Nonlinear Distortion Compensation Technologies In OTFS Mobile Communication System

In high-speed mobile scenarios,traditional Orthogonal Frequency Division Multiplexing system cannot simultaneously satisfy requirements of Time-frequency dual-selected channel.In contrast,the new multi-carrier modulation technique Orthogonal Time Frequency Space has attracted extensive attention due to its high-speed transmission capability,resistance to multipath transmission and high Doppler frequency shift,as well as compatibility with OFDM system.However,the OTFS system itself suffers from high out-of-Band emission power and high Peak-to-Average Power Ratio,which may lead to in-band distortion and out-of-band spectrum expansion,thereby affecting the system transmission efficiency.Therefore,the nonlinear distortion and memory effect compensation processing of power amplifier of OTFS system are studied,and the OTFS system memoryless power amplifier receiver compensation and memory power amplifier adaptive predistortion are realized.Firstly,the necessity of nonlinear distortion compensation for OTFS system is analyzed,followed by the construction of a model and basic architecture for the system.Additionally,the systematic mathematical representation and matrix discretization of the OTFS system are presented.The theoretical advantages of the OTFS system are described and a mathematical derivation of the channel response matrix is provided.Moreover,the influence of memoryless and memory nonlinear models on the OTFS signal is characterized separately,by considering the distortion characteristics of the power amplifier system and the nonlinear memory effect.Secondly,a nonlinear distortion compensation algorithm for the OTFS system caused by the PA nonlinear characteristics is proposed.The OTFS signal is modeled based on the behavior model of the PA system and the OTFS channel model.The equivalent OTFS symbol in the Time-Frequency domain is calculated using a complex coefficient polynomial model.The distortion expression of OTFS symbols is obtained using the Bussgang theorem,The distortion item average energy is used to estimate the channel state information,and the distorted signal is reconstructed by updating the polynomial normalization coefficient estimation value.Furthermore,the iterative zero-forcing equalization algorithm was employed to complete adaptive compensation analysis of the nonlinear distortion at the receiver.The results show that the nonlinear influence on transmission signals is reduced,which caused by Saleh and Rapp behavior models of power amplifier,and BER performance of OTFS system is improved to 10^-5 in the proposed algorithm.Finally,the memory effect of HPA is addressed by using a generalized memory polynomial to approximate the parallel Hammerstein model.Due to the near-singular autocorrelation matrix of the system,it is difficult to obtain an inverse directly,so the ridge regression method is used to estimate the GMP coefficients and improve the convergence of the traditional LS algorithm.However,the PSD of the predistorted signal obtained by the ridge regression method is still not ideal,so the model is further simplified by integrating the Gram-Schmidt orthogonalization into the OMP algorithm to reduce the number of coefficients and the complexity of the DPD algorithm.Additionally,the Moore-Penrose pseudo-inverse is utilized to further enhance the stability of the algorithm.After pruning the DOMP algorithm,the indirect learning structure of the predistortion is updated iteratively using the recursive prediction error training algorithm to achieve the correction and compensation of nonlinear distortion signals in the OTFS system.The complexity of the DOMP algorithm is reduced by using the pseudoinverse of the extended matrix.Simulation results demonstrate that the adaptive predistortion algorithm can optimize the nonlinear characteristics of the HPA in the OTFS system,and the processed OTFS signal has a certain out-of-band suppression effect.