Interference suppression with antenna arrays in OFDM systems under transceiver I/Q imbalance

In this paper, we address the effects of radio frequency (RF) transceiver in-phase/quadrature-phase (I/Q) imbalance in transmission systems which are utilizing orthogonal frequency-division multiplexing (OFDM) waveforms. Special emphasis is on the analysis of external interference sources and their mitigation with receiver antenna array processing, assuming independent fading for the antenna elements. In addition, I/Q imbalance is assumed to be arbitrarily frequency selective and independent in different transceiver branches. We show that I/Q imbalance is especially harmful in the presence of strong interferers when conventional per-subcarrier processing is implemented on the receiver side. Based on these results, we propose a joint subcarrier processing where each of the subcarrier signals is combined with the signal at the image carrier. Such processing is shown to be very efficient in I/Q imbalance mitigation as well as in the total interference suppression.