Research On EVM (Error Vector Magnitude) Optimization Method For OFDM System

Orthogonal frequency division multiplexing(OFDM)technology is a kind of multi-carrier modulation technology.It is widely used in multipath environments and high-speed data transmission in fading channels,with strong anti-fading capabilities and extremely high spectrum utilization.However,OFDM modulated signals have the higher peak-to-average power ratio(PAPR)characteristics.And its envelope variation range is large,when this modulated signal passes through a high power amplifier(HPA),it will cause serious in-band distortion and out-of-band radiation,thereby reducing the bit error rate(BER)performance and increasing the sub-channel Interference.Therefore,reducing PAPR becomes the most difficult problem of OFDM system.This article has carried out specific research on the above problems.(1)The PAPR optimization method based on the linear alternating direction multiplier(LADMM)algorithm is designed.This paper considers the PAPR optimization problem with EVM constraints.Communication standards have strict limits on out-of-band radiation and in-band distortion.In the optimization process,idle subcarriers are usually used to meet the requirements of spectrum mask to limit out-of-band radiation,and EVM of data subcarriers is used to quantify in-band distortion.For simplicity,the out-of-band radiation is assumed to be zero.The second order cone programming(SOCP)algorithm has also used to minimize PAPR in OFDM signals under EVM constraints.However,the complexity of this method is relatively high,so this solution is not applicable in actual systems.The LADMM algorithm is a simple and efficient algorithm.The algorithm has a low computational complexity,and only a pair of FFT-IFFT calculations is performed during each iteration.At the same time,the paper proves LADMM algorithm's convergence and perform simulations.Compared with the existing algorithm,the simulation results show that the LADMM algorithm not only obtains a larger PAPR reduction,but also greatly improves the performance of the bit error rate.(2)An EVM optimization method based on the forward and backward splitting(FBS)algorithm of OFDM system is designed.Compared with the previous iterative clipping filtering(ICF)and optimized iterative clipping filtering(OICF),the ICF algorithm can reduces out-of-band radiation,but it can't reduce in-band distortion.OICF solves the problem of in-band distortion,but the computational complexity is very high.Both of thesealgorithms need to choose the optimal clipping threshold,which is difficult to choose.In practical applications,the number of different subcarriers and modulation methods will seriously affect the selection of the optimal clipping threshold.In order to overcome the limitation of pre-determined optimal clipping threshold,our proposed FBS algorithm is an efficient iterative method that does not require clipping and filtering operations.The computational complexity of this algorithm is as low as the LADMM algorithm.Numerical experiments show that the algorithm is superior to existing algorithms in PAPR and bit error rate(BER).