Suppression Of Narrow-band Interference In Spread Communications In HF

HF employs Spread Spectrum, it has many advantages that include resistance to jamming and noise, tolerance of multipath effects, security and low spectral density, code division multiplexing, and so on. Although spread-spectrum systems are naturally resistant to narrowband interference, it has been known that active methods of NBI suppression can significantly improve the performance of such systems. Not only does active suppression of NBI improve error-rate performance, it also can improve the performance of acquisition.This paper uses predictive techniques to suppress NBI, and analysis the least mean square algorithm. The linear LMS filter is designed to suppress the narrow band interference, it predicts the received signal based on previously received value, and subtracts the predicted value obtained at each sampling instant from the signal received during the subsequent instant and use the resulting prediction error as the input to the correlator, the filter coefficients are updated using the Normalized LMS algorithm. The ACM filter is introduced, and the nonlinear LMS predictor is designed to suppress narrow band interference. The performance of filter suppressing interference is evaluated by the SNR improvement that it can get.With the FPGA using massively, the paper discuss the QRD-LSL filter that can be implement by FPGA. Representing the most fundamental form of an order-recursive adaptive filter, the QRD-LSL algorithm combines highly desirable features of recursive least squares estimation, QR-decomposition, and a lattice structure., displaying better numerical properties and rate of convergence and computational efficiency. The QRD-LSL filter can be used to suppress narrowband interference.