Multi-transmit Waveform Optimal Design Algorithms For Cognitive Radars

The multi-transmit waveforms, transmitted independently by multiple antennas ormultiple pulse intervals, have the extensive application prospects of obtaining thespatial waveform diversity, resolving range ambiguity and also countering the deceptionjamming of distance. Specifically for cognitive radar, the optimal design ofmultiple-transmit waveforms and pulse compression (PC) filter with the priorinformation are critical technology. The main contributions of this thesis aresummarized as follows:1. The issue of the multi-transimit waveform optimal design for differentapplication backgrounds is considered. For the multi-transmit ground moving targetindication (GMTI) radar systems, the requirements of multiple orthogonal waveformsare analyzed in detail. Focusing on this novel scheme, an orthogonal waveform optimaldesign method is also proposed. Under the constraints of the correlation properties andorthogonality of multiple waveforms, the performance of one-dimension images, i.e. theshape of its mainlobe and the performance of its sidelobe, is considered as theoptimization criteria. The effect of the orthogonality of waveforms and the integratedsidelobe level of one-dimension images, on the detection performance of the GMTI, isstudied. It is shown that larger improvement factor (IF) can be obtained with transmitwaveforms that have better orthogonality. Meanwhile, the wider scope of the detectedspeed can be reached with transmit waveforms that have better ISL performance of theextracted signals. Furthermore, the time-varying multiple orthogonal waveforms areused to reject the deception jamming of distance. The waveform design method and theanti-jamming strategy are also researched. According to the principle ofpermutation and combination, the more the waveform-hopping order for anti-jammingis, the higher the probability of anti-jamming can be obtained. Simulations demonstratethe feasibility and validity of the algorithm for anti-jamming.2. It is impossible to design a set of orthogonal waveforms with ideal impulsiveautocorrelation and ideal zero cross-correlation properties, thus the orthogonalsequences with part zero correlation zones are considereed. Instead of all sidelobe zones(concluding the auto-correlation sidelobes and crosscorrelation sidelobes), the constraintto the sidelobe region nearby the mainlobe is adopted. Based on this idea, thezerocorrelation zone-orthogonal sequences (ZCZ-OS) is proposed. The waveform isapplied to the multi-transmit-GMTI radar system, and then the characteristics of aperiodic correlation performance and one-dimension images are analyzed. Themain-lobes and the side-lobes nearby the mainlobe of the PC results are almost identical.Therefore they are effective on the cancellation of strong clutter near the moving target.For the time-varying multi-transmit waveform system, it has been proved that thecomposite ambiguity function of ZCZ-OS is with all zero sidelobe region at zeroDoppler frequency, and is almost all zero sidelobe when the Doppler frequency is small.3. In practice, the environment information changes constantly. Thereby, theadaptive waveform design can meet the actual requirements better. In the case ofdynamic environment, the controllable low sidelobes (or zero sidelobes) orthogonalwaveforms are studied and a method for designing the PC filter based on priorenvironmental range knowledge is presented. Low sidelobes are designed in the areawhere strong clutter may appear, and high sidelobes are designed in the area whereweak clutter or no clutter may appear. Based on the binary ZCZ-OS, two methods fornew controllable low sidelobe orthogonal sequences are proposed, i.e. cyclic shiftZCZ-OS and ternary ZCZ-OS. The cyclic shift digit does not affect the zero sideloberegion of periodic correlation functions, but the position and size of the zero sideloberegions of aperiodic correlation functions (ACF), especially the region nearby themainlobe. Therefore, the ACF properties of the obtained cyclic shift ZCZ-OS areanalyzed. And then the results of the cyclic shift digit and the zero sidelobe regions arerecorded in a library, respectively. By selecting the cyclic shift digits, the proper zerocorrelation zones are designed to inhibit the strong clutter or jamming (priorknowledge). According to the rule similar to the array configuration of minimumredundancy linear arrays, the ternary ZCZ-OS are obtained by inserting several zeros inthe proper positions of the binary ZCZ-OS or cyclic shift ZCZ-OS. It is shown that theternary waveforms have lower peak sidelobe level (PSL) and still have the zero sidelobezones. In the applications of cognitive radar, the two new ZCZ-OS with flexible zerosidelobe zones can adapt to the dynamic jamming environment. Given the prior targetand clutter environmental range knowledge, the desired time response mask inverselymatched to the environment can be obtained. The idea of adding the artificialinterference is introduced to design the PC filter. Iterations, performed on the addedpower of artificial interference (weighting coefficients) into the sidelobe region, ensurethat the obtained PC result tends to the desired time response mask. Theenvironment-based matched filter can be used not only for single waveform, but also formultiple-transmit waveforms. 4. For a multiple-transmit radar system, the signal processing procedures, includingthe PC and transmit beamforming (TBF), are operated in turn. They are coupled witheach other. Replacing the PC results by signal processing results (including PC andTBF), the sidelobes performance is studies. The multiple elements of antenna arraytransmit ZCZ-OS independently. The analyses on the gain and sidelobe level of TBFoutput are presented. The range sidelobe level within the main beam (in angle domain)is focused on. For the normal direction, due to the inherent phase property of ZCZ-OS,the TBF output has part zero sidelobes area, of which the distribution characteristics arediscussed. For the other directions, to improve the range sidelobe performance withinthe main beam, a systematic search algorithm to optimize the transmission order ofsignals is addressed for an optimal relationship chart of DOA and transmission order. Aclass of ZCZ-OS with window shaped envelope is proposed to improve the PSL. Thewindow shaped envelope and TBF are exploited to accumulate receiving power andreduce the range sidelobe levels, and hence to enhance target detection performance.Numerical experiments show that the window shaped envelope is superior to therectangular envelope, with lower sidelobe levels and a very small loss of gain and rangeresolution.