Research On LFM Interference Suppression Technology In DSSS System

Spread spectrum communication technology has been widely applied in communications for its advantage of low launched power spectral density, strong anti-jamming capability, low intercept possibility, multiple access capability, better anti-fading capability, and so on. Specially, DSSS technology is the most widely used. But in the system of DSSS, when the external interference exceeds the system's interference tolerance, it will deteriorate system performance rapidly. So it's necessary to take corresponding measures to suppress the external interference. At present, the researches of interfering suppression focus on narrowband interference, but less on broadband non-stationary interference suppression; and in Broadband non-stationary interference, one of the most typical kinds is linear frequency interference. Instantaneous frequency of LFM is a linear function of time. But Fourier transform is used for global transformation, so it can't describe the time-frequency characteristics of LFM.Based on the above theory, first of all, this paper introduces an interference suppression method based on fractional Fourier transform. Using the characteristics that linear frequency modulation interference can generate energy aggregation in a fractional Fourier transform domain, the location of LFM interference is detected through corresponding fractional Fourier transform of DS signals that mixed with interference, and the effect of LFM interference on system performance can be reduced by using narrow-band band-stop filter device to achieve suppression of the interference. Then, in the light of the leakage of the spectrum due to cutting off the signal, it processes the signal for adding windows. The analysis of adding windows characteristics of linear frequency modulation interference in the domain of fractional Fourier transform shows that the energy of interference is relatively more concentrated. Finally, considering that there will be a sectional energy loss of the signal because of adding windows, this paper uses overlapping sub-windowed approach to achieve interference suppression. The result shows that overlapping sub-windowed approach is easier to implement than the band stop filter and it can improve system performance of interfering suppression more effectively.