Transceiver Inphase/Quadrature Imbalance, Ellipse Fitting, and the Universal Software Radio Peripheral

In this paper, we introduce a method for inphase/quadrature imbalance parameter estimation based on ellipse fitting. The performance of the method is analytically derived. In particular, it is shown that the method exhibits a small bias (which can be negligible under some standard practical conditions) and a variance slightly above the Cramér-Rao bound. The method is then applied to measurements from a contemporary BiCMOS transceiver which is used on one of the most popular daughterboards of the universal software radio peripheral. In our measurements, the phase skew varies up to 5^° with the baseband frequency, while the amplitude imbalance varies between 0 and 0.3 dB over carrier frequencies and across hardware units. The time variation, however, is only 0.004 dB in amplitude and 0.06 ^° in phase. This indicates that the units could either be calibrated online when there is no transmission (in a two-antenna MIMO system, one antenna could transmit a calibration signal to the other), or they could be calibrated during production, in which case a table with different carrier and baseband frequencies would be needed. However, there is no need to estimate the parameters on every burst.