Study On Transform Domain Narrow-band Interference Excision Techniques In Direct Sequence Spread Spectrum Systems

Currently, Spread-Spectrum(ss) technique has been widely applied both in military and in civilian communications for its many advantages, which make it suitable for the environments of wireless communication. The best advantage is its outstanding ability to anti-jam if the processing gain is large enough, however, its ability of interference suppression is always limited. The system performance is dramatically degraded and its capacity is largely reduced if the processing gain is not sufficiently large and no other methods can be adopter to suppress strong interference.This thesis firstly simple introduces the theories of spread spectrum communication system and narrow-band interference suppression in direct sequence spread spectrum, After discussing the advantages and disadvantages of transform and filter-bank based transform domain interference excision techniques. When we design a polyphase structure based perfect reconstruction filler banks, we construct analysis filter bank from a low-pass filter by means of modulation rather than design and optimize each filter in the analysis and synthesis filter bank independently. Lapped transform, which is indeed cosine modulated filter bank, is chosen in this dissertation accordings to the above rule.Based on the characteristic of lapped transform and advantages of narrow-band interferences in DSSS, a interference excision scheme is proposed by the author. In the part of narrow-band interferences, modified median filter (MMF) algorithm is designed for narrow-band interference excision. It performs the transform domain filting twice. The first filtering locates the position of interference and mitigates most of them, the second filtering is performed in a small neighborhood of the located interference. So lapped transform domain MMF algorithm can completely mitigate the interference without distorting thedesired signal. This algorithm also has the merits of short processing time and insensitivity to jammer power and frequency. Simulation results demonstrate that our approach has significantly improved BER performance and more practical implementation.In this dissertation, the theories of the filter-bands and lapped transform with narrow-band excision algorithm are important part of transform domain interference excision, we use two ways to evaluate system performance, computer simulation and analytical expression. We can easily see that lapped transform is good at suppressing narrow-band, it will be widely applied.