Research On Blind Processing Of GNSS Multipath Signals In Urban Environment

The Global Navigation Satellite System(GNSS)has now developed into an indispensable infrastructure for human social life.In recent years,the public life field is becoming a new growth point in the satellite navigation industry.However,in a complex urban environment,signal power attenuation and multipath effects have become major factors affecting GNSS receiver performance.The multipath effect is mainly caused by the reflection of buildings near the antenna and the diffracted signals from different paths.This may cause interference between the paths,resulting in ranging errors of up to tens of meters or even higher,seriously affect the positioning accuracy and reliability of the system.This topic takes GNSS signals in urban environment,especially Beidou signals as the research object,considering that it is difficult to obtain prior information accurately in practical applications,and focuses on the use of blind processing technology to solve the problem of multipath interference in navigation signals.First of all,the topic takes the Beidou signal as an example to study the structure,components and modulation of the GNSS signal,establishes a simulation mathematical model of the Beidou intermediate frequency B1 signal,and analyzes the various factors that affect the Beidou multipath model.The correctness of the model is analyzed and verified from the perspective of probability and statistics.Based on the research needs of the project,an array signal reception model is introduced on the basis of the foregoing work and the simulation data is captured and estimated the Direction of Arrival(DOA).The DOA estimation verifies the correctness of the signal simulation scheme and satisfies the needs of subsequent research objects.Secondly,in order to provide the direction of arrival information for the beamforming operation,the paper first uses the representative MUSIC algorithm as an example to analyze the DOA estimation algorithm,and then analyzes the principle,characteristics and applicable conditions of Independent Component Analysis(ICA)algorithm.Based on the above,a DOA estimation scheme based on ICA algorithm is proposed and compared with the MUSIC algorithm.The results show that the performance of the proposed scheme is obviously better than that of the traditional MUSIC algorithm.Thirdly,on the basis of the foregoing work,the paper has conducted in-depth research on the DOA estimation method for multipath signals.By introducing the idea of sparse signal reconstruction in compressive sensing,theory and combining it with ICA algorithms,a new one is proposed.The DOA method for direct and multipath components can be estimated and compared with the classical spatial smoothing combined with MUSIC algorithm.The results show that the new scheme has better estimated performance than the classical algorithm and can be used to solve the underdetermined problem which other methods can't handle.In addition,because the two-dimensional DOA estimation is more common in practical applications,based on the newly proposed DOA estimation method for one-dimensional multipath signals,a two-dimensional multipath signal DOA estimation method based on L-shaped array is proposed.This method reduces the dimensionality of overcomplete dictionary in compressive sensing theory by defining the composite angle,and converts the two-dimensional DOA estimation problem into a one-dimensional problem,so that the computational complexity is greatly reduced.Simulation results demonstrate the effectiveness of the method.Then,because the GNSS received signal strength is weak in the urban environment,and the DOA estimation of the multipath signal requires a higher SNR of the received signal from the array,it is difficult to estimate the arrival direction of the signal and then suppress the multipath.This topic proposes a beamforming method based on post-correlation techniques based on the characteristics of the GNSS signal being processed.In this method,the beamforming algorithm is added to the tracking loop.The post-correlation technique is used to process the received signals on all the array elements.The signal-to-noise ratio is improved by using the spreading gain,and then the multipath DOA estimation is performed on the despread signal.The DOA estimation results are used to beamform the despread signal to achieve the purpose of suppressing multipath.Simulation results show that this method can effectively suppress short-path multipath,medium-length and long-path multipath interferences when the signal-to-noise ratio of satellite navigation signals is as low as-35 dB.The obtained results are not affected by multipath delay and multipath attenuation.The bit error rate is 0.Within the range of-35 to 0 dB of the signal to noise ratio of the received signal,the signal to noise ratio of the navigation message can be further increased by 4 to 6 dB after the spreading gain.Finally,based on the foregoing research,in order to give full play to the advantages of ICA technology in signal separation,this paper proposes the application of ICA technology to post-correlation multipath suppression.Similar to the post-correlation beamforming method,this method uses the ICA algorithm to directly extract the direct signal component from the spreading signal and achieve multipath suppression.Because the ICA algorithm itself has the advantage of filtering out noise,the signal-to-noise ratio of the navigation message is further improved after the spreading gain.The simulation results show that this scheme can effectively suppress multipath interference in the case of a signal-to-noise ratio as low as-35 dB,and also removes noise and system noise partly generated in the correlation process,resulting in great improvement of navigation signal quality.In the signal-to-noise ratio of the received signal range of-35 ~ 0dB,the signal-to-noise ratio of the navigation message can be further increased by 5 to 11 dB after the spread gain.Because it is also based on the post-correlation technique,this scheme and the previous post-correlation processing scheme can be used for multipath suppression in a GNSS weak signal environment,and it is simpler to implement and will not introduce DOA estimation errors.Although this scheme can not form a gain in the desired direction,the ICA method can use the characteristics of the algorithm itself to filter out noise,so it is slightly better than the former in improving the signal quality and has better multipath signal suppression ability.