Investigating the USRP: I/Q imbalance

THE development of contemporary and future wireless communication systems puts high demands on accurate and timeefficient test methods for production and product validation. Reducing the time for a test is essential and the accuracy of the applied method with respect to the length of the recorded data sequence is an important figure of merit to trade test time versus accuracy of the output of the test. Efficient measurement methods are also required as a prerequisite for digital correction to combat impairments produced by the analog circuitry – in line with the dirty radio frequency (RF) paradigm [1]. Rapid prototyping of new digital communication technologies is essential, and often relies on software defined radio implementations. A widespread hardware platform is the available variant of the universal software radio peripheral (USRP) by Ettus Research LLC, which is becoming a common technology for research, education, and development [2]. We have proposed a least-squares approach to determine the I/Q imbalance of a direct conversion receiver, based only on receiver baseband data. We use I/Q imbalance parameters defined as in [3]. Under a Gaussian assumption, the accuracy of the method has been addressed, yielding closed form results. Experimental data from four different USRP receivers has been collected. It has been shown that gain imbalance and quadrature skew are accurately estimated employing data that covers only a handful of full periods of the test stimuli, which highlights the practical relevance of the derived test method. It has also been shown that that estimating the LO leakage is a more complicated problem, not due to the accuracy aspects but to the systematic errors, providing errors in order of 10 − 20 dBs in scenarios where gain imbalance and quadrature skew are accurately estimated. By examining samples of the USRP receiver, we have not only validated its data-sheet performance, but also observed some outlier performance of a sample receiver. The evaluation of USRP receivers