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According to the radar theory [1], it is well-known that the pulse-compression techniques are widely employed inside of advanced military radar systems to increase the range resolution. Whereby the range resolution is inverse proportional with the frequency band of the transmitted signals, in the last period of time, in special literature, a lot of suitable wideband signals (e.g., frequency modulated (FM) signals etc.) were analyzed as performance level. In addition, one of the most important requests imposed to the wideband signals is to assure for the sidelobes of the compression filter response the lowest level. The presence in the response of significant sidelobes may cause interference with other near echo signals, and having unwanted effects in the detection process and ambiguities in the estimating of the target range. Accordingly, a major research direction in the high-resolution radar theory refers at the designing of improved waveforms with rectangular envelope, but with suitable modified FM laws, so that the matched filter response to contain lower sidelobes than in the standard linear FM (LFM) case [2]. Generally, in this research domain, the nonlinear FM (NLFM) signals represent an important family of continuous (pure) modulation waveforms with applicability inside of pulse-compression radar systems. They have been claimed to provide a high-range resolution, an improved SNR, low cost, good interference mitigation, and a spectrum weighting function inherently in their modulation function which offers the advantage that a pure matched filter gives low sidelobes. However, the major drawback of the most part of the common NLFM waveforms seems to be their Doppler intolerance which requires for example, using of several (bank) filters at the receiver [3]. Although in pulse-compression radar literature a lot of interesting research works to investigate and design optimal (as sidelobe level) NLFM waveforms have been done [4, 5], one of the most important methods as application area, is focused on stationary phase technique (SPT) use [3, 6, 7]. In addition, as a common design characteristic, all these approaches have as starting point the choice of a desired energy/power spectral density (E/PSD) shape and next, they gradually perform the specific calculus of NLFM laws, but no details about the parameters involved in this synthesis process (e.g., correction of the frequency deviation, NLFM signal base etc.) or a comprehensive study (as sidelobe suppression efficiency, Doppler tolerance etc.) among some interesting spectrum shapes of NLFM signals. This paper aims at presenting an extended comparative study as sidelobe reduction among some very promising NLFM laws achieved by applying of an appropriate SPT synthesis algorithm. In addition, some important aspects related to the suitable choice of the specific parameters involved in this designing process of NLFM signals are also indicated. Consequently, in the first part of the paper, a synthetic overview of the SPT synthesis algorithm used in this NLFM comparative analysis is indicated. In the next largest part of the paper, a comprehensive study as sidelobe suppression, Doppler errors etc. among some well-known PSD functions and some proper designing rules of NLFM laws, respectively are presented. Finally, the most important conclusions are also discussed. References |
