| In recent years,the idea of providing internet access from space has resurfaced in order to meet the growing global demand for high-speed communication.The key advantage of non-ground networks lies in their ability to offer extensive global coverage,thereby improving and strengthening the coverage of ground networks in low-coverage areas such as air,sea,forests,and remote regions.In the realm of network communication,Low Earth orbit(LEO)communication satellite clusters have regained prominence due to their unique features and technological benefits.To provide faster and more affordable communication resources,bespoke satellites are required,with beam coverage design being a crucial aspect of user customization.This design can not only achieve centralized communication,but also facilitate flexible beam coverage,making it an essential component in many disciplines related to electromagnetic waves,including phased array ultrasonic waves,optics,and high-speed wireless communication.As we approach the next generation of communication,improvements in flexibility and throughput of large-scale satellite internet are expected to occur at a rapid pace.However,due to limited radio frequency resources,these systems tend to utilize millimeter wave bands.The present thesis examines the beam coverage technologies for future large-scale satellite constellation networks based on user distribution variability.Due to the constraints of low Earth orbit satellite communication,challenges associated with current beamforming techniques are analyzed,including broadband beam obliquity and high-precision beam scanning.Additionally,the thesis delves into an in-depth study of optical control array signals for the next generation of space multiplexing technology,which is based on an optical microwave processor for RF orbital angular momentum beam generation.This technology still faces challenges related to mutual interference between modes caused by phase instability.Furthermore,the complexity and power consumption of such large phased arrays represent key issues that must be addressed.To overcome these challenges,this thesis presents research content that includes:1)By combining the user traffic demand model and the LEO satellite beam coverage model,this paper analyzes the impact of beam coverage characteristics on the performance of LEO satellite systems.The user traffic model is based on the simulated distribution of users with uniform and normal distribution,and also on the geographical distribution of users based on historical AIS and ADS-B data acquired by real-time LEO satellites.Based on this user distribution and traffic demand model,beam coverage modeling of One Web(representing fixed beam coverage)and Space X(representing flexible beam coverage)LEO satellites is carried out in this thesis with the available information.The variation of user link performance caused by atmospheric conditions is also considered.Based on these models,this paper simulates these two LEO satellites in terms of throughput,delay,and access probability,and finally summarizes and discusses the sensitivity of beam coverage to user diversity.2)An optically controlled phased array using an array of waveguide gratings for beamforming assisted by millimeter waves has unique advantages in a low-orbit communication satellite network with interplanetary links.It can overcome the strabismus problem of broadband beamforming by using real time delay lines.It is also combined with the advantage of achieving ultra-high precision beam scanning interval(beam switching interval <1.8°,beam scanning range ±40°,no beam squinting for2 GHz broadband signal during scanning)with the help of high precision RF phase shifter.In this thesis,an optical-controlled millimeter-wave phased-array-based beamforming system is proposed and verified by theoretical simulation and experiment.Further exploiting the compatibility of the system in multi-hop transparent transmission LEO satellites,the advantage of the system is that the coverage of the beam can be adjusted by WDM,where one wavelength of the array corresponds to one coverage scan angle.This optically controlled beamforming system reduces the complexity of implementing hopping beams in transparent transmission LEO satellites and extends the flexibility of transparent transmission LEO satellite networks.3)The vector synthesis optical phased array technique is proposed,and the vector synthesis technique can greatly reduce the number of controllers.The technique can be implemented by optical power management of two passive delay networks and does not require a phase shifter,and the feasibility of the scheme has been tested experimentally.The technique reduces the complexity of the on-board payload and indirectly allows the two vector beams to achieve a beam scan within ±10° by adjusting the intensity of the light,and a beam scan accuracy of <0.4° can be achieved.4)A new scheme of RF angular momentum generation and beam steering system based on dual wavelength control using a circular antenna array is proposed,using an integrated chip containing an array waveguide grating structure and a microwave photon phase shifter,by which microwave photon phase shift and optical true time delay line can be realized.Demonstrates the variation of RF-OAM beam direction from0° to 14° in the E-plane and H-plane.The simulation results verify the steering capability of the RF-OAM beam generated by the photonic chip-based dual-wavelength control scheme in the two-dimensional direction.It also overcomes the problem of mode mixing and pointing shift of RF-OAM beams caused by the introduction of phase shift by conventional RF devices when the RF-OAM beam transmits broadband signals.5)A new reduced-dimensional phased-array antenna is proposed,which innovatively maps the beam pointing and phased-array weights in a matrix.This matrix is compressed by using the concept of singular value product in order to reduce the number of active devices for large phased arrays.The thesis simulate the matrix compression process to find out the constraints and optimization objectives of the theory in practical applications,and use the particle swarm optimization algorithm and deep neural network Transformer to optimize and accelerate the computation of the compressed matrix.Based on this theoretical framework,a 4×16 RF phase shifting experimental board is designed to verify the theoretical framework.And the rationality of the theory is demonstrated by the beam pointing accuracy and the side flap height under different compression ratios(16 arrays/4 phase shifters,8 array units/3 phase shifters,4 arrays/2 phase shifters). |
