近年來已有一些研究針對採用混合主動元件(active elements；AEs)與被動元件(passive elements; PEs)配置之可重構智慧反射面(reconfigurable intelligent surface；RIS)架構的毫米波無線通訊系統，探討通道估測問題；在這些研究中，AEs通常是隨機分佈在RIS上，而此種安排通常會導致令人不滿意的通道估測效能。為了處理這個問題，本論文提出一種新的AEs選擇方法，以強化通道估測效能；此方法透過全球導航衛星系統獲得用戶設備(user equipment；UE)的即時位置，並據以設計RIS中的AEs佈局。我們將RIS劃分為多個子區域，再根據UE的位置從每個子區域中策略性地選擇適當個數的AEs；我們還提出一種貪婪座標下降(greedy coordinate descent)演算法，以降低AEs選擇的運算複雜度；為了迅速調整AEs的位置以因應連續移動的UE，我們進一步將強化學習技術應用於AEs選擇；最後，我們利用所選擇之AEs和壓縮感知技術進行UE與RIS之間的通道估測。電腦模擬結果顯示，相較於隨機AEs選擇機制，所提出的AEs選擇方法可明顯改善通道估測效能；此一優越效能特點使得採用混合式AEs與PEs配置之RIS架構極有潛力應用於實際無線通訊系統。

Recently, some investigations have been made on channel estimation for millimeter wave wireless communication systems using the reconfigurable intelligent surface (RIS) architecture with a hybrid configuration of active elements (AEs) and passive elements (PEs). In these works, AEs were typically randomly distributed on the RIS, and this arrangement often caused unsatisfactory channel estimation performance. To deal with this issue, this thesis proposes a novel scheme for selection of AEs to enhance the channel estimation performance, where the layout of AEs is well designed according to the real-time location of user equipment (UE) obtained from the Global Navigation Satellite System. The RIS is partitioned into multiple subregions and an appropriate number of AEs are selected strategically from each subregion according to the UE location. A greedy coordinate descent algorithm is also proposed to reduce the computational complexity of the selection process. To adjust the positions of AEs quickly for a continuously moving UE, reinforcement learning is further used for the selection of AEs. With the selected AEs, channel estimation is finally performed between the UE and RIS based on compressed sensing. It is demonstrated by computer simulation results that the proposed AEs selection scheme induces a significant performance improvement in channel estimation over the random AEs selection approach. The excellent-performance feature makes the RIS architecture with a hybrid configuration of AEs and PEs highly promising for use in practical wireless communications.

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