Optimized Channel Estimation for OFDMA Uplink with Frequency-Dependent I/Q Imbalance

I/Q imbalance (IQI) is one of the major front-end imperfections affecting the performance of communication systems. Due to frequency-dependent (FD) IQI, channel is elongated and is split into direct and image channels. We consider the problem of joint estimation of the channel and I/Q parameters for OFDM based systems. Time-domain estimation of the effective channel taps poses a question: what is the optimal number of taps that need to be estimated for the best estimation performance? More the estimated taps, more is the noise contribution to the estimator. Lesser the estimated taps, less is the precision due to unestimated taps. In this paper, we address this issue by deriving analytic expressions for mean squared errors (MSE) of estimates of the effective channels and conditions for their optimization. In the absence of channel knowledge, we propose a choice of number of taps to improve estimation performance.