Frequency-Selective I/Q Imbalance Compensation for OFDM Transmitters Using Online Frequency-Domain Adaptive Predistortion

I/Q imbalance in practical direct-conversion transmitters is a major impairment in OFDM systems. The demand for transmit OFDM signals with low error vector magnitude (EVM) while using increasing bandwidths illuminate the need for robust and high performance frequency-selective I/Q imbalance compensation in OFDM direct-conversion transmitters. Inspired by the 2008 work of Zou, Valkama, and Renfors, this paper extends an approach for compensating frequency-selective I/Q imbalance using a frequency-domain adaptive predistortion scheme and loopback signal path from RF to baseband. The proposed extended method is able to operate fully online (i.e., in normal transmission mode) and recursively using an operational OFDM signal and does not require a special calibration signal or calibration mode. The online and recursive nature of the proposed extended algorithm allows for seamless tracking of any dynamic characteristics associated with I/Q imbalance and simplifies transmitter operation. For the specific frequency-selective I/Q imbalance case considered, simulations show a significant improvement in the image rejection ratio (IRR) over the signal band, showing performance on par with that reported in the cited 2008 work which used a non recursive version of the method based on offline operation, a calibration mode, and a customized pilot signal. Several aspects of the approach are tunable for any given application, allowing the system designer to balance achieved IRR against convergence time, tracking speed, and computational complexity.