Research On Symbol Timing Synchronization Technology Applicable To Covert Communication System

Covert communication technology has received more and more attention in recent years,because it can effectively ensure that the communication process is not discovered by eavesdroppers,solving the problem of secure transmission of secret content,and meeting the more stringent requirements of today’s users for wireless communication security.However,since the covert communication signal is completely submerged by noise,covert transmission needs to be completed at a very low signal-to-noise ratio.Traditional synchronization methods are difficult to synchronize such signals,and new synchronization techniques are urgently needed.On a given covert communication scheme,this paper conducts a preliminary study on the symbol timing synchronization of covert communication.Firstly,this paper introduces a covert communication scheme based on the combination of QC-LDPC and symbol spread spectrum,and analyzes the problems existing in traditional synchronization methods.On this basis,a synchronization scheme based on spread spectrum training sequence and a correlation synchronization determination method based on Euclidean Distance（ED）and Posterior Probability（PP）are designed,namely EDC Synchronization method and PPC synchronization method.The scheme firstly transmits the training symbols by spread spectrum,finds the timing synchronization point by sliding despreading at the receiving end,uses the Euclidean distance and a posteriori probability to compute the similarity between the despread training symbols and the known training symbols,and determines Timing synchronization point.Secondly,a relatively comprehensive simulation and comparison of the synchronization performance of the EDC synchronization method and the PPC synchronization method is carried out.The simulation results show that when the number of sub-carriers and the spreading factor are 576,the EDC and PPC synchronization methods can achieve the correct acquisition rate of timing start point greater than or equal to 90%at the symbol signal-to-noise ratio of-18 d B and-20 d B,respectively.It can be seen that the PPC method can achieve the same synchronization performance when the signal-to-noise ratio is reduced by 2 d B compared with the EDC,which proves that the PPC method is more advantageous for the symbol timing synchronization of the low detection probability signal.Finally,the above two synchronization methods are used in a covert communication scheme based on the combination of QC-LDPC and symbol spread spectrum,and the synchronization performance is simulated and analyzed.On the basis of satisfying the system reliability(bit error rate of the receiver is lower than 10^5-)and concealment（the error detection probability of illegal monitors is at least 89%）,the EDC method can be used when the signal-to-noise ratio is arbitrarily greater than-17.5 d B and a small frequency offset,it can meet the system reliability and the false detection probability of illegal monitors is84.9%.When the PPC method satisfies the system reliability,the false detection probability of illegal monitors is 89.9%under any signal-to-noise ratio greater than-19.25 d B,and the frequency offset adaptability is better.Comparing the two synchronization methods in the system experiment results,the PPC symbol timing synchronization method is more suitable for covert communication systems.