Adaptive compensation of frequency selective IQ imbalance and carrier frequency offset for ofdm based receivers

Direct conversion architectures are currently receiving a lot of interest in OFDM based wireless transmission systems. However such systems are very sensitive to front-end non-idealities such as in- phase/quadrature-phase (IQ) imbalances and carrier frequency offset. In this paper the joint effect of frequency selective IQ imbalance at the receiver under carrier frequency offset is studied. We also consider the case when the cyclic prefix is not sufficiently long to accommodate the channel impulse response combined with the receiver filter, which results in inter-block-interference (IBI) between the OFDM symbols. We propose a frequency domain per-tone equalizer (PTEQ) based receiver structure obtained by transferring a time domain equalizer (TEQ) to the frequency domain. In addition to the frequency domain PTEQ design procedure, a training-based RLS type initialization scheme for direct per-tone equalization is proposed. The algorithm involved provides a very efficient post-FFT adaptive equalization and front-end compensation performance which is shown to be close to the ideal case.