Optimal pilot based frequency-dependent I/Q imbalance compensation for wideband direct-conversion transmitters

Current trends in low-cost and low power consumption transmitters in state-of-the-art wireless systems is to use the direct-conversion principle. Such transmitters, however, suffer severely from the I/Q imbalance effect, which introduces mirror-frequency interference and degrades data detection. This paper proposes a novel method for frequency-dependent I/Q imbalance estimation and compensation. The I/Q imbalance is estimated jointly in the frequency domain with very low complexity and minimum error variance, by relying on a specially designed pilot and its optimization. The performance and complexity of the algorithm are evaluated through both theoretical analysis and computer simulations. The simulation results are compared with those of existing techniques and show considerable advantages of the proposed algorithm. Moreover, a new time domain I/Q imbalance compensator structure is developed which delivers ideal compensation performance as well as the low complexity benefit. Both of the proposed estimation and compensation algorithms are implemented and their respective operation is verified on a test bed utilizing a commercial wideband direct-conversion based signal generator. Experimental results show excellent performance improvement in image rejection in the imbalanced modulator and a resultant very low EVM values below 0.5% across a frequency band of 90 MHz.