Modelling, estimation and compensation of imbalances in quadrature transceivers

[摘要] ENGLISH ABSTRACT: The use of the quadrature mixing topology has been severely limited in the past due toits sensitivity towards mismatches between its signal paths. In recent years, researchershave suggested that digital techniques can be used to compensate for the impairments inthe analogue quadrature mixing front-end. Most authors, however, focus on the modellingand compensation of frequency-independent imbalances, reasoning that this approach issufficient for narrow band signal operation. This common assumption is, however, becomingincreasing less applicable as the use of wider bandwidth signals and multi-channel systemsbecomes more prevalent.In this dissertation, baseband equivalent distortion models are derived, which modelfrequency-independent, as well as frequency-dependent contributions towards the imbalancesof the front-end. Both lowpass and bandpass imbalances are modelled, which extends currentmodelling approaches found in literature. The resulting baseband models are shown to becapable of explaining the imbalance characteristics observed in practical quadrature mixingfront ends, where existing models fail to do so.The developed imbalance models is then used to develop novel frequency-dependent imbalanceextraction and compensation techniques, which directly extract the exact quadratureimbalances of the front end, using simple test tones. The imbalance extraction and compensationprocedures are implemented in the digital baseband domain of the transceiver anddo not require high computational complexity. The performance of these techniques aresubsequently verified through simulations and a practical hardware implementation, yieldingsignificant improvement in the image rejection capabilities of the quadrature mixingtransceiver.Finally, a novel, blind imbalance compensation technique is developed. This techniqueis aimed at extracting frequency-independent I/Q imbalances in systems employing digitalmodulation schemes. No test tones are employed and the imbalances of the modulator anddemodulator are extracted from the second order statistics of the received signal. Simulationsare presented to investigate the performance of these techniques under various operatingconditions.

[发布机构] Stellenbosch University