I/Q Imbalance Compensation in Wideband Millimeter-Wave Transmitters Using a Single Undersampling ADC

In this paper, a new method to identify and compensate for the I/Q imbalance in wideband millimeter-wave transmitters using a single undersampling analog-to-digital converter (ADC) is proposed. The proposed method alleviates the need for an I/Q demodulator; hence, simplifying the structure and calibration procedure of the required transmitter-observation receiver (TOR). In addition, an algorithm for estimating the delay and phase offsets between the transmitter and the single-ADC TOR is devised. The presented algorithm improves upon the exhaustive search methods employed in the literature. Measurement results using an off-the-shelf I/Q modulator, driven by an 800 MHz test signal, show about a 5% reduction in the root normalized mean-square error (RNMSE) of its output signal when the proposed method is utilized. Further measurements were conducted to assess the performance of this method, in conjunction with digital predistortion (DPD), when a power amplifier (PA) is added to the transmitter chain. When compared with the application of DPD only, the I/Q imbalance correction allowed additional reductions of about 4% in the RNMSE and 1.5 dB in the adjacent-channel power ratio of the PA output signal. These improvements were obtained with ADC sampling rates as low as 20 times below the Nyquist rate.