High Robust Narrowband Interference Suppression Algorithm For OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM)has been widely used as a multi-carrier modulation technique in broadband systems,and OFDM modulation will also be used in 5G.However,due to the increasing lack of frequency domain resources and complex wireless communication environments,OFDM systems will be affected by narrowband interference,which may result in the system not being able to communicate properly.This paper mainly studies the narrowband interference suppression problem of OFDM systems.The purpose is to ensure the normal communication of the system in the presence of narrowband interference.First,the OFDM system model and the interference model are built.The narrowband interference is modeled as a form of single-tone interference and multi-tone interference.The effects of narrowband interference with different positions,amplitudes and quantities on system performance are studied.It is found that in the presence of strong narrowband interference,the system's bit error rate performance is very poor,and normal communication cannot be performed,indicating the necessity of narrowband interference suppression.Secondly,the interference detection and classification algorithms are studied.In the interference detection,the principle of the threshold method based on mean and variance and the double-threshold method based on forward consecutive mean excision(FCME)are introduced.Then the FCME-based double threshold method is improved and proposed.Then an improved FCME-based double threshold method is proposed,which reduces the computational complexity of the FCME-based double threshold method.Comparing the threshold method based on mean and variance,the FCME-based double threshold method and the improved FCME-based double threshold method,the detection performance of the double-threshold method is significantly better than the single-threshold method,and the improved FCME-based double threshold can achieve good interference detection performance with lower calculation amount.In the interference classification,the interference is divided into type 1 interference and type 2 interference,and it is proposed to distinguish it by the low threshold of double threshold method.Then,the frequency domain narrowband interference suppression method is studied for type 1 interference.The principle of interference suppression in the frequency domain nulling method is explained in combination with channel decoding theory.The principle of narrowband interference suppression method based on interference alignment(IA)is analyzed and an interference frequency estimation method based on amplitude ratio method and Chirp-Z transform(CZT)spectrum refinement is proposed.Finally,the performance of the frequency domain nulling method,the interference alignment method based on the amplitude ratio method and the interference alignment method based on the CZT spectrum refinement are compared.Since the interference frequency estimation accuracy is higher,the interference alignment method based on the CZT spectrum refinement is better than the other two algorithms.Finally,a narrowband interference suppression method combining time-frequency domain is proposed for type 2 interference.The reason why time domain filtering can avoid spectrum leakage is analyzed and the design method of time domain notch filter is given.A time domain adaptive notch filtering method is introduced,and its convergence performance is analyzed.It is found that this method has a long time in the tracking process of interference frequency,so it is not suitable for time-varying interference.Aiming at the mutual influence of multiple interferences,combined with the advantages of time domain and frequency domain interference suppression methods,a narrowband interference suppression method combining time-frequency domain is proposed.The method combines the time domain notch filter and the interference frequency estimation based on CZT spectrum refinement to achieve accurate estimation of the interference frequency,and then filters out the interference through the time domain notch filter.The suppression performance of different algorithms for type 2 interference is compared,and the superiority of time-frequency domain combination method for type 2 interference suppression is illustrated.Finally,the performance of the overall interference suppression algorithm is simulated under different interference combinations,which shows the high robustness of the interference suppression algorithm in complex interference environments.