I-Q Mismatch Estimation and Compensation in Millimeter-Wave Wireless Systems

A salient feature of millimeter-wave (mmW) wireless systems is their large bandwidths. A direct consequence is that radio frequency (RF) non-idealities, such as phase noise, I-Q imbalance, and non-ideal frequency response of bandpass filters, become more pronounced compared to existing systems operating below 6 GHz. However, investigations of the impact of such nonidealities and techniques for their compensation are limited. In this paper, the problem of I-Q mismatch is investigated, motivated by the authors' recent work on mmW prototype design and development. First, a model for the non-ideal system is developed by modeling the in-phase (I) and quadrature (Q) passband channels separately, rather than the common complex baseband representation. The resulting channel matrix reveals the structure of interference introduced across I-Q channels and frequencies. Second, a new approach is developed for estimating the non-ideal channel using the new model. Third, a linear receiver architecture is developed to compensate for the interference caused by the I-Q mismatch. Finally, the performance of the new proposed system is compared to a baseline conventional system which ignores I-Q mismatch. The results indicate significant loss in performance even for modest values of I -Q mismatch parameters. In particular, the baseline system exhibits a saturation of output signal-to-interference-and-noise ratio (SINR) regardless of the input SNR. The new proposed system does not suffer from such saturation and its SINR can be increased indefinitely by increasing the input SNR. The analytical results are validated with experimental evaluation on a 28 GHz mmW wireless testbed.