Compensation of Transmitter I/Q Imbalance in Millimeter-Wave MIMO Systems Using a Single Transmitter Observation Receiver

Abstract

This article proposes a new method to concurrently identify and compensate for the I/Q imbalance in millimeter-wave (mm-wave) multiple-input multiple-output (MIMO) direct-conversion transmitters (Txs) using a single transmitter observation receiver (TOR) fed with the combined outputs of the individual Tx chains. In addition, a signal training approach is proposed that minimizes in-band distortion while maintaining acceptable performance in the out-of-band region. The proposed I/Q imbalance mitigation method is validated by both the simulation and measurement results, using quadrature modulators arranged in one, two, and four Tx configurations for a compensation bandwidth of 4 GHz. The simulation results demonstrate the ability of the proposed method to obtain the same performance for one, two, and four Txs after compensation using an ideal power combiner. Simulations also reveal the sensitivity of the output signal quality to the isolation of the combiner. The experimental measurement results show a normalized mean-square error improvement from 14.5% to 2.22% and 3.46% for the one and four Tx configurations, respectively, using a power combiner with limited isolation. Finally, the method's ability to improve compensation accuracy is demonstrated as part of the digital predistortion (DPD) linearization of an mm-wave power amplifier (PA).