Baseband compensation principles for defects in quadrature signal conversion and processing

Van Rooyen, Gert-Jan (2004-04)

Thesis (PhD)--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: Keywords: software-defined radio, SDR, quadrature mixing, quadrature modulation, quadrature demodulation, digital compensation, software radio, direct-digital synthesis, DDS. An often-stated goal of software-defined transceiver systems is to perform digital signal conversion as close to the antenna as possible by using high-rate converters. In this dissertation, alternative design principles are proposed, and it is shown that the signal processing techniques based on these principles improve on the prior system's accuracy, while maintaining system flexibility. Firstly, it is proposed that digital compensation can be used to reverse the effects of hardware inaccuracies in the RF front-end of a software-defined radio. Novel compensation techniques are introduced that suppress the signal artefacts introduced by typical frontend hardware. The extent to which such artefacts may be suppressed, is only limited by the accuracy by which they may be measured and digitally represented. A general compensation principle is laid down, which formalises the conditions under which optimal compensation may be achieved. Secondly, it is proposed that, in the design of such RF front-ends, a clear distinction should be drawn between signal processing complexity and frequency translation. It is demonstrated that conventional SDR systems often neglect this principle. As an alternative, quadrature mixing is shown to provide a clear separation between the frequency translation and signal processing problems. However, effective use of quadrature mixing as design approach necessitates the use of accurate compensation techniques to circumvent the hardware inaccuracies typically found in such mixers. Quadrature mixers are proposed as general-purpose front-ends for software-defined radios, and quadrature modulation and demodulation techniques are presented as alternatives to existing schemes. The inherent hardware inaccuracies are analysed and simulated, and appropriate compensation techniques are derived and tested. Finally, the theory is verified with a prototype system.

AFRIKAANSE OPSOMMING: Sleutelwoorde: sagteware-gedefinieerde radio, SDR, haaksfasige menging, haaksfasige modulasie, haaksfasige demodulasie, digitale kompensasie, sagteware-radio, direk-digitale sintese, DDS. 'n Gewilde stelling is dat digitale seinomsetting in sagteware-gedefinieerde kommunikasiestelsels so na as moontlik aan die antenna moet geskied deur gebruik te maak van hoëspoed omsetters. Hierdie verhandeling stel alternatiewe ontwerpsbeginsels voor, en toon aan dat hierdie beginsels die eersgenoemde stelsel se akkuraatheid verbeter, terwyl stelselbuigsaamheid gehandhaaf word. Dit word eerstens voorgestel dat digitale kompensasie gebruik word om die effekte van hardeware-onakkuraathede in die RF-koppelvlak van sagteware-gedefinieerde radio's om te keer. Nuwe kompensasietegnieke, wat seinartefakte weens koppelvlak-onakkuraathede kan onderdruk, word aangebied. Die mate waartoe hierdie artefakte onderdruk kan word, word slegs beperk deur die akkuraatheid waarmee dit gemeet en digitaal voorgestel kan word. 'n Algemene kompensasiebeginsel word neergelê waarin die voorwaardes vir optimale kompensasie vasgelê word. Tweedens word voorgestel dat 'n duidelike onderskeid getref word tussen seinverwerkingskompleksiteit en seinverskuiwing in RF-koppelvlakke. Daar word getoon dat konvensionele SDR-stelsels dikwels nie hierdie beginsel handhaaf nie. 'n Alternatief, naamlik haaksfasige menging, word voorgehou as 'n tegniek wat duidelik onderskei tussen seinverskuiwing en seinverwerking. Akkurate kompensasietegnieke is egter nodig om effektief van sulke mengers gebruik te maak. Haaksfasige mengers word voorgestel as veeldoelige koppelvlakke vir sagteware-gedefinieerde radio's, en haaksfasige modulasie- en demodulasietegnieke word voorgestel as plaasvervangers vir bestaande tegnieke. Die inherente hardeware-onakkuraathede word geanaliseer en gesimuleer, en geskikte kompensasietegnieke word afgelei en getoets. Laastens word die teoretiese resultate met 'n praktiese prototipe bevestig.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/49869
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