Direction Finding Method Research For Communication Signal Based On Of Time Reversal Focusing

In the complex electromagnetic environment,with a simple algorithm and precise performance to detect the communication signal and the direction of source is a goal for all countries in the world in the electronic direction finding equipment research field.As an important part in the electronic warfare,communication electronic warfare plays an important role for controlling the war.Communication and data link system are the infrastructure to support the information transmission system.And they are the nerve center to connect command and control centers to a variety of electronic warfare equipment,or connect the electronic warfare equipment and equipment.Once an important node of the communication system and data link is found,it will have been destructed and attacked fatally,even lead to the failure of the war directly.So detecting the direction of source that emits communication signal becomes a very important part in the modern electronic warfare,which cannot be ignored.The current methods for detection the direction of communication signal has been mainly includes the amplitude method,the phase method and the space spectrum direction method,but these methods have a lower precision or a larger amount of calculation to varying degrees.Using the environment adaptive focusing characteristic,time reversal technology is introduced into the field of detecting the direction of source,which can improve the performance of direction finding system in complex environment.Time reversal process is reverse the order of received signal on the time axis,then the time reversed signal is transmitted into the same propagation medium,and the focusing of signal energy is realized.Time reversal process will show more stable survival ability in the more complex environment,so it can effectively overcome the phase aberration and waveform distortion in the bad condition.Whether it is a homogeneous medium or inhomogeneous medium,the signal which is processed by time reversal technique has the synchronous focusing characteristic in time and space,this time-space focusing characteristic is environment adaptive,that is the focusing does not need the prior knowledge,such as the array distribution and the characteristics of transmission medium.So the time reversal technique has a huge potential advantage in the source direction finding field.Based on the classic time reversal theory,the time reversal focusing and communication signal direction finding method are studied.The time reversal direction finding method can accurately detect the direction of blind signal which without prior knowledge and the weak signal which with a low signal to noise ratio(SNR)in the complex electromagnetic environment.The main studies content as follows:In order to study the applicability of time reversal theory in the electromagnetic field,the electromagnetic time reversal detection method is proposed based on the basic principle of the classical time reversal theory.Firstly,the time reversal solution of plane electromagnetic wave equation is deduced based on the Maxwell equations,which verifies the time reversal invariance of the wave equation.According to the time reversal invariance of the wave equation,the basic principle of time reversal focusing is analyzed,the expression of target signal energy is deduced,and the algorithm steps of active time reversal focusing are given.In order to verify the effectiveness of this algorithm,the simulation experiment is carried out,and the simulation results verify the correctness and effectiveness of the algorithm.Then two kinds of focusing algorithm based on time reversal operator are given: decomposition of time reversal operator selective focusing algorithm and high resolution time reversal operator decomposition focusing algorithm.Eigenvalue decomposition of the time reversal operator is the basis of the two algorithms,and the number of eigenvalues is corresponding to the number of targets.The first method uses a single eigenvector to derive the focusing objective function and it can realize the selective focusing.The second method uses all noise eigenvectors to derive the focusing objective function and it can realize the focusing on each target.Compared two methods,more information about eigenvector is used in the second method than the first method,so it can get higher resolution.Simulation results verify the accuracy and effectiveness of these two methods.In order to realize the passive detection of source,a kind of passive time reversal focusing method is given.This method solves the problem of time reversal signal back propagation physically,changes the working mode,and it can work with a passive mode.In the passive time reversal focusing,the source signal is received and recorded by receiving array,and then the received signal is time reversed.Through the theoretical analysis,a conclusion is given: the time that the time reversed signal of n-th receiving antenna is maximum is only related to the emitted signal from source,and the time that the time reversed signal of every receiving antenna is maximum is same,the time reversed signal of every receiving antenna is maximum when the direction of observed point is same as direction of source.So the direction of source can be obtained by calculating the received time reversed signals of N antenna elements at each observed point and searching the maximum energy,and the direction of observed point that has the is the direction of source.So the physical transmission process of time reversed signal in the classical time reversal method can be instead of using the time reversed signal to calculate the received signal energy of every observed point in the given area,and the virtual focusing on virtual source is passive.Finally,the effectiveness of the algorithm is proved by experiment and simulation.Based on the passive time reversal focusing method,a virtual time reversal passive direction finding algorithm is proposed.In this algorithm,the receiving array is a uniform circular array(UCA)with eight elements,the theoretical formula of time reversal focusing is derived,and the search models and schemes of azimuth angle and elevation angle are given.For solving the time-consuming problem in direction search algorithm,a kind of method that using one-dimensional search to replace the two-dimensional direction search is proposed,and the correctness of this simplified method is verified by mathematical proof.Then a scheme of virtual time reversal direction finding algorithm is discussed,the corrective measure to correct the signal propagation delay that is affected by signal frequency conversion.Finally the implementation steps of the algorithm are given.The simulation results show that the virtual time reversal passive direction finding algorithm can get better accuracy in the condition of low SNR,it indicates that this method is more suitable for detecting the direction finding the weak signal in the complex environment.At the same time,the results also show that the search area can be simplified greatly.Finally,the virtual time reversal algorithm complexity is analyzed,the result shows that the computational complexity of the algorithm can be simplified to a term that only relates to the length of the signal with parallel processing in hardware,and the virtual time reversal passive direction finding algorithm can get much more simplified complexity than MUSIC algorithm.