Research On Synchronization And Access Technologies For Satellite OFDM Systems

OFDM(Orthogonal Frequency Division Multiplexing)technology has a high spectrum utilization efficiency,which can effectively combat frequency selective fading and is suitable for asymmetric data transmission.OFDM technology is applied to satellite mobile communication systems,which can improve spectrum efficiency and increase system capacity.Due to the high-speed movement characteristics of low-earth-orbit satellites,in order to maintain the orthogonality of OFDM,relevant measures need to be taken in the satellite communication system to resist the influence of frequency and timing deviation.OFDM synchronization technology currently focuses on the study of frequency offset estimation and timing offset estimation.Position information compensation and frame structure-based subcarrier spacing adjustment are also currently hot topics.On the other hand,in order to meet the growing demand for Internet of Things access and reduce the access delay of a large number of ground terminals,the scarce time-frequency resources in satellite communication systems need to be fully utilized.Therefore,this thesis studies the synchronization technology and access technology under high Doppler frequency offset.This thesis first analyzes the current classic OFDM synchronization technology,and then proposes a weighted sequence synchronization method that resists frequency offset.By weighting the PN sequence and the synchronization sequence,using differential cross-correlation and a series of frequency offset estimation algorithms,better synchronization performance is got.With reference to the algorithm for estimating uplink timing advance and Doppler frequency offset based on position information,considering the effects of residual frequency offset,satellite beam position,and terminal motion speed,the optimal configuration of the satellite communication subcarrier interval in the Ka band is analyzed.In the process of random access of low-earth-orbit satellites,the joint transmission algorithm of preamble sequence and control information that can achieve fast access is studied.By analyzing the traditional joint transmission and superimposed training sequence transmission methods,an implicit data transmission based on weighted sequences is proposed.This method uses the cyclic shift value of the constant envelope zero auto-correlation CAZAC(Const Amplitude Zero Auto-Corelation)sequence to represent information data,and can guarantee the orthogonality between the hidden data and the synchronization sequence.Compared with traditional methods,it can simultaneously meet the needs of synchronization and data transmission.