Mixer Imbalance Correction in Wireless OFDM Systems

In this paper we present an improved method for correcting mixer imbalance in wireless systems using Orthogonal Frequency Division Multiplexing (OFDM). Mixer imbalance causes a nonlinear distortion to complex baseband signals. We start by introducing several mathematical models for mixer imbalance in the time domain. Then we select the most appropriate one of the time domain models and use it to develop a frequency domain model. We show that when observed in the frequency domain, mixer imbalance causes intersubcarrier interference as well as interference between the in phase and quadrature components of each individual subcarrier. Our improved method of correcting for these effects takes advantage of symmetries in the interference parameters. These symmetries allow for better estimation of the interference which in turn allows for better recovery of the transmitted symbols. We show that for a given set of mixer imbalance parameters there is a minimum SNR at which imbalance correction will improve symbol recovery. The exact value for the threshold SNR depends on the imbalance parameter estimation method used. The method proposed here lowers that threshold SNR at which improved symbol recovery begins compared to a previously proposed method. Error Vector Magnitude (EVM) and Symbol Error Rate (SER) are the metrics we used to evaluate our proposed method.