Contents I. Introduction
The direct conversion transmitter topology [1] gains significant popularity for multi-standard, multi-bands applications due to smaller number of components and ease of use. However, it suffers from an unequal complex gains of the in-phase (I) and quadrature (Q) paths, which depend on operating temperature and frequency [2]. This imbalances caused by the modulators have a significant effect on the quality of the output radio frequency (RF) signal. Furthermore, the Power Amplifiers (PA) stage of the transmitters is well known for their nonlinear behavior, which degrades the output signal quality and introduces out-of-band power emission. To improve the signal quality and compensate for the out of band power, the digital predistortion technique [6], [7] is currently the most popular approach to compensate for the PA nonlinearities. Though, as reported in [8] and [9], the I/Q imbalance at the transmitter would significantly degrades the linearization capacity of the digital predistorter. In this regard, a number of I/Q imbalance estimation/compensation methods at the transmitter side have been reported in the literature [2]–[5]. Hilborn [2] proposed an I/Q imbalance estimator based on the statistical properties of the modulator's in-phase and quadrature input and output signals where an ideal demodulator is assumed in the feedback path. Cavers [3] proposed a method to estimate and correct adaptively for the imperfections of the quadrature modulators and demodulators. Windisch and Fettweis [4] proposed a simple and efficient blind technique to compensate for the I/Q imbalance for low-IF (intermediate frequency) transmitter architectures based on minimization of the image-to-signal power ratio. More recently, Angrisani et al. [5] proposed a clustering-based method to detect the I/Q impairments of the modulator which consists of three stages: signal demodulation, clustering and an evaluation of the impairment amount. But similar to [2], the demodulator impairments were not taken into account in the signal demodulation stage of the algorithm. Although the I/Q imbalance estimation/compensation can be performed at the receiver side [10], [11], this paper adopts an I/Q imbalance compensation technique at transmitter side to guarantee an acceptable transmitting signal quality and out of band emission level. This paper presents a novel I/Q imbalance estimator/compensator method which estimates and separates the respective I/Q imbalances of the quadrature modulator and demodulator. In the remaining of this paper, the architecture of the new concurrent modulator and demodulator I/Q imbalance estimator/compensator is studied. Then, detailed analyses, supported by several simulations, are carried out to evaluate the accuracy and robustness of the proposed algorithm when applied to a practical modulator and demodulator that exhibit extra imperfections, such as direct current (DC) offsets, phase offset, and propagation delay.
