New I/Q imbalance modeling and compensation in OFDM systems with frequency offset

In this paper, a new model and compensation scheme of quadrature imbalance in analog I/Q OFDM transceivers is presented. Classically, the effect of I/Q imbalance is modeled by a crosstalk between pairs of symmetrical OFDM sub-carriers. We show that this model is not valid when the carrier frequency offset is compensated after the source of I/Q mismatch. Assuming a perfect digital compensation of the frequency offset in baseband, the I/Q imbalance generates interference between all sub-carriers and not only symmetrical ones. When analog components, multipath propagation and frequency offsets are taken into account, the complete matrix analysis has to make use of mathematical results such as the diagonalization of pseudo-circulant matrices. The existing models happen to be a sub-case of this new more general result. A new compensation scheme applying to every case is proposed as well as a receiver autocalibration procedure. It is shown that system-specific assumptions can simplify the implementation of the compensation. These assumptions are validated by simulation in the framework of a 5 GHz WLAN transceiver design with realistic production spread of the analog components.