Sidelobe Level Reduction in the ACF of NLFM Signals Using the Smoothing Spline Method

The high level of sidelobes in the autocorrelation function of the nonlinear frequency modulation signal is a challenge. One of the conventional methods to reduce the sidelobe levels is to use the principle of stationary phase. In this method, the frequency function is calculated using a selection window. The signal frequency function cannot be obtained in closed form and numerical methods must be used to find it. This is usually done using the polynomial curve fitting. In this paper, the frequency function of the signal has been obtained using the smoothing spline method. The simulation results show an improvement of 10 dB to 20 dB in the peak sidelobe level of the autocorrelation function of the nonlinear frequency modulation signal compared to the previous methods. Moreover, the sidelobe arrangement in the autocorrelation function becomes more regular with the smoothing spline method, improving range resolution. This improvement is highly applicable in various telecommunication fields, particularly for target discrimination.