Research On OFDM Anti-interference Method In Time-varying Fading Channel

In the 4G era,orthogonal frequency division multiplexing(OFDM),as a mature multicarrier technology,has been widely used in wireless communication systems such as long term evolution advanced(LTE-A)and Wi-Fi.In addition,in the current stage of evolution from 4G to 5G,OFDM has also received a lot of attention.Related studies include improved waveform technology based on OFDM,intelligent communication,cognitive radio,visible light communication and so on.It can be seen that OFDM is still one of the hot topics in mobile communication research.With the change of future traffic environment,wireless communication technology under high-speed movement has received more and more attention.For OFDM systems,high movement under time-varying fading channel will cause Doppler effect,leading to inter-carrier interference(ICI).Besides,complex communication environments will cause multipath effects,if cyclic prefix(CP)is insufficient,inter symbol interference(ISI)will occur.In view of the above problems,this paper has carried out research on anti-interference methods for OFDM system.Aiming at the interference caused by Doppler,this paper adopts a weighting matrix to suppress ICI.However,unlike the existing methods,this paper degenerates the original full-dimensional weighting matrix into a diagonal weighting matrix,whose purpose is to significantly reduce the complexity of the system implementation and directly improve the bit error rate(BER)performance of the system rather than signal to interference plus noise ratio(SINR).In the optimization design of the diagonal weighting matrix,the modeling of SINR was modified,where the variables to be optimized are transformed into the diagonal non-zero coefficients in the diagonal matrix.Finally,the average subcarrier BER was used as the cost function and the gradient algorithm was applied to iteratively update the diagonal coefficient to its optimum.Moreover,for practical applications,polynomial-basis expansion model(P-BEM)is used to realize the estimation and reconstruction of the channel coefficient matrix,and is further used in SINR calculation and optimization design.From the contrastive simulation of OFDM system,it can be seen that the diagonal weighting method can significantly reduce the implementation complexity of the system without causing significant performance degradation,meeting the performance requirements.For the interference caused by insufficient CP in multipath channels,related studies have proposed a channel compression technique based on the principle of maximum shortening signal noise ratio(MSSNR),but this technique is not suitable for time-varying situations and has the problem that the system upgrade is not obvious.Therefore,this paper proposes a Toeplitz matrix design to suppress ICI and ISI in multipath time-varying fading channels.Similar to the design of diagonal weighting matrix,the SINR and BER modeling are also performed in the design of the Toeplitz matrix,and the optimization variables are converted into the shortened impulse response filter(SIRF)coefficients which constitutes the Toeplitz matrix.Meanwhile,the gradient optimization algorithm is performed at the cost function of the average BER.For practical anti-interference of OFDM system,the P-BEM technology is also adopted to estimate and reconstruct the channel coefficient matrix.The difference is that the Toeplitz matrix is applied to the head of the receiver and the internal structure of the matrix is different,so the modeling and effects of the two designs are essentially different.Through simulation comparison,it shows that the Toeplitz matrix design can not only deal with the interference problem caused by insufficient CP in multipath channels,but also can handle with the interference due to Doppler shift.