Compensation of frequency-selective I/Q imbalances in space-time coded multi-antenna OFDM systems

In multi-antenna transmission systems, multiple parallel radio implementations are needed. To keep the overall transceiver implementation feasible, this poses big restrictions on the size and cost of individual radios, resulting in various imperfections in the analog radio front-ends. 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 considers this I/Q mismatch problem in space-time coded multi-antenna OFDM system context, taking both transmitter and receiver sides into account and assuming the challenging yet practical case of frequency-dependent I/Q mismatches. Practical pilot- based I/Q imbalance compensation scheme is proposed, stemming from the derived signal and system models and proper pilot allocation. The performance of the compensator is analyzed using extensive computer simulations and shown to virtually reach the perfectly matched reference system performance with low pilot overhead. Also some important practical issues such as pilot interpolation as well as the effects of channel estimation errors in the imbalance compensation context are addressed.