Analysis And Compensation Of IQ Imbalance In OFDM System

In recent years,people are increasingly pursuing higher speed and higher quality of communication.Orthogonal frequency division multiplexing(OFDM)has been adopted as the air interface technology in 5G standard.As a kind of Multi-Carrier Modulation(MCM)technology,OFDM has the ability to implement highspeed parallel transmission of serial data.The advantages of OFDM system include good resistance to frequency selective fading and inter-symbol interference(ISI),and high efficiency of spectrum utilization.The transmission quality of an OFDM system depends on the performance of its RF transceiver.Direct frequency conversion transceivers(zero intermediate frequency transceivers)have gradually become the mainstream development direction of wireless communication systems due to their advantages of small size,low energy consumption,and low cost.However,direct conversion transceivers have limited performance due to the non-idealities of analog devices.The reason is that the 90 degree phase difference between the in-phase and quadrature branches cannot be guaranteed when up-converting and down-converting,which is called IQ(In-phase and Quadraturephase)imbalance.How to reduce the performance loss caused by IQ imbalance to OFDM system is a very important research area.Therefore,the estimation and compensation algorithms of IQ imbalance in OFDM system have been considered in this paper.The main research contents are listed as follows:(1)Taking the IQ imbalance in both transmitter and receiver of the OFDM system as the research object,this paper analyzed the system model and proposed a joint channel estimation framework.With the help of the definition of the channel residual energy(CRE),by minimizing the CRE,a channel estimation scheme is presented when both the transmitter and the receiver have IQ imbalance.Simulation results show that the scheme has good performance.(2)Aiming at the situation that the receiver has both IQ imbalance and carrier frequency offset(CFO)in the OFDM system,we firstly analyze the system model and then the joint estimation problem is transformed into an optimization problem in a specific situation according to CRE.Taking advantage of the universality of the blind estimation algorithm,a joint estimation scheme for IQ imbalance,CFO and channel response is proposed.The case where the residual CFO is small and the application in multiple OFDM blocks is also discussed.The simulation results verify the performance advantages and robustness of the algorithm.(3)We propose an efficient IQ imbalance compensation scheme to jointly estimate the IQ imbalance and channel taps.Specially,the system model is firstly reduced to a non-convex optimization problem and then a non-linear least squares(NLS)algorithm is introduced for local linearization of nonlinear systems to achieve sub-optimal solutions.The NLS algorithm can quickly converge to the corresponding Cramer-Rao lower bound(CRLB)after three iterations,which guarantees low complexity and high estimation accuracy.The simulation results are consistent with the theoretical analysis conclusions.