Compensating I–Q Imperfections in Hybrid RF/Digital Predistortion With an Adapted Lookup Table Implemented in an FPGA

The performance of hybrid RF/digital predistortion (RF-DPD) is limited, due to in-phase (I) and quadrature-phase (Q) imperfection in its key component, the RF vector multiplier, and the associated circuitry. These imperfections cause errors, in terms of implemented gain and phase of the predistortion function. This brief presents the methodology of implementing hybrid RF-DPD with a lookup table (LUT) adapted to compensate for hardware related I-Q imperfections of the RF vector multiplier within the digital signal processing domain. This modified LUT will accurately compensate for I-Q imperfection, without needing a precise tuning of the control voltages at the pins of the RF vector multiplier. This brief also presents the test setup for characterizing the RF-DPD system to obtain the I-Q imperfections within it and utilizes this information to modify the LUT to compensate for these imperfections. To verify the capability of the modified LUT in compensating for the I-Q imperfections, an experimental validation is carried out by linearizing a class-AB base station power amplifier using the hybrid RF-DPD system developed with an Altera Stratix field-programmable gate array (FPGA) evaluation board. In addition to the 12-dB adjacent-channel leakage ratio obtained using static RF-DPD, an improvement of 2.5 dB is achieved using the proposed I-Q compensation technique.