LTE-Advanced中利用動態頻率重用及階層式群集多點協作以提升傳輸效能之研究

Abstract

長期演進技術演進版本(LTE-Advanced)利用載波聚合(Carrier Aggregation)、多天線輸出輸入(MIMO)與多點協作傳輸(CoMP)技術大幅提升上傳與下載的傳輸速率及加強基地台服務的覆蓋率。LTE-Advanced以其技術上的優勢，已儼然成為第四代行動通訊系統的主流技術。 Femtocell是為了克服室內環境通訊品質不佳而於2009年由3GPP Rel-8訂出的標準。Femtocell具有低功率、即插即用、低成本、強化行動寬頻網路室內覆蓋率、提升使用者傳輸效能(throughput)及緩解Macrocell之loading等許多優點。Femtocell是用戶只要自行接上線路就可使用的「Zero-Touch」模式。雖然設定方便，但會有以下問題：Femtocell與Femtocell間的干擾、Femtocell與外部Macrocell的干擾、何時需調高功率以提升訊號強度、何時需調降功率以減少干擾等。本計劃擬提出適用於LTE Macrocell及Femtocell的動態頻率重用技術，動態分配及重用頻段以有效提升細胞邊緣(cell edge)及相鄰扇行區(adjacent sector)的傳輸效能。此外，我們再搭配改良後的多點協作傳輸(CoMP)的「多點聯合處理(JP)」與「協調式排程/波束成型(CS/CB)」，並利用多天線輸出輸入(MIMO)通道技術來加強傳輸效能(throughput)。LTE Direct是3GPP在Rel-12所制訂，利用現有LTE頻譜支援裝置與裝置(Device to Device，D2D)通訊的技術。LTE Direct有助於達到強化系統容量(system capacity)及行動數據分載(mobile data offload)的目的。LTE Direct雖然俱有技術及應用上的優勢，但因其使用現有的LTE頻譜來傳輸資料，若共用頻譜缺乏妥善管理，D2D UE會因額外的干擾而降低SINR。目前關於D2D頻率分配的相關研究大多屬於靜態頻率重用，在本計劃中，我們擬研究並提出適用於LTE Direct的動態頻率重用技術來降低干擾，以達到更高的傳輸效能。CoMP技術是3GPP於Rel-11所制訂，適用於LTE-Advanced傳輸的多點協作傳輸技術。CoMP可藉由協調多點傳送與接收以提升基地台服務的覆蓋範圍及強化細胞邊緣的傳輸效能。然而若基地台的頻率重複使用，則來自其它基地台的干擾將會影響該基地台的傳輸效能。在本計劃中，我們擬研究並提出階層式群集多點協作傳輸(Hierarchical Clustering based CoMP)技術，透過動態協調多個基地台以互相調整訊號，將來自其它基地台的干擾轉成對細胞邊緣有用的訊號，以達到最佳的傳輸效能。 總結來說，本計劃針對在LTE-Advanced結合Femtocell與LTE Direct的架構，透過動態頻率重用及階層式群集多點協作傳輸技術達到有效分配頻譜、降低干擾、提升傳輸效能、增加系統容量的目的。我們透過實驗與模擬兩種方法驗證本計劃中所提出之方法的有效性。關於實驗部分，我們量測多支手機在不同LTE-Advanced基地台內不同地理位置的干擾問題。關於模擬部分，我們透過支援LTE-Advanced、Femtocell、LTE Direct模組的網路模擬器模擬各種不同的網路情境。

LTE-Advanced distinguishes itself as the main 4G technology because it employs Carrier Aggregation (CA), Multi Input Multi Output (MIMO) and Co-ordinated Multi-Point Transmission (CoMP) state of the art technologies to significantly enhance the uplink and downlink transmission performance and is able to largely extend service coverage. Femtocell, specified in 3GPP Rel-8, is used to overcome the low communication quality in indoor environment. Femtocell has the property of low power consumption, plug and play, low cost, coverage enhancement, mobile data offload, etc. Femtocell can be easily installed by end users in a ”Zero-Touch” scenario. However, it has to face the following problems: interferences between femtocell / femtocell and macrocell / femtocell, when to increase the power to enhance signal strength and when to reduce the power to prevent interference. In this research, we proposed a dynamic frequency reuse technique and hierarchical clustering based CoMP for LTE Macrocell and Femtocell to enhance the cell edge and adjacent sector transmission performance. LTE Direct, specified in 3GPP Rel-12, uses the same LTE spectrum and is able to support Device to Device (D2D) communication. LTE Direct has the advantages in increasing system capacity and mobile data offloading. Since LTE Direct shares the same spectrum as that of LTE, spectrum management becomes one of the most important issues. In the literature, most of the frequency allocation strategies are static, we proposed a dynamic frequency reuse technique suitable for LTE Direct in this research. CoMP, specified in 3GPP Rel-11, is used for LTE-Advanced co-ordinated multipoint transmission. CoMP is able to extend service coverage and cell edge transmission. In this research, we proposed a hierarchical clustering based CoMP to enhance transmission performance through coordinating multiple points. In summary, this research proposed a dynamic frequency reuse and hierarchical clustering based CoMP to optimize frequency allocation, reduce interference, and enhance transmission and system capacity under LTE-Advanced / Femtocell / LTE Direct architecture. Finally, we used both experiment and simulation to verify the effectiveness of our proposed methodology.