Performance Evaluation of Combination of Gaussian NLFM and Rayleigh NLFM Signals

Pulse compression achieved with un-weighted linear FM (LFM) suffers from relatively high autocorrelation function (ACF) side-lobes. Since the ACF is the inverse Fourier transform of the power spectrum, the ACF sidelobes could be reduced by shaping the signal according to one of the many sidelobe suppression techniques. The shaping is performed by changing the pulse amplitude along the time axis. In LFM instantaneous frequency is linearly related to time. Indeed, the resulting spectral shaping is very close to the desired spectrum. But it has a serious drawback. In a matched transmitter-receiver pair, it results in variable amplitude of the pulse transmitted. Variable amplitude requires linear power amplifiers which are less efficient than saturated power amplifiers. This problem can be alleviated by performing amplitude weighting only at the receiver. The resulting mismatch causes SNR loss. In linear FM the transmitter spends equal time at each frequency; hence the nearly uniform spectrum will result. Another method for shaping the spectrum is to deviate from the constant rate of frequency change and to spend more time at frequencies that need to be enhanced. This approach is termed as nonlinear FM (NLFM). In this paper an attempt is made to compare the performance of Gaussian NLFM, Rayleigh NLFM and the combination of both these NLFM signals with respect to their ambiguity plots and range resolution plots.