Frequency offset estimation in OFDM systems in presence of IQ imbalance

The orthogonality between sub-carriers in OFDM systems is disrupted by offsets between the carrier frequencies in the transmitter and receiver, as well as by the so-called IQ mismatch in the mixers. When treated separately, effective algorithms exist for estimation and compensation of frequency offset as well as IQ imbalance. However, with both effects present, such algorithms do not lead to useful estimates of the related parameters. We propose a novel iterative technique for the estimation of the frequency offset in the presence of IQ imbalance. The proposed algorithm is tested on an OFDM communication system designed based on the specifications of IEEE 802.11a for wireless local area networks. Simulation results show that the frequency offset estimation algorithm works well even with large values of gain and phase imbalances in the receiver mixer. When an estimate of the frequency offset is obtained, and the complex baseband samples are corrected accordingly, any effective technique for the compensation of IQ imbalance can be applied.