分頻雙工巨量多天線系統中多使用者之下行波束成型

Abstract

本論文中探討在分頻雙工巨量天線系統中針對多位使用者在下行傳輸之波束成型設計之議題。由於巨量天線系統具備大幅提高頻寬效益及傳輸能量效率之能力，已被視為實現下一代無線通訊需求之重要技術。在此系統下，如何使基地台獲取與使用者間下行通道資訊為此系統效能提升之關鍵。分時雙工模式基於通道互惠之特性，能夠使基地台有效獲取下行通道資訊，因此被目前與巨量天線系統相關之研究廣泛採用。雖然分頻雙工模式並不具有通道互惠性，它卻更適合用於需要低時間延遲之通訊服務，因此分頻雙工模式在目前的行動通訊系統中被大量採用。下行資料傳輸效能可藉由下行波束成型設計以達到最佳化。而波束成型之設計通常需要下行通道之資訊。在上一代通訊系統為分頻雙工系統獲取通道資訊的機制中，下行訓練及上行回饋為必要之手段。然而，在分頻雙工巨量天線系統中，使用以往獲取通道資訊的機制會產生大量的頻寬損耗。因此，此論文中提出了不需下行訓練及上行回饋之波束成型設計方法。這些方法更適合用在需要極度低延遲的通訊應用中。除此之外，本論文也提出了能夠大幅降低為了下行訓練及上行回饋產生的頻寬損耗之通道估計方法。此方法能顯著提升分頻雙工多天線系統中下行傳輸的頻寬效益。 關鍵字：巨量多天線系統、波束成型、分頻雙工

This dissertation addresses the fundamental issues of beamforming designs in massive multi-input-multi-output (MIMO) systems, especially for these systems operated under the frequency-division duplexing (FDD) mode. Massive MIMO is regarded as a crucial technology in enabling the next generation mobile communications due to its capability of enhancing system capacity and improving energy efficiency. Acquiring accurate downlink (DL) channel state information (CSI) at the transmitter for performing DL beamforming is regarded as a key of realizing the advantages of massive MIMO. The majority of studies concerning massive MIMO adopted the time-division duplexing (TDD) mode because it enables the base station (BS) to acquire DL CSI efficiently due to channel reciprocity. FDD massive MIMO is also considered because FDD is more suitable for providing low-latency services, and that is why it is widely deployed in nowadays cellular networks. Nevertheless, channel reciprocity no longer holds in FDD systems. Therefore, DL training and uplink (UL) feedback prior to DL data transmission are usually required for CSI acquisition at the BS. Compressing the overhead of DL training and UL feedback is considered utterly critical to the validation of FDD massive MIMO. In the literature, many channel estimation and beamforming schemes aimed at improving the efficiency of DL training and UL feedback have been proposed. However, these schemes all result in more round-trips between the BS and UEs than what TDD massive MIMO possesses. This implies longer latency of DL transmission. In this dissertation, beamforming schemes that need neither DL training nor UL feedback are proposed. Therefore, the proposed schemes are more suitable for extremely low-latency applications, which play an important role in the next generation wireless communication systems. The proposed schemes take advantage of the angular reciprocal property of FDD systems, which was suggested by a number of channel measurement studies. This property is called angle reciprocity in the literature. The simulation results provided in this dissertation indicate that the proposed schemes possess comparable performance to previous schemes that need DL training and UL feedback. Furthermore, a two-stage beamforming scheme with significantly reduced overhead of training and feedback is proposed by utilizing angle reciprocity. It is demonstrated that the proposed two-stage beamforming scheme outperforms some other schemes in achievable spectral efficiency. An optimization method for multicarrier new transmission waveform design is also proposed in this dissertation. Keywords: Massive MIMO, robust beamforming, beam-time block coding, no CSI feedback, two-stage beamforming, new waveform.