| GNSS provides passive positioning service by transmitting radio signal with theon-orbit satellites which makes signal quality play a crucial role for the systemperformance. Practical transmitted navigation signal is the product of ideal signal whichexperienced a series of linear and non-linear distortions due to the non-idealities of coredevices in the on-board payload, modernized navigation signal is affected by suchnon-idealities more significantly because of its wider bandwidth compared with traditionalGPS signal. The distorted navigation signal may lead to the degradation of positioningprecision and accuracy or even disable the system from positioning, thereforecompensation for the distortion properties of signal transmitting channel is of greatsignificance.For the purpose of improving the deterioration effect on signal quality introduced bythe non-idealities of on-board transmitting channel, this thesis studies the channelcompensation algorithm by means of signal predistortion and evaluates its improvementfor signal quality. In this paper: generalized navigation signal model is studied as well asthe receiving theory, influence of signal distortion over receiving performance is analyzed;quality evaluation indices used for navigation signal are introduced, relation betweenabove evaluation indices and receiving performance is analyzed; core devices of theon-board payload are modeled respectively including memoryless high power amplifier,high power amplifier with memory, filter, their influence on navigation signal quality isanalyzed; predistortion algorithm based on lookup table is proposed for memoryless highpower amplifier, where on-line learning and off-line learning schemes are included andtheir linearization performance is verified; predistortion algorithm for high poweramplifier with memory is proposed including an indirect-learning-based memorypolynomial method and a filter lookup table cascading method; predistortion method forfilter is proposed and its improvement made for frequency response is verified;predistortion algorithms for each single non-ideal devices as well as the wholetransmitting channel are simulated and verified to examine their improvement for signalquality. Study of this paper has shown that the group delay ripple of the on-board filterintroduces significant impairment in positioning accuracy and that the non-linearityproperty of high power amplifier would bring in out-of-band spectrum regrowth, besidesthe degradation of positioning precision and demodulation performance can be induced bythose devices. The compensation for channel distortion property by signal predistortioncan result in comprehensive and significant improvement of navigation signal quality andfinally presented as the improvement of GNSS service performance. |
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