適用於毫米波無線電通道之利用雙極化天線陣列可重組式混合波束成型系統

Abstract

本論文研究重點是考慮在毫米波(Millimeter Wave, mmWave)的環境中使用雙極化天線陣列的可重組式混合式波束成型 (Reconfigurable Hybrid Beamforming) 架構。混合式波束成型架構好處是可以有效率的實現大型多天線陣列系統，傳統上要實現多輸出輸入陣列天線系統的做法為每一單元天線皆有其專屬射頻和基頻之間的轉換電路，然而在大型陣列天線上這樣的實作方式極為昂貴而且耗能。為了有效降低其成本和使用多輸入輸出技術所帶來的好處，我們所發展的混合式波束成型架構僅使用少數的轉換電路，一方面可利用射頻波束成型的波束增益來克服毫米波通訊的高訊號衰減率，另一方面於基頻端仍保有一定的自由度對訊號串流進行MIMO訊號處理。另外由於單一饋入方向的貼片天線在空間中具有傳輸死角，為了彌補極化在空間上的死角與毫米波環境中訊號的高相關性問題，我們使用雙極化的貼片天線來降低這兩者所帶來的影響。 傳統固定式混合波束成型系統，對於資料串流數目和射頻基頻轉換電路與天線陣列之間連接皆為固定，然而當通道條件或是使用者位置改變，系統可能無法達到最好通訊品質。基於傳統式架構，本論文提出一種可重組式混合波束成型架構，在基頻與射頻轉換電路和天線陣列之間加入類比多工器，使得在相同數目的射頻基頻轉換路徑和天線下可以動態轉換，改變連接方式，並且根據使用空時編碼(Space-time code)或空間多工(Spatial multiplexing)在基頻端設計前置編碼器，最後設計一套訓練流程，讓系統在各種情境中都可調整至最好的傳輸模式並獲得最大通道容量。由模擬結果可得到，系統根據不同基地台與使用者位置、傳輸功率和傳播路徑，由使用者回傳的通道資訊來設計前置編碼器，並透過訓練流程調整至最好的傳收模式，通道容量都有顯著提升。相較於固定式的架構，可重組式架構更有彈性且有額外自由度，並且能夠提升通道容量，對於下世代行動通訊是一種具有潛力的技術之一。

This thesis studies the reconfigurable hybrid beamforming architecture with dual-polarized phased antenna arrays for millimeter wave (mmWave) communication systems. Traditionally, to implement a MIMO antenna array system, every single antenna requires a RF chain which is a very expensive and high power consumption approach to achieve large-scale multi-antenna array system. In contrast, the hybrid beamforming architecture is a rather cost-effective option since less RF chains are needed. Besides, it not only compensates high path loss in mmWave, but also exploits the degree of freedom in signal processing. Furthermore, we use dual-polarized patch antenna to reduce the effect from the dead zone of single-polarized patch antenna and from the high spatial correlation issue in mmWave communication systems. For the traditional fixed hybrid beamforming system, the connecting of RF chains and antenna arrays is fixed. When the channel conditions or the positions of users change, the system may not achieve the best quality of communication. Based on the traditional architecture, this thesis proposes a reconfigurable hybrid beamforming architecture. Combine the analog multiplexers between RF chains and antenna arrays, let it can dynamically adjust the connecting of RF chains and antenna arrays and conduct space-time code or spatial multiplexing and design their corresponding baseband precoder. Finally, design a training flow to let the system can choose the best operating mode. By the simulation results, the system will adjust to the best operating modes according to the training flow for different channel conditions and the capacity will significantly increase. Comparing with the fixed architecture, the reconfigurable one is more flexible and has another degree of freedom to increase the capacity. For the next generation mobile communication, it is one of the potential techniques.