| The shortwave communication has the characteristics of strong anti-destructive ability and long transmission distance.It plays a vital role in regional emergency communications and military communications.Due to the rapid changes in the ionosphere and severe signal fading,shortwave communications have the difficult problems in signal synchronization detection and low synchronization accuracy.This thesis studies the synchronization and diversity reception technology of shortwave narrowband communication systems,and improves the communication quality of shortwave communication by improving the synchronization detection ability and signal anti-fading performance of the communication system.The specific research contents are as follows:Aiming at the problems of low signal-to-noise ratio and strong multipath propagation that cause shortwave signal synchronization detection difficulties and low synchronization accuracy,this thesis researched a time-frequency joint synchronization detection method based on Costas sequence.This method uses the excellent autocorrelation characteristics represented by the Costas sequence matrix for signal detection and synchronization,and extends the traditional sliding window method from the time domain to the time-frequency domain,which can obtain the estimated value of time and frequency synchronously.At the same time,in order to reduce the amount of calculation,a precise estimation method using discrete spectrum analysis is given.The simulation results show that,compared with the traditional PN code synchronization method,the method adopted in this thesis has better anti-noise and anti-frequency offset performance.On the mid-latitude shortwave channel given by the International Telecommunication Union（synchronization code length is 1.28s）,this synchronization method can achieve a detection rate of 98%(the false alarm probability is less than 10-2),the time synchronization error does not exceed 12ms,and the frequency estimation error is up to about 1 Hz.Aiming at the problem of poor estimation accuracy caused by the spectral line positioning error of the discrete spectrum analysis method,this thesis proposes an improvement scheme based on iterative optimization.When the maximum number of iterations is limited to 10,the time estimation error is reduce to 5ms,and the frequency estimation error is reduce to 0.2Hz.In order to further improve the gain of diversity receiving technology,this thesis studies a diversity combining scheme based on Bayesian data fusion,which extracts soft symbol information based on the time spectrum of the synchronized signal,and converts the soft symbol information into probability through the maximum likelihood criterion.Then Bayesian fusion is performed on the multi-channel signals.The simulation results show that the gain of the double diversity scheme based on Bayesian data fusion in the short-wave harsh channel is about 10d B(the bit error rate is 10-2).In addition,the error leveling phenomenon in the single-channel demodulation and decoding in the bad shortwave channel has also been improved.Compared with Equal Gain Combining,the method proposed in this thesis is about 3d B higher(bit error rate is 10-2).Finally,this thesis has carried out practical tests on the researched synchronization method and diversity combining scheme,selected two communication links from Chongqing University of Posts and Telecommunications to Youyang and Chongqing University of Posts and Telecommunications to Wanzhou as the actual communication links.At sunrise and sunset times when the ionosphere changes drastically,he bit error rate after double classification is an order of magnitude lower than that of a single channel signal,and the average bit error rate is about 10-3.At the same time,the signal detection rate has reached 94.34% and 98.26%. |
PDF Full Text Request