Exact SINR Analysis of OFDM Systems under Joint Tx/RX I/Q Imbalance

The direct-conversion architecture used for Orthogonal Frequency Division Multiplexing (OFDM) systems suffers from intercarrier interference (ICI) between image subcarriers in each OFDM symbol due to I/Q imbalance between the in-phase (I) and quadrature (Q) branches at the transmit and receive sides. One of the widely-used metrics to evaluate the performance degradation due to I/Q imbalance is the signal to interference plus noise ratio (SINR). The instantaneous sub-carrier SINR can be represented as a ratio conditioned on a particular channel realization and the exact SINR is evaluated by averaging over the channel realizations. In the literature, it is common to approximate the SINR by taking the average of the numerator and denominator separately although the two are not independent. In this paper, we calculate the exact SINR under the assumption that each pair of interfering subcarriers experiences uncorrelated channel realizations. We show that the SINR grows as the logarithm of the input SNR when the receiver has I/Q imbalance. On the other hand, under transmit-only I/Q imbalance, there is an SINR ceiling for large input SNR. Therefore, transmitter side I/Q imbalance is more harmful, in terms of average SINR, than receiver-only I/Q imbalance. We also show that, the approximate SINR expression, commonly used in the literature, is not accurate for large input SNR values, except for the case of transmit-only I/Q imbalance.