| Summary: | Thesis (PhD)--University of Stellenbosch, 2005. === 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.
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