Joint Compensation of OFDM Frequency-Selective Transmitter and Receiver IQ Imbalance

Abstract

Direct-conversion architectures are recently receiving a lot of interest in OFDM-based wireless transmission systems. However, due to component imperfections in the front-end analog processing, such systems are very sensitive to in-phase/quadrature-phase (IQ) imbalances. The IQ imbalance results in intercarrier interference (ICI) from the mirror carrier of the OFDM symbol. The resulting distortion can limit the achievable data rate and hence the performance of the system. In this paper, the joint effect of frequency-selective IQ imbalance at both the transmitter and receiver ends is studied. We consider OFDM transmission over a time-invariant frequency-selective channel. When the cyclic prefix is long enough to accommodate the channel impulse response combined with the transmitter and receiver filters, we propose a low-complexity two-tap equalizer with LMS-based adaptation to compensate for IQ imbalances along with channel distortions. When the cyclic prefix is not sufficiently long, then in addition to ICI there also exists interblock interference (IBI) between the adjacent OFDM symbols. In this case, we propose a frequency domain per-tone equalizer (PTEQ) obtained by transferring a time-domain equalizer (TEQ) to the frequency domain. The PTEQ is initialized by a training-based RLS scheme. Both algorithms provide a very efficient post-FFT adaptive equalization and their performance is shown to be close to the ideal case.