Research On Waveform Design For Ambiguity Suppression Under Discontinuous Spectrum

With the development of electronic technology,the electromagnetic environment faced by radar systems is becoming more and more complex.The wider operating bandwidth that required in modern radar systems to improve system performance,the continuous growth of radar users,both lead to the spectrum congestion problem more and more serious,and the radar system faces problems such as insufficient available continuous bandwidth or in-band spectrum interference in the working band.The discontinuous spectrum prvides high autocorrelation sidelobes to the radar waveform,which greatly affects the detection performance of the radar system.Besides,the coherent processing method used by the radar system also lead to the range and Doppler ambiguity problem.This problem will not only lead to the ambiguity of target detection.Moreover,the the target will be submerged in the clutter that aliased by range and Doppler ambiguity problem,which make the target undetectable.The existing signal processing and waveform design methods only deal with a single problem,while the environment faced by the actual radar system is complex and variable,and it is common for both situations to occur simulataneously.Thus,this thesis focuses on the maximum unambiguous range/velocity extended waveform design in a discontinuous spectrum environment to improve the ability of radar system to cope with complex environments.Therefore,this thesis attempts to solve both of the them at the same time,giving the radar system a wider application space.Firstly,according to the existing modulation methods,two kinds of signal forms in discontinuous spectrum environment are given,and the spectral constraints under discontinuous are also described,which provides the basis for the subsequent discontinuous spectrum waveform design.In addition,in order to achieve a reliable evaluation to the performance of the discontinuous spectrum waveform,the performance metrics,i.e.,the mainlobe resolution,the side lobe level,the range ambiguity and Doppler ambiguity,based on the periodic ambiguity function are given.At the same time,for each performance metric,the effect of discontinuous spectrum is discussed.In the design of discontinuous spectrum waveform,this thesis presents the peak sidelobe optimization method based on two kinds of discontinuous signal models,and discusses the controllable parameters in the discontinuous spectral waveform design methods in detail.Then the single-stopband analysis model and the double-stopband analysis model are established respectively.Based on the discontinuous spectral waveform design methods,the performance distribution map of the peak sidelobe of the radar waveform with respect to the stopband width and position is given.Therefore,the influence of the width and position of the stopband on the sidelobe performance can be obtained from the performance distribution map,which provides a reference for the selection of the working band to the radar system under complex electromagnetic environment.The optimization of frequency hopping waveform under discontinuous spectrum,while solving the in-band spectral interference problem,is limited by the degrees of freedom,so that the waveform only has the ability to optimize the center autocorrelation sidelobes.The larger the number of frequency hopping pulses,the better the sidelobes performance after waveform design.However,when the period is constant,the increase of the pulse number will reduces the pulse interval,which causes the range ambiguity problem to be more serious.In order to solve the range ambiguity problem,the waveform is required to have inter-pulse orthogonality.It is well known that orthogonal complementary codes in phase coded waveforms do well for this.However,the phase-coded waveform must occupy a continuous bandwidth.In the discontinuous background,if the frequency hopping is introduced between the pulses to achieve discontinuous spectrum,its orthogonal complementary performance will be lost.In fact,frequency hopping between the pulses,and phase coding inside the pulse,provides the waveform high design potential.However,the now available waveforms with frequency steping between pulses and identical phase codding inside pulse are simply combining the two techniques,which is far from achieving the optimal performance of the waveform.In view of this,an optimiz ation method of phase-coded frequency-hopping(PCFH)waveforms will be given in this thesis.The PCFH waveform not only has the frequency agility capability provided by frequency hopping,but also has the high design freedom provided by phase coding.Therefore,while suppressing the sidelobes of the center autocorrelation function,the periodic recurrent sidelobes can also be included in the design.Thus alleviate the range ambiguity problem and solve the clutter aliasing problem caused by range ambiguity.In addition,the discontinuous spectral frequency hopping design method can also be introduced into the PCFH waveform design,which provides the ability for the PCFH waveform to deal with the in-band interference.Therefore,the discontinuous spectrum application of the waveform is further realized on the basis of ambiguity range extension.As for the Doppler ambiguity problem in radar waveforms,this thesis firstly derives an improved range-Doppler(RD)processing method with Doppler extension based on the traditional RD processing method.Obviously,not all the waveforms are suitable for the improved RD processing method.It can be known from the radar periodic ambiguity function that the recurrent Doppler sidelobes depend on the Doppler cut of the single-periodic ambiguity function at integer multiples of the inverse of the waveform period.For the pulse waveform,since the pulse width is much smaller than the pulse period,this will result in several Doppler cuts inside the Doppler mainlobe,that is,the periodic ambuity function must have several Doppler ambiguity gratinglobes.Therefore,in order to solve the Doppler ambiguity problem,the pulse width is required to be equal to the pulse period.Thus,the periodic Doppler cuts is located exactly at the zero point of the single-periodic ambiguity function,which will eliminating the Doppler ambiguity grating lobes and solving the Doppler ambiguity problem.Therefore,for the improved RD processing,this paper presents a suitable phase-coded continuous waveform design method to solve the Doppler ambiguity problem while ensuring the sidelobe performance in the radar system.In addition,in order to cope with the complex electromagnetic environment,this thesis extends the above design method to the design of constant complex sequence,which combines with the discontinuous spectrum FM waveform design,provides a discontinuous spectrum PCFM waveform design for high-speed targets.