Research On Carrier Synchronization Technology Of LEO Satellite Spread Spectrum Communication System

With the development of satellite technology,satellite communication has gradually begun to affect all aspects of people's life.The low orbit satellite has the characteristics such as short path delay,low power consumption and multi-satellite network.And it can combine with ground mobile communication technologies such as cellular,multiple access and beamforming.This dissertation is about the low earth orbit satellite spread spectrum communication system's physical layer.And I design the downlink system and study the carrier synchronization for hardware implementation in the future.The key research is about how the system modular influence carrier synchronization,and how to synchronize under the influence of doppler.Mobile communication is developing rapidly,so the design can refer to ground mobile communication technologies.This system choose orthogonal variable spreading factor as spreading codes and Gold sequence as scrambling codes.In addition to the user data channel,the system includes the synchronous channel and pilot channel for the system's acquisition and tracking.This dissertation mainly carry out the research of carrier synchronization which is based on the design of the system.Based on the two-dimensional time and frequency search,I use fast fourier transformation for preliminary estimation of frequency offset.After analysis of common phase and frequency locked loop,I choose two-order phase and frequency locked loop to track the carrier.Because the system have some influence on carrier synchronization,so I design one way to eliminate the interference.Based on the design of the system,system modeling is done by simulink's s function which is parallel processing.Then the research for the influence of carrier synchronization include the factors of the shape matching filter and the multi-logical channel.I test the system with carrier synchronization modular,and the results show that the two order frequency locked loop and phase locked loop can solve the doppler problem.Frequency locked loop interference cancellation algorithm can enhance synchronization performance in the system,and the system's bit error rate is slightly bigger than theoretical value.but the trend is same as theoretical value and it can meet the requirements to guide the actual hardware system design.