On I/Q imbalance effects in MIMO space-time coded transmission systems

Through the introduction of multiple antennas in future wireless communications systems, the requirements for the power consumption, cost and size of individual radio front-ends become even more challenging than in the current single antenna systems. In this context, the so called direct-conversion front-end architecture is of big interest for building compact radios. One practical issue in this context is the so called I/Q imbalance problem related to the amplitude and phase matching of the I and Q branches of each analog front-end. In this paper, we analyze the I/Q imbalance effects in a multiantenna setup and show that I/Q imbalance is relatively even bigger problem in MIMO systems than in their traditional single antenna counterparts. As an example, the basic 2×1 Alamouti transmit diversity scheme is examined in detail and a closed-form solution for the resulting signal-to-interference ratio at the output of the receiver combining stage is derived as a function of imbalance properties. The obtained results indicate that I/Q imbalance can easily become a limiting factor in multiantenna systems, and should carefully be mitigated using proper analog or digital signal processing. In general, the analysis results are verified using extensive computer simulations.