I/Q mismatch compensation in zero-IF OFDM receivers with application to DAB

This work addresses the I/Q mismatch problem which arises due to analog component tolerances in zero-IF OFDM receivers. The application of this work is to digital audio broadcasting (DAB). The approach is to employ a decision directed LMS-based frequency-adaptive equalizer, applied to OFDM demodulated carriers. The equalizer is trained initially using the phase reference symbols in each frame and is switched to a decision directed mode when the error reaches a defined level. In the decision directed mode, the output is taken prior to the threshold decision device in order to present to the subsequent Viterbi decoder a signal suitable for soft decisions. Simulation results demonstrate convergence within six frames; the particular convergence rate depending on choice of step-size and number of carriers per group employed. Even with large phase and amplitude imbalances, 25° and 5 dB respectively, in a DAB system the compensation algorithm attains a reduction in raw BER from 10^-1.5 to 10^-5 with a 15 dB SNR.