可重構智慧表面是由大量的被動反射元件組成，並且可以藉由調整各個元件的相位，將入射的電磁波訊號反射至特定的方位。由於此項特性，可重構智慧表面在近幾年備受關注，而且被視為在未來的無線通訊架構下極具潛力的節能技術。在本篇論文中，我們考量一個由可重構智慧表面輔助的多用戶通訊場景，在此架構下，基地台的波束成形以及可重構智慧表面的相位普遍藉由最佳化理論來設計。雖然現存的研究證實了可重構智慧表面帶來的巨大增益，不過由於基地台、可重構智慧表面與用戶之間的耦合通道，同時設計基地台與可重構智慧表面的波束成形會產生非常高的複雜度。此外，可重構智慧表面的次波長結構以及傳播環境下散射的不足都會產生空間相關性，進而導致嚴重的用戶間干擾。在此篇論文中，我們提出兩種波束成形設計方式，相較於利用最佳化理論的設計策略，此兩種設計方式擁有較低的複雜度。同時我們也利用使用者分群技術來降低空間相關性帶來的嚴重干擾，由於使用者分群技術的應用，可重構智慧表面的配置模式也根據是否參考分群結果來調整相位而分成兩種。藉由模擬結果可以發現，對比於未使用分群技術的情況，我們提出的方式可以得到更高的整體傳輸率。除此之外，我們推導出一個通用的表示式來代表可重構智慧表面輔助系統中相關係數的期望值，並藉由數值結果來證實此理論分析的準確性，甚至可以直覺地觀察可重構智慧表面輔助系統的通道硬化特性。

This thesis considers the reconfigurable intelligent surface (RIS)-assisted multi-user communication scenario, where an RIS is used to assist the base station (BS) for serving multiple users. The RIS consisting of passive reflecting elements can manipulate the reflected direction of the incoming electromagnetic waves and thus it offers a new design dimension to the system designer. To jointly determine the BS beamforming and RIS phase shifts, the tools from optimization theory is commonly used. While the existing work has shown significant gain of RIS, the complexity of solving the joint BS and RIS beamforming is quite high due to the coupled structure of the cascade channel from the BS through RIS to the user. In addition, the sub-wavelength structure of the RIS and the insufficient scattering in the propagation environment introduce spatial correlation that may cause strong interference to users. In this thesis, we propose two beamforming design strategies with much lower complexity than that using the optimization theory. To handle strong interference due to spatial correlation, we also consider user grouping that leads to two configuring modes for RIS depending on whether the grouping result is taken into account when determining the RIS phase shifts. Simulation results are presented to demonstrate the superior sum rate achieved by the proposed methods than that without user grouping. Moreover, we derive a general expression to represent the expected value of the correlation coefficient in the RIS-assisted system. The numerical results depict the accuracy of our analysis and provide insights into property of channel hardening in RIS with a large number of antenna elements.

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