Performance analysis of space-time coded MIMO-OFDM systems under I/Q imbalance

The combination of orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) techniques has been widely considered as the most promising approach for building future wireless transmission systems. In such systems, the limited implementation resources for the radio parts cause big restrictions on the size, cost, and quality of the individual radio transceivers. This implies that there are several imperfections at the analog radio front-end stages, which in turn can severely limit the overall system performance. One good example is the so called I/Q imbalance problem related to the amplitude and phase matching of the transceivers I and Q chains. This paper studies the performance of space-time coded (STC)-OFDM systems under I/Q imbalance. As a practical example, a 2 x 1 STC-OFDM system is examine in detail and a closed-form solution for the resulting signal-to-interference ratio (SIR) due to I/Q imbalance at the output of the receiver combining stage is derived. The analytical outcomes are verified using extensive computer simulations, and can easily be extended to multi-antenna receiver cases as well. In general, the obtained results indicate that I/Q imbalance can easily become a limiting factor in practical STC-OFDM systems and should be carefully mitigated using proper digital and/or analog signal processing.