Research And Implementation Of Channel Reciprocity Calibration Scheme Based On I/Q Imbalance Compensation In TDD Massive MIMO System

In recent years,massive multiple-input multiple-output(MIMO)is one of the key technologies for the fifth generation of wireless communications(5G)because of its advantages of large throughput,high frequency spectral rate,etc.In massive MIMO system,the premise of obtaining its great performance is that the base station can get accurate channel state information(CSI).A series of key technologies in massive MIMO system,such as precoding,power distribution,modulation,etc.are based on accurate channel state information(CSI).The channel reciprocity in time division duplexing(TDD)system allows the base station to obtain the CSI of the downlink channel through the estimation results of the uplink channel.Compared with the frequency division duplexing(FDD)system,it can greatly reduce the resources required for CSI feedback.Despite the fact that physical channels over the air are reciprocal,such reciprocity cannot be directly extended to the interfaces between the digital basebands of RF devices.The undesirability of analog RF devices such as the imbalance between the In-phase(I)and Quadrature-phase(Q)branches will destroy the channel reciprocity.Finally,it will lead to channel non-reciprocity.If the channel reciprocity is not calibrated,the performance of the TDD massive MIMO system will be greatly reduced.In this artical we study the impact of two types of I/Q imbalance—frequency-independent(FI)I/Q imbalance and frequencydependent(FD)I/Q imbalance,on channel reciprocity in TDD massive MIMO system.Meanwhile,two channel reciprocity calibration schemes are proposed for these two types of I/Q imbalance,respectively.The main research work of this paper is as follows:Firstly,the problem of channel reciprocity calibration is studied when the receiver is affected by the frequency-independent I/Q imbalance.The channel reciprocity calibration scheme is designed by introducing the time domain calibration matrix into the base station and mobile station respectively.At the same time,a low-complexity estimation method is also proposed.Numerical results confirm that the proposed scheme can restore the channel reciprocity and provide reliable performance.Secondly,considering the channel reciprocity calibration problem when there is frequencydependent I/Q imbalance at both transmitter and receiver.A low-complexity calibration scheme is proposed according to the characteristics of frequency-dependent I/Q imbalance,and reciprocity is calibrated for each antenna separately.The coefficients of calibration matrix are estimated using the coordinate ascent(CA).Numerical results confirm that the proposed scheme can well restore the channel reciprocity and improve the performance greatly.