| Accompanied with the development of the communication countermeasure technology and the electromagnetic environment of the battlefield deteriorate gradually,Direct Sequence Spread Spectrum(DS-SS) communication as an important anti-jamming communication mechanism,it has been applied widely in the battlefield confidential communications, satellite navigation, missile guidance and other fields. At the same time, jamming and anti-jamming technologies in DSSS have been a hot issue in communication countermeasure area. Frequency Modulation (FM) jamming which can be characterized by their instantaneous frequency are typical wideband non-stationary jamming, and they are also important targets of anti-jamming technology research. In the current FM interference rejection technique research, the Linear FM (LFM) interference has been fully studied because of its clear interference model and simple modulation rule. But Non-Linear FM (NLFM) interferences still lack of effective interference suppression measures for their complex modulation laws, so it is essential to study Non-Linear FM interferences suppression methods. This paper summarizes the existing FM interference suppression methods, based on these, the Non-Linear FM interference suppression techniques are studied, the main work are summarized as follows:(1) The characters of NLFM jamming and the effects of different jamming parameters have been discussed in the paper. Firstly, the corresponding receiver model of the actual DS system is established, and then the effects of Jamming-Signal Ratio, relative frequency sweep bandwidth and center frequency of the interference have been simulated based on Monte Carlo method. Simulation results show that the SNR is the main factor affecting interference effects, and the wideband NLFM jamming has higher power efficiency than the narrowband interference, which further demonstrate the necessity of countermeasures for NLFM interference.(2) The suppression methods for mono-component and mixed-order multi-component Polynomial Phase jamming have been proposed in the interference parameter estimation error case. Polynomial Phase jamming is a typical class of NLFM jamming; the existing interference suppression algorithms for polynomial phase interference are not effective in jamming mitigation because of the parameter estimation error, besides, corresponding suppression methods are needed for mixed-order multi-component Polynomial Phase jamming. To solve these problems, parameter estimation methods for mono-component and mixed-order multi-component Polynomial Phase Signal are studied in this paper first, then the corresponding anti-jamming measures are proposed which based on instantaneous frequency demodulation and adaptive jamming elimination algorithm. Simulation results show that the proposed algorithm can effectively reduce the influence of parameter estimation error, and mixed-order multi-component Polynomial Phase jamming has been completely removed form the received signal. The effectiveness and robustness of anti-jamming method have been dramatically increased(3) Suppression methods for interference with unknown frequency modulation model have also studied in the paper. Time-Frequency Distributions(TFD) are often employed to suppress NLFM jamming which have unknown frequency modulation model, but the common TFD methods are usually not ideal in dealing with NLFM, the Time-Frequency concentration property is always poor in Time-Frequency domain. All of these shortcomings restrict the performance of TFD-based anti-jamming methods. This paper has introduced Chirplet Transform (CT) to solve this problem, and fast algorithm of CT is firstly studied. Then a CT based interference suppression method is proposed by considered that Chirplet atoms have strong relativity with NLFM jamming but weak relativity with DS signal and noise. Simulation results show that the proposed method can effectively suppress mono-component and mixed multi-component NLFM jamming, and the applicability of the algorithm is quit satisfying. |
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