Research On Subcarrier Interference Analysis And Suppression Technology In OFDM System

Orthogonal Frequency Division Multiplexing(OFDM)is a special multi-carrier transmission scheme.The system has strong anti-interference ability and high spectrum utilization,so it has been widely adopted by many digital communication standards,and has become one of the key technologies of 5G communication.The OFDM system requires precise frequency localization to ensure the orthogonality between its sub-carriers.Due to factors such as fast time-varying channels and Doppler frequency spreading,the frequency offset of the OFDM system sub-carriers is generated.It destroys the orthogonality between subcarriers,resulting in inter-carrier interference(ICI)in OFDM systems,which reduces the signal detection performance at the receiving end.Therefore,in a fast time-varying channel environment,how the OFDM system suppresses the impact of ICI will have very important practical significance.At present,ICI suppression technology mainly includes ICI self-cancellation algorithm;frequency domain equalization;time domain windowing method.This paper will focus on the analysis of frequency domain equalization techniques and propose improved algorithms.The frequency domain equalization algorithm can be divided into two major categories: linear and nonlinear.In linear algorithms,zero-forcing(ZF)and minimum mean-squared error(MMSE)are commonly used,but in time-varying channel environments,linear equalization algorithms are not ideal.In the nonlinear ICI suppression algorithm,most of them adopt iterative feedback as the main scheme.The typical is the minimum mean square error order detection equalization algorithm(MMSE-SD),which combines the minimum equalization error equalization(MMSE)with serial interference cancellation(SIC).In time-varying channels,The Bit error rate performance of this algorithm is very good.A large increase,but its computational complexity is high,and it is not easy to be applied to wireless communication systems.Therefore,the ICI suppression algorithm needs to weigh the relationship between complexity and performance.Based on the above problems,this article proposes a minimum mean square error equalization algorithm(MMSE-RC)based on reliability constraints,which combines the minimum mean square error(MMSE)with the Maximum Likelihood(ML)idea.Based on the constellation distance limiting structure(PD structure),the OFDM system receiving end divides the MMSE equalized output signal into two categories: reliable signal and unreliable signal,and the reliable signal is directly quantized and output;the unreliable signal is re-determined by the RC algorithm,which greatly improves the detection accuracy of the received signal.At the same time,MMSE-RC frequency domain equalization has another advantage: the algorithm can flexibly adjust the relationship between algorithm complexity and bit error rate performance by changing the threshold size in the PD structure to meet the more needs of the communication system.Compared with the MMSE-SD equalization algorithm,this algorithm performs better overall performance in a fast time-varying channel environment.