本論文中針對了空中可重構智慧反射表面的位置不確定性問題作探討，建立了一個由空中可重構智慧反射表面輔助的三維下行傳輸系統。儘管空中可重構智慧反射表面相比於地面可重構智慧反射表面有著全向性反射以及可自由調整其位置的好處，它必須被裝設在氣球或無人機等空中平台上以確保它維持在一定高度的天空中，而這導致了額外的功能消耗，同時它的實際位置也可能因晃動或估測錯誤而導致與系統認知的位置不同。對於搭載著龐大數量之被動式反射元件的可重構智慧反射表面來說，些微的位置變動都會使表現大幅下降，因此需要特別針對因位置不確定性而產生的向為誤差作處理。本篇論文提出了兩種被動波束成形設計方法去降低相位誤差對表現所帶來的影響，其一是使用最佳化方法將問題轉化成凸優化的半正定規劃問題，其二則是波束平坦化技術，目的在於拓寬由空中可重構智慧反射表面建構的波束以提升系統對相位誤差的容忍程度。在晃動程度到一定程度下，這兩種方法所建構的強韌被動波束成形可以提供更好的傳輸品質。

This thesis considers a 3D aerial reconfigurable intelligent surface (ARIS) downlink system with ARIS location uncertainty. Compared with the terrestrial RIS, ARIS can provide panoramic full-angle reflection which requires only one reflection to serve the nodes in the coverage area. Furthermore, the adjustable location and height of ARIS makes the deployment and beamforming design more flexible. However, ARIS must be equipped on the aerial platforms such as balloon or unmanned aerial vehicle (UAV) to maintain its height. This requirement causes more energy consumption and ARIS location uncertainty. For the RIS with numerous passive reflecting elements, the beamforming performance is sensitive to the location variation. Thus, it's necessary to address the effect of the phase error caused by the ARIS location uncertainty. In this thesis, we propose two methods to design a robust passive beamforming of the ARIS which aims to reduce the effect of the phase error. One is the optimization method which transforms the non-convex problem of passive beamforming design into a convex semi-definite (SDP) problem. Another is beam flattening technique, a unique concept that broadens the beamwidth of the beam constructed by the ARIS to increase the tolerance for phase error. With a robust passive beamforming design to deal with the ARIS location uncertainty, the quality of service (QoS) can be improved when the location variation is sufficiently large.

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