Quadrature mixer imbalances in digital TDMA mobile radio receivers

The impact of imbalances in nonperfect quadrature mixing stages on the performance of a TDMA (time-division multiple-access) mobile radio receiver with adaptive Viterbi-MLSE (maximum-likelihood sequence estimation) equalization is studied. Amplitude and phase imbalances in quadrature mixers give rise to nonlinear signal distortions which can be described using a simple mirror signal model. The mirror signal acts as a correlated interference signal superimposed on the wanted signal and, hence, degrades receiver performance. In other words, the output signal of a quadrature mixer with amplitude and phase imbalances can be decomposed into a wanted signal and a mirror signal. It is shown that this decomposition can be utilized for a rough performance analysis which, in general, overestimates the performance degradation. Simulation results reveal that receivers in digital TDMA mobile radio systems are rather insensitive to quadrature mixer imbalances, which is a consequence of the robustness of the digital modulation schemes applied in such systems