatible implementation structure,the design of the constant envelope waveform in the CP-OFDMA framework is investigated.Starting from a general FDMA signal model with constant envelope property,a compatible implementation structure in the CP-OFDMA framework is derived.As the equivalent pulse-shaping filter in the frequency domain for constant envelope waveforms becomes no longer strictly band-limited,the frequency domain filters are properly designed to alleviate the multiple access interference(MAI).A direct way to avoid residual multiple access interference is to allocate enough guard band between users,which leads to the loss of the spectrum efficiency.In order to achieve constant envelope waveform while avoiding the spectrum efficiency loss,an alternative subcarrier mapping scheme is further proposed,where the adjacent users have a partial overlap of frequency bands.Unlike the existing constant envelope OFDM waveforms that require a complicated non-linear receiver,the proposed schemes allow a low-complexity frequency domain widely linear receiver to combat both inter-symbol-interference(ISI)and MAI.Simulation results verify that the proposed waveform combined with an iterative receiver can achieve high power efficiency with negligible loss of spectrum efficiency and bit error rate performance.Then,the optimal design of the robust downlink beamforming under per antenna power constraint(PAPC)in multi-beam low-earth-orbit(LEO)mobile satellite system is studied.Because of the strong spatial directivity in the satellite channels,the downlink beamforming can be efficiently designed based on the angle of departure(Ao D)-based channel state information(CSI).However,acquiring perfect Ao D-based CSI is challenging due to the angle estimation error and attitude jitter of the LEO satellites.In light of this,the beamform design for the LEO satellite downlinks with imperfect Ao D-based CSI is investigated.For robust design,the angle deviations of each user are treated as random variables,which are supposed to exhibit a particular distribution and integrated within a certain range to achieve the robustness of the proposed downlink beamforming method against the angle mismatch.Instead of using the sum-power constraint on the transmit antennas,a more realistic per-antenna power constraint is adopted.Accordingly,the ergodic sum-rate maximization problem of the system is proposed.By leveraging the corresponding Lagrangian formulation and identifying the optimal beamforming structure as the solution to a generalized eigenvalue problem,an iterative algorithm for the robust beamforming design is derived.To reduce the computational complexity,a low-complexity version of the proposed algorithm is implemented based on deep learning techniques to meet the high demand for real-time applications in LEO satellite systems.Simulation results verify the effectiveness of the proposed beamforming approach.Finally,the optimal design of the robust hybrid beamforming based on the low-resolution phase shifters(PSs)under PAPC for the high-frequency multi-beam LEO satellite system is studied.The transmission model for the multi-beam LEO satellite downlinks under the framework of the partially-connected architecture is established.On this basis,the ergodic sum-rate maximization problem under the PAPC and the discrete phase constraint is established.Since the considered sum-rate maximization problem is a non-convex optimization problem and involves two optimization variables,the digital beamforming matrix and the analog beamforming matrix,it is difficult to obtain the optimal solution directly.To address this problem,the sumrate maximization problem is transformed into a quality of service problem through reasonable equivalence and approximation,and the digital beamforming matrix and the analog beamforming matrix are designed by alternate iterative optimization.By leveraging the corresponding Lagrangian formulations and modeling the optimal digital beamforming matrix and the optimal analog beamforming matrix as the solution to a generalized eigenvalue problem,respectively,an alternate iterative design algorithm for the robust hybrid beamforming design is derived.According to the derived channel autocorrelation matrix expression based on the distributions of angle deviations,the proposed hybrid beamforming algorithm achieves robustness to the angle mismatch of each user.Simulation results verify the proposed hybrid beamforming approach’s effectiveness and robustness to the imperfect Ao D-based CSI.