| The Global Positioning System(GPS)is widely used worldwide due to its highprecision,all-weather and other characteristics.However,in indoor environments such as underground parking lots,tunnels or shopping malls with a large number of people,the signal for satellite is weak,and the signal cannot be accurately captured and tracked,which seriously affects the positioning requirements in special environments.As a ground transmitter capable of transmitting GPS-like signals,pseudolites have a wide range of wide coverage,strong adaptability,and can act in a variety of special environments,especially in complex indoor environments.Pseudolite systems can play an important role.This article is based on the traditional pseudolite signal system,according to the characteristics of the indoor environment,and the traditional signal system is improved,and the ultimate goal is to realize the pseudolite transmitter.The specific research works are as follows:(1)In view of the problems of multipath and near-far effects,the traditional pseudolite signal system is improved.In order to solve the problem of multipath effect in indoor environment,the code rate and modulation method of pseudolite signal were re-determined.The test results show that the pseudolite signal with a code rate of10.23 MHz and BOC(10,10)modulation has better anti-multipath performance than traditional pseudolite signals.Aiming at the near-far effect of pseudolites,a pulse signal modulation method is proposed to solve the problem.The pulse signal generation process is introduced in detail,and the principle of the pulse signal against the near-far effect is explained.The analysis results show that the pulse modulation method can effectively alleviate the pseudolite multipath effect.For the indoor environment,various system loss models are studied,and the link analysis of the pseudolite system is performed according to user needs to determine the range of action and signal power of the system.Finally determine the complete pseudolite signal system.(2)Build an experimental platform and design a pseudolite transmitter.Use Vivado platform to write FPGA partial logic on Zedboard development board.Through the design of the clock module,C/A code generator module,navigation message module,subcarrier generation module,pulse signal module,carrier generator module and modulation module,the generation of digital baseband signals is completed.In addition,the AD9361 radio frequency agile transceiver is used in conjunction with the Zedboard development board to realize high-speed digital-toanalog conversion and up-conversion process,and finally realize a complete pseudolite transmitter.(3)Use the integrated logic analyzer(ILA)to capture the clock signal,C/A code signal,spread spectrum code signal and modulation signal,etc.,to verify the accuracy of the output results of each module.Use IIO Oscilloscope spectrum oscilloscope to analyze the time domain and frequency domain of the received signal,and finally verify the accuracy and feasibility of the radio frequency signal of the pseudolite transmitter.The final result shows that the entire pseudolite signal transmitter can meet the design requirements.In this paper,through the design of the pseudolite signal system,a pseudolite signal simulator based on Zedboard and AD9361 is built.The test results show that the pseudolite signal simulator can successfully send a pseudolite signal with a radio frequency point of 2.4GHz,the spectrum bandwidth of the two main lobes is about40 MHz,and the signal duty cycle is 10%.The entire pseudolite signal transmitter can meet the design requirements. |
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