Research On IQ Imbalance Compensation Algorithms In OFDM Systems

Orthogonal frequency division multiplex(OFDM)has been adopted as the key technology in broadband wireless communication systems because it can meet the high bit rate demand for wireless communication with the robustness against frequency selective fading channel and the efficient utilization of frequency.RF front end is the most important part of communication receivers and decides the transmission performance.With the increasing development of integrated circuit technology,the direct conversion receiver with the characteristics of simple structure,easy integration,low cost and power consumption,has been widely concerned and commercialized in recent years.Using DCR in OFDM systems has an important significance for system integration.However,DCR introduces some RF impairments like IQ imbalance to destroy the orthogonality between the subcarriers in the OFDM systems.and decreases the performance of the system.Therefore,it is very important to study the estimation and compensation of IQ imbalance in OFDM systems.In this paper,the problem is studied and the main work is as follows:Firstly,compared with advantages and disadvantages of the direct conversion receivers and superheterodyne receivers,the time domain and frequency domain model of IQ imbalance are established to describe the received signal under IQ imbalance.On this basis,the accuracy effect of OFDM synchronization with IQ imbalance is further discussed.IQ imbalance can reduce the traditional algorithm for fractional carrier frequency offset(CFO)estimation accuracy,laying the theoretical foundation for the subsequent algorithm design.Secondly,taking into account the advantages of the blind estimation algorithm,a new algorithm of IQ imbalance blind estimation is designed based on high order circularity of complex random signal.First of all,the method to describe the complex signal of high order circularity is discussed in detail,and prove that M-PSK and M-QAM signals possess the character and analyze the influence of multicarriers modulation on the high order circularity.Further more,complex signal of high order circularity for IQ imbalance is analyzed and provide a theoretical basis for estimation.Secondly,analyzing the typical blind estimation algorithm based second order cirvularity,the algorithm has the disadvantage of slow convergence.A new IQ imbalance estimation and compensation is based on sixth order circular moment using Newton iterative method.Finally,simulation experiments show that the proposed algorithm has a faster convergence rate and higher image rejection capability.Finally,the existence of IQ imbalance and CFO is considered,and two kinds of algorithms based on training sequence are designed for the joint of estimation and compensation of IQ imbalance and CFO.The first algorithm makes use of the second order circularity of complex signals,and uses the training sequence with two segment repetition structure,which can be used to estimate the IQ imbalance and CFO.The algorithm has low complexity.To improve estimation accuracy,the second algorithm uses rotation invariant subspace algorithm to construct the principle,with rotation invariance of the sub array using training sequence four repeats,decomposition of CFO is estimated by characteristics of the correlation matrix,based on the simplified approximation.The closed solution of IQ imbalance parameters is acquired.Simulation results show that the proposed algorithm is superior to several typical algorithms in the estimation of frequency offset,and the bit error rate is the lowest when the existence of IQ imbalance and CFO.