The combinational modulated radar signal by linear frequency modulation and pseudo random code can increase signal complexity and reduce the probability of signal interception. However, this combination keeps the disadvantage of high sidelobe of linear frequency modulation and cannot overcome the increase of main lobe width caused by adding window function. Using the advantages of nonlinear frequency modulation signal generated by traditional stationary phase principle in suppressing sidelobe, a method which combined piecewise fitting nonlinear frequency modulation with Barker code modulation was proposed, and by using their respective advantages, the proposed method can not only reduce the ambiguity between distance and velocity, but also suppress the sidelobe. Finally, the simulation curve of explicit function model of this signal and ambiguity function were analyzed. The simulation results show that this method not only further reduces the ambiguity between distance and velocity and improves the performance of low probability signal interception, but also produces lower sidelobe than that of traditional nonlinear frequency modulation, which illustrates the feasibility and efficiency of the method used for radar signal.

Focusing on the issue that the Signal-to-Clutter-and-Noise Ratio (SCNR) of echo signal is low when cognitive radar detects extended target, a waveform design method based on SCNR was proposed. Firstly, the relation between the SCNR of cognitive radar echo signal and the Energy Spectral Density (ESD) of transmitted signal, was gotten by establishing extended target detection model other than previous point target model; secondly, according to the maximum SCNR criterion, the global optimal solution of the transmitted signal ESD was deduced; finally, in order to get a meaningful time-domain signal, ESD was synthesized to be a constant amplitude based on phase-modulation after combining with the Minimum Mean-Square Error (MMSE) and iterative algorithm, which met the emission requirements of radar. In the simulation, the amplitude of time-domain synthesized signal is uniform, and its SCNR at the output of the matched filter is 19.133 dB, only 0.005 dB less than the ideal value. The results show that not only can the time-domain waveform meet the requirement of constant amplitude, but also the SCNR obtained at receiver output can achieve the best approximation to the ideal value, and it improves the performance of the extended target detection.

Concerning the problem that the weak target might be covered by the range side-lobes of the strong one and the range side-lobes could only be suppressed to a certain value, an improved Kalman-Minimum Mean-Square Error (K-MMSE) algorithm was proposed in this paper. This algorithm combined the Kalman filter with the Minimum Mean-Square Error (MMSE), and it was an effective method for suppressing range side-lobes of adaptive pulse compression. In the simulation, the proposed algorithm was compared with the traditional matched filter and other improved matched filters such as MMSE in a single target or multiple targets environments, and then found that the side-lobe levels, the Peak-SideLobe Ratio (PSLR) and Integrated SideLobe Ratio (ISLR) of the Point Spread Function (PSF) were all decreased obviously in comparison with the previous two methods. The simulation results show that the method can suppress range side-lobes well and detect the weak targets well either under both the condition of a single target and the condition of multiple targets.