Research On Techniques Of Narrowband Interference Suppression Based On Transform Domain And FPGA Design

Direct sequence spread spectrum (DSSS) communication systems have the ability of low probability of intercept (LPI) transmission and multipath interference suppression and can be used as CDMA. It is widely used in the military communication systems and the third generation communication systems. Generally speaking, the jamming margin of the DSSS communication systems mainly depends on the spread spectrum processing gain. However, when the interference's bandwidth is much narrower than that of the DSSS signal and can be seen as a narrowband interference, the jamming margin can be improved significantly by the preprocessing of the narrowband interferences without increasing the processing gain. Therefore the preprocessing of the narrowband interferences places an important role in the study of the spread spectrum communication. Because of the preprocessing of the narrowband interferences, DSSS communication systems not only can tolerate strong jamming in military communication but also can be used as overlapped communication systems. Narrowband interferences not only include singleton interference, polyphony interference, CW pulse interference and narrowband Gaussian noise, but also include some other narrowband signals which need to be excised, such as narrowband digital modulation signals.There are two kinds of main techniques in the narrowband interference suppression. One is adaptive filter in time domain. The other is interference excision in transform domain. Both of them have their own advantages and disadvantages. The advantage of the later one is that it is easier to suppress several narrowband interferences without increasing the complexity. This thesis mainly studies the later one. The main research contents and contributions are summarized as follows:1) The DFT Filter Bank and the Windowed Lapped FFT are studied. Based on the introduction of their principles, structures and processing procedures, simulations are carried out. The simulation results demonstrate that those two kinds of techniques all have good BER performance This thesis also points out that the DFT Filter Bank has better performance in spectrum leakage.2) After the introduction of threshold processing algorithms, according to the characteristics of narrowband interference, a new algorithm based on Adaptive Double Thresholds is presented. The simulation results are provided which show this algorithm has good performance.3) The design of the algorithm based on DFT filter bank is implemented in FPGA. The simulation and validation are performed on the platform of QuartusII.