| Multiple-Input Multiple-Output(MIMO) technology in modern wireless communication applied to radar systems has developed a new concept of MIMO radar, which is one of the most important researching directions both in china and abroad. MIMO radar divided into two types, that is, statistical MIMO in which the antennas are widely spaced, and MIMO radar with collocated antennas. The transmit signal of the statistical MIMO radar system has the characteristics of waveform diversity and spatial diversity. The transmitting and receiving antennas array collocated are called monostatic MIMO radar and the antennas array divided are called bistatic MIMO radar. At present, the research of MIMO radar system is in the theoretical verification stage, the main research directions are orthogonal waveform design, digital beam forming, Direction of Arrival(DOA) and Direction of Departure(DOD) estimation, and etc. The nearly orthogonal waveform using frequency modulation, phase encoding and their combination are designed in this paper. The nearly orthogonal waveforms were evaluated by using ambiguity function, and the performance of several nearly orthogonal waveforms was verified by MATLAB simulation.The main work and contributions are summarized as follows:1) The signal model of bistatic MIMO radar is analyzed and the parameter information of each array reception signal is obtained. Presenting each signal should have ambiguity features in order to accurately separate information of each emission signal from the combined signal when matched filtering.2) Nearly orthogonal waveforms of Orthogonal Frequency Division-Linear Frequency Modulation(OFD-LFM), Orthogonal Frequency Division-Non-Linear Frequency Modulation(OFD-NLFM) and discrete frequency coding are designed by frequency modulation method. OFD-LFM nearly orthogonal waveform has good pulse compression performance, and has large Doppler frequency shift tolerance. But its Ambiguity Sidelobe Peak(ASP) is higher, and the Cross-ambiguity Peak(CP) value of the adjacent signals is also higher. We put forward a method to design OFD-NLFM nearly orthogonal signal by using the method of inverse evaluate window function and orthogonal frequency division. Under the small frequency modulation interval, the OFD-NLFM has lower ASP value and CP value than the same OFD-LFM signal of frequency modulation interval. The signal model of discrete frequency coding nearly orthogonal waveform is established, and its ambiguity function is calculated. The main factors affecting the nearly orthogonal waveform of the discrete frequency coding are analyzed.3) In the use of genetic algorithm to design discrete phase encoding nearly orthogonal waveform, the relationship between the auto-ambiguity and cross-ambiguity of the discrete phase coded signal is analyzed, and it is concluded that the ambiguity sidelobe energy and the cross-ambiguity energy are minimum when the ambiguity sidelobe peak is equal to the cross-ambiguity peak. Therefore, a new objective function is designed by minimize the auto correlation and cross correlation energy, the ASP equal to CP, the stablilization of ASP and CP. The nearly orthogonal waveforms are designed by using the objective function, and the MATLAB simulation is carried out.4)Based on the advantages and disadvantages of LFM nearly orthogonal signal and PC nearly orthogonal signals, puts forward the combination of LFM and PC, OFDM to design the Orthogonal Frequency Division-Non-Linear Frequency Modulation– Polyphase Coding(OFD-LFM-PC) nearly orthogonal waveform, in which orthogonal frequency division of the difference signals, discrete phase coding of the LFM signal. The signal model of OFD-LFM-PC waveform is established, and the ambiguity function of the signal model is derived. It is concluded that the coding length, the frequency modulation interval and the number of phase are the main factors that affecting the ambiguity performance of OFD-LFM-PC signals. MATLAB simulation was carried out by setting different frequency modulation interval and coding length. |
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