| Wireless communication technology has developed with an unprecedented speed in recent years,but also faces many challenges at the same time.In order to increase the transmission rate and reduce the production cost,much radio equipment generally is made to combine direct-conversion transceiver with low-cost wireless electronic technology.However,there are many non-linear problems in the direct-conversion structure,in which the I/Q imbalance problem is the main object of our concern.I/Q imbalance means there are phase and amplitude mismatches between the two quadrature signal branches in the transceiver,and the mismatches will induce mirror interference to the desired signal.The research work carried out in this paper is to research the blind compensation algorithm to cancel the I/Q imbalance in the general broadband multi-carrier direct-conversion receiver based on the 802.11ac protocol.After summarizing the previous research results and deeply understanding the relevant basic theory,this paper first builds models for frequency independent I/Q imbalance and the frequency-dependent I/Q imbalance,and then researches I/Q imbalance cancellation algorithm based on the classical adaptive filtering algorithm LMS.The algorithm is extended to the frequency-independent I/Q imbalance and frequency-dependent I/Q imbalance,respectively.The mathematical model of the I/Q imbalance is the basis,the widely linear model is used as the filter structure of the cancellation filter,the objective is to recover the 'properness'of the signal.The emphasis is on the derivation of the adaptive process of the filter,that is,the update rule of the filter tap weights.Finally,the process of the cancellation algorithm is summarized in frequency-independent and in frequency-dependent situation,and the effect of algorithm is verified by computer simulation.Then,in order to improve the gradient noise amplification problem in LMS algorithm,this paper researches an I/Q imbalance cancellation algorithm based on NLMS,which is also extended to the frequency-dependent I/Q imbalance cancellation,and also analyzes its MSE and IRR performance.The performance of MSE has regularity,which can be expressed by closed mathematical expression,and the relationship between the number of iterations to the final steady state and the step parameter is obtained.This relationship can be used to determine the step parameter,so that different values of the step parameter can meet the requirements of adaptive filter's steady-state MSE and the convergence rate.The step parameter is calculated and summarized.The effect of the algorithm is also verified by computer simulation.In order to further improve the performance of the I/Q imbalance cancellation algorithm,two advanced I/Q imbalance cancellation algorithms based on adaptive filtering are proposed,which are based on affine projection adaptive filtering and RLS adaptive filtering,respectively.Firstly,the I/Q imbalance model and adaptive theory is used to derive the algorithm which is applicable to the actual situation.Then,the stability of the algorithm is analyzed,and the parameters of the algorithm are determined,so the details of the whole algorithm are completed.The concept of regularization is introduced,and the regularization of the algorithm is completed to ensure the robustness of the algorithm.Finally,the effect of the two algorithms is verified by computer simulation,and the influence of the parameters on the algorithm performance is studied.The performance of the two algorithm is compared with the traditional cancellation algorithm.Finally,the performance of the algorithm based on affine projection adaptive filtering is better than that of NLMS.The accuracy of algorithm based on RLS adaptive algorithm is better than that of based on NLMS. |
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