Quantization effects on beamforming in dense phased arrays

Wilke, Cornelis Rossouw (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Reflector-based radio telescopes have traditionally been the instrument of choice in Radio Astronomy since around the 1960’s. However, in particular for frequencies below around 1 GHz, a new generation of aperture array radio telescopes are being developed internationally. These aperture arrays consist of phased antenna arrays that are electronically steered by making use of analogue and digital beamforming techniques. The implementation cost of an analogue beamformer is dominated by its complexity of which its phase resolution has a significant contribution. On largescale systems such as the Square Kilometre Array, it is of substantial interest to investigate the possibility of reducing subsystem cost, which additionally serves as the motivation of this thesis. Reducing the phase resolution of an analogue beamformer can have an impact on its implementation cost, but also its performance in an array. Important beamforming performance metrics for Radio Astronomy are identified as the main beam pointing accuracy and the side lobe level. A methodology is developed by which these performance metrics can be evaluated as a function of the array parameters and phase resolution of the beamformer. Initially, the performance is determined using array factor based beamforming on a forced excitation model, with phase weights quantised according to a certain phase resolution. The performance is, therefore, determined purely as a function of the array configuration without the effects of mutual coupling. In this way, the impact of phase quantization is directly observed that would otherwise be obscured by the contribution of mutual coupling. As a conclusion, a demonstrative study is performed on a free excitation planar model, which implies that mutual coupling is incorporated. To obtain coupling parameters, a Dense Dipole Array (DDA) prototype tile is built and measured. The performance metrics were then evaluated, although to a lesser extent compared to the forced excitation model, due to limitations in the practical measurements of the DDA. Where applicable, the results were successfully explained from insight obtained in the forced excitation model, which further confirmed the use of the proposed methodology presented in this thesis. The performance metrics were evaluated by simulation based on array sizes that accommodate the available processing power and, for the free excitation models, the angular resolution in the practical measurements. Therefore, the thesis serves as a demonstration of a proposed methodology. The results, however, did provide valuable insight on quantization effects that will be applicable in larger arrays.

AFRIKAANSE OPSOMMING: Vanaf omtrent die 1960’s was skottel-gebaseerde radio teleskope normaalweg die instrument van keuse wanneer dit kom by Radio Astronomie. Vandag word daar egter reg oor die wereld ’n nuwe generasie van gefaseerde antenna samestelling radio teleskope ontwikkel. Hierdie samestellings word elektronies gestuur deur gebruik te maak van digitale en analoog bundelvorming tegnieke. Die implementering koste van ’n analoog bundelvormer word oorheers deur sy kompleksiteit, waarvan sy fase-resolusie ’n groot bydra het. Op groot-skaal sisteme soos die Vierkante-Kilometer Samestelling is dit van belang om die moontlikheid van ’n reduksie in subsisteem koste te ondersoek. Deur die fase-resolusie van ’n analoog bundelvormer te verlaag kan ’n impak op sy implementering koste gemaak word, maar ook sy optrede. Belangrike optree-aspekte van ’ bundelvormer in Radio Astronomie is geïdentifiseer as die hoof lob wys-akkuraatheid en die sy lob vlak. ’n Metode is ontwikkel waarmee die optree-aspekte van ’n samestelling bepaal kan word in terme van sy fisiese eienskappe en die fase-resolusie van die bundelvormer. Die optrede is aanvanklik bepaal deur gebruik te maak van skikking faktor gebaseerde bundelvorming op ’n geforseerde-opwekking model, met fase gewigte gekwantiseer volgens ’n bepaalde fase-resolusie. Die optrede is daarvoor bepaal slegs as ’n funksie van die samestelling se eienskappe, sonder die effekte van wedersydse koppeling. Op hierdie manier word die effekte van fase-kwantifiseering direk vasgestel wat andersins verduister sou wees deur die bydra van wedersydse koppeling. ’n Finale studie is gedoen gebaseer op ’n vrye-opwekking model, wat impliseer dat wedersydse koppeling in ag geneem word. Om koppeling parameters te verkry is ’n Digte Dipool Samestelling (DDS) prototipe teël gebou en gemeet. Die optree-aspekte is toe bestudeer, hoewel tot ’n mindere mate in vergelyking met die geforseerde opwekking model, asgevolg van beperkings in die praktiese metings van die DDS. Waar van toepassing, was die resultate verduidelik deur gebruik te maak van die insig wat verkry is in die geforseerde-opwekking model. Dit het ook verder die metodiek wat in hierdie tesis aangebied word bevestig. Die optree-aspekte is bestudeer deur simulasies te gebruik wat gebaseer is op samestelling groottes wat beide die beskikbare rekenaarkrag en die hoek resolusie van die praktiese metings wat in die geforseerde-opwekking model gbruik is, akkommodeer. Die resultate het wel waardevolle insig in die effekte van fase-kwantiseering voorsien wat van toepassing sal wees op groter samestellings.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103460
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