Research On Multiplexing Technology Of Bistatic/multistatic Sonar Channel

Multistatic sonar system is considered more advantages and potential than monostatic sonar in anti submarine and other fields.However,the new system is also facing new problems.For separated source and receiver,the receiving station will be seriously disturbed by the direct path wave of neighboring units,the scattering and echo from other multi target,et al.In that case,to achieve efficient reuse of underwater acoustic channel is a fundamental prerequisite.The impact of interference on the multistatic detection performance was discussed,and the channel reuse methods were proposed in time,code and space domains in this paper.Based on the system model,the basic working principle of multistatic sonar is introduced.In order to compare the monostatic sonar and multistatic sonar,the detection performances of four kinds of network forms with the probability of starting the track were compared,multistatic with the distributed network structure is the best.According to the problem of the inexactitude definition of the coverage,a novel definition was proposed based on Neiman Pierson criteria.Then the advantages on coverage of multistatic compared with monostatic in searching and positioning modes were pointed out.The coverage of two typical models is analyzed,and the influence of different factors on the coverage is studied.The three interference characteristics of multistatic sonar are compared,and the applicable environment of four channel multiplexing methods is analyzed.It is considered that the direct wave interference suppression should be given in the time domain and the detection resluts without being affected by the direct wave should be fully utilized.This paper analyzes the characteristics of periodic direct wave interference,and proposes guard rings and blind rings as the basis of time division multiplexing.The random multistatic sonar network model and its related conclusions are discussed.The calculation method of the optimal array area under direct wave interference is given.In order to increase the number of effective acoustic contact to the moving target and improve the continuous monitoring ability of the target,a pulse parameter optimization method based on the maximum number of cumulative effective detection is proposed.We propose a method of optimizing the location parameters of sonars under the criterion of maximum guard area through the rational placement of multistatic sonar to increase the guard area in the fixed area.In the detection of multistatic sonar,it is necessary for each transmitting station to designmutually orthogonal signals in order to distinguish the echo from multiple source.It is considered that the good orthogonality(weak correlation)is the core requirement for the signal of the transmitting station.In addition,multistatic detection signals should also meet the special requirements for special application environments or special tasks.Aiming at the bad propagation conditions of shallow sea,a design method of space-time code is proposed.The spatial diversity gain is obtained by using the vertical transmitting array,which can effectively resist the sub-channel fading.The excellent orthogonality can be obtained by modulating the space-time code by the spread spectrum signal.The problem of loss of array gain is analyzed when the main correlation peak can not focus caused by the different arrival delay of each subchannel,especially when the array manifold is mismatched.The time reversal correlation detection technique is proposed,and obtain the best matching detection signal of the real receiving signal by using the designed training signal to estimated the channel.The simulation results show that the space-time code detection signal can resist sub-channel fading and be suitable for shallow sea environment and that time reversal correlation detection technology can greatly reduce the error rate of space-time code,but more sensitive to the degree of interference to the traning signal.Aiming at the low intercept capability requirement,a multistatic Costas low intercept signal design method is proposed.The Orthogonal Costas Signal(OCS)signal generation method under Doppler restraint is given,the ability of several coding to enhance low interception is compared.The method of generating Orthogonal Code Costas Signal(OCCS)which modulated by Loosely Synchronous(LS)and Noise Frequency Modulation(NFM)signals is proposed,and generalized to the Double OC Signal(DOCS).Compared with the DOCS signal modulated by Walsh code,the DOCS signal modulated by LS code has the ideal cross-correlation performance and has the better autocorrelation performance;the DOCS signal modulated by NFM signal has the stable cross-correlation performance,the excellent self-correlation performance and scalability.DOCS signal modulated by LS code and NFM signal were verified by pool and field tests.Multistatic sonar receiver can be interfered by the strong direct wave of neighboring units.In this paper,the method of subspace focusing zero constraint is studied aiming at side lobe interference.Its performance is similar to that of the conventional wideband zero constraint,but it can improve processing speed.Then a subspace focusing blocking matrix and a diagonal focusing blocking matrix are proposed aiming at the main lobe interference.Compared with the conventional broadband blocking matrix,the subspace focus blockingmatrix focuses the signal subspace to the same frequency point and then blocks it,which can effectively reduce the sidelobes and improve the detection effect of the desired signal.When the number of array elements is small,the performance degradation of subspace focus blocking array is serious,and a diagonal focus blocking array is proposed.Diagonal focus blocking array focuses two times to block the interference and estimate the target azimuth.Under the condition of far field plane wave,the traditional array signal processing technology can not suppress the interference signal in the same direction as the echo.Aiming at the difficulties that the traditional interference suppression method can not suppress the co-directional interference,this paper proposes a transmitting acoustic shielding technology.The transmitting acoustic shielding technology uses the vertical emission array,and the subchannel multi-path structure between the each transmitting element and the receiving station is not the same.Shielding weight is calculated at the transmitting end using the estimated channel state information,and the original transmitting signal is preprocessed to obtain a self-shielding transmitting signal.The self-shielding signals of each transmitting elements are canceled when passing by the direct path at the receiving station,and the receiving end does not need the process again,which corresponds to shielding the received signal response at the receiving station position.The echo is not canceled for it transmits in a different path.The simulation results under several different channel conditions show that the transmitting acoustic shielding can effectively suppress the direct wave energy and not weaken the target echo intensity.The benefits of transmitting acoustic shielding for the low-intercept signal design are discussed.Finally,the feasibility of the transmitting acoustic shielding is verified by the pool test.The results show that the acoustic shielding can suppress the response of the received signal at the specific position,and it shields the fixed point rather than the fixed directional.