I/Q Imbalance Identification and Compensation for Millimeter-wave MIMO Systems

Abstract

Today’s fourth generation (4G) cellular mobile communication networks are tasked with providing service for an ever increasing number of mobile users and their demand for increased data rates. The fifth generation (5G) of cellular mobile communications will be required to be able to handle the burden currently on 4G networks and also service new technologies as they are introduced. Massive multiple-input multiple-output (MIMO), Millimeter Wave (mmWave) and beamforming have recently been identified as a key enabling technologies for the fifth generation (5G) of cellular mobile communications. Current transmitter typologies exhibit non-idealities that are non-negligible in practical hardware, especially when transmitting wideband mmWave signals. This leads to the requirement that RF building blocks, such as PAs and quadrature modulators, and their respective nonlinearity, and I/Q imbalance must be corrected for. This thesis proposes a new method to concurrently identify and compensation the I/Q imbalance in mmWave MIMO direct-conversion transmitters (Tx) using a single transmitter observation receiver (TOR). New 5G standards for mm-wave transmitters have strict error vector magnitude (EVM) requirements; however, adjacent channel power ratio (ACPR) requirements are typically relaxed. Therefore, this thesis also proposes judiciously engineered uncorrelated training signals for minimizing the error vector magnitude (EVM) while maintaining acceptable performance in the out-of-band region. The latter is necessary to ensure proper Tx linearization when applying digital predistortion (DPD). The proposed method was validated using a 4 GHz signal in ADS simulation for 1, 2, 4 and 8 Tx chains as well as in measurement using a custom built transmitter comprised of 1, 2 and 4 mm-wave Tx chains utilizing commercially available quadrature modulators. NMSEs of 19.9% before and 2.25% after I/Q imbalance compensation were obtained. Finally, the compensation accuracy of the proposed method was further confirmed when the I/Q compensation filters are calculated in back-off and applied during the DPD linearization of a mm-wave power amplifier (PA).