Iterative pre-compensation of I/Q imbalances in an adaptive 2.4 GHz MIMO-OFDM system

Currently evolving communication standards, especially those using multi-antenna (MIMO) multicarrier modulation techniques, are based on the exploitation of channel state information (CSI) at the transmitter. The utilization of adaptive transmission schemes comes with a more sensitive behavior with respect to front-end imperfections. Direct-conversion architectures in turn allow for low-cost transceiver solutions but introduce higher imbalances of the in-phase (I) and quadrature (Q) branches. For adaptive systems with high data rates severe performance losses are observed with respect to bit error rates (BER). In this paper the influence of transmitter side I/Q imbalances is investigated with respect to a MIMO hardware demonstrator, which applies adaptive Orthogonal Frequency Division Multiplexing (OFDM) strategies. An enhanced algorithm for blind online pre-compensation is presented. Comparable single-tone low-IF measurement results indicate an improved image signal suppression and higher convergence robustness while the system shows decreased downlink BER after compensation.