頻域與用戶域之排程技術於合作式多天線與多基地台之研究

Abstract

在近幾年來，無線通訊傳輸對於頻寬的需求與日俱增，因此，多天線傳輸技術被提出用來改善系統的效能，以增加傳輸信號的可靠度以及提升通道的容量，然而，在多使用者環境下，使用者之間彼此的傳輸通道狀況並不相同，因此，在有限的傳輸頻寬下，為了確保系統的效能最佳化同時維持使用者間的公平性，我們也需要設計有效率的排程技術。在本論文中，我們將深入的研究進階的多天線傳送技術，並設計適合的排程技術以輔助多天線技術來提升系統的效能，探討在有限的傳輸頻寬下，除了確保系統的效能最佳化同時維持使用者間的公平性。本論文廣泛研究及設計進階多天線傳送技術於不同的無線通訊網路，包含(a)蜂巢式正交分頻多重接取系統、(b)多細胞網路，以及(c)異質性網路。所設計的進階多天線技術包含了(a)天線分集技術，以及(b)天線集束技術，可以有效的提升於無線通訊環境下傳輸信號的可靠度以及提升通道的容量。而所提出的排程技術則多面向的探討實體層與媒體存取層排程技術，探討在多使用者環境下，將有限的資源最作有效的利用。論文內容主要分成三個方向，分別簡述如下： 首先，在第一部分，我們研究在蜂巢式正交分頻多重接取下行系統中，利用循環延遲分集技術輔以多使用者頻率選擇性排程，以提升系統的效能。研究主要分成兩個方面：針對每根傳送天線設計最佳的循環延遲以及設計有效率的頻域排程技術。在設計最佳的循環延遲方面，我們針對不同使用者的通道狀況以及複雜度的考量，提出多個循環延遲設計的方法，來有效率的設計對於每個傳送天線最佳的循環延遲以提升子通道的容量。同時，再輔以多使用者頻率選擇性排程，可以顯著的提升系統的效能。 其次，在頻域排程技術方面，考量使用者們如何在有限的資源下有效率的回報排程所需的資訊，我們提出了基於通道選擇性的使用者排程回報技術來最佳化頻域排程的效率，最佳化頻域排程包含了決定最佳的總回報資源、考慮比例公平的機制分配回報資源給每位用者以及最佳化每位使用者的回報排程資訊。相較於先前的研究，在相同的回報資源下，我們所提出的使用者排程資訊的回報技術可以有效的提升系統的效能以及使用者間的公平性。 在第二部分，考慮在異質性網路的環境中，為了解決不同基地台之間的干擾問題，我們採用了干擾對齊技術，藉由在傳送端以及接收端的天線設計專門的集束向量，使得所有從鄰近基地台過來的干擾信號在接收端可以被對齊在一個子空間，來達到干擾消除的目的。我們提出了基於小細胞群的干擾對齊技術，來消於在階層式異質性網路的環境中的跨層以及單層細胞間干擾問題，我們所提出的干擾對齊技術可以有效率的消除大基地台以及小基地台之間的干擾，同時也消除了有合作的小基地台間的干擾。此外，考量到最大化自身的接受信號強度，我們提出了兩個低複雜度的媒體存取層排程技術，以配對最適當的基地台給使用者。相較於傳統的干擾對齊技術，我們所提出的方法可以有效率的改善系統的效能，尤其對於位在細胞邊緣地帶的使用者有顯著的改善。 在第三部分，我們研究機會干擾對齊技術，並結合實體層與媒體存取層排程技術來提升系統的效能。相較於在第二部分所提出的干擾對齊技術，所有合作的基地台需要分享彼此所服務的使用者的通道資訊來設計傳送端以及接收端的集束向量，因此在非理想的後端網路下會因為傳輸延遲而影響干擾對齊技術的效能；機會干擾對齊技術則是利用使用者們回報鄰近基地台對於其接收信號的干擾程度，並選擇所受干擾程度最小使用者來達到近似於干擾消除的目的。但傳統的機會干擾對齊技術其效能依賴於使用者的數目多寡，此外，在高訊號雜訊比時，殘餘的干擾信號會限制系統的效能，因此在第三部分，我們提出了新的機會干擾對齊技術，考慮在上行鏈路多細胞正交分頻多重接取系統中，針對不同的子通道，每位使用者會動態的設計傳送集束向量到特定的一個基地台的干擾子空間以消除對該基地台的干擾。我們所提出的機會干擾對齊技術可以改善受到嚴重干擾的基地台，同時也確保了基地台間資源分配的公平性。其次，在實體層與媒體存取層排程技術方面，實體層排程分配子通道給最適當的使用者，而媒體存取層排程在於配對最適當的基地台給使用者。然而，隨著使用者以及基地台的增加，實體層與媒體存取層排程技術將會耗費相當高的運算複雜度，因此，我們提出了基於基因演算法的實體層與媒體存取層排程技術，可以在系統效能以及系統複雜度間提供一個良好的取捨。 總結來說，在本論文中，我們研究了不同的排程策略對於進階多天線技術的重要性。除了提出多天線傳輸技術來改善系統的效能外，並設計適合的排程技術以輔助多天線技術來最佳化資源的分配及管理。

Recently, demands for broadband transmission have grown exponentially.To meet the requirement, multiple antenna techniques have received high attention in the last decades for their capabilities to improve the overall system performance, which not only increase the reliability of the communication but also impressively boost the channel capacity. However, advanced multiple antenna techniques alone do not ensure effective utilization of the available resources due to the channel randomness among users. Efficient scheduling policies are required to ensure data streams from the base station, the fundamental transmission unit of cellular networks, to various users efficiently and fairly, and vice versa. Therefore, in this dissertation, we investigate the cross-layer design for joint multiple antenna techniques and scheduling policies in wireless communications systems to enhance the system performance. The dissertation includes three issues: • Frequency-Domain Scheduling for Cyclic Delay Diversity Antenna System. • User-Domain Scheduling for Interference Alignment-based Antenna Beamforming in Multiple Base Stations. • Joint Frequency and User Domain Scheduling for Opportunistic IA-based Antenna Beamforming in Multiple Base Stations. In the first part, we investigate the cyclic delay diversity aided frequency selective scheduling for the OFDMA downlink system. The investigations are done from two aspects: cyclic-delay search and efficient frequency domain scheduling. For each user, several cyclic-delay search methods are proposed in efficiently searching the cyclic-delays for transmit antennas to maximize the sub-channel capacity. For efficient frequency domain scheduling, we jointly consider the channel dependent feedback method and frequency-selective scheduling to facilitate the scheduler, where the channel dependent feedback method includes the available total feedback resources, the feedback resources allocated to each user, and the optimal feedback information for each user. The proposed cyclic-delay values search methods provide good tradeoff between system performance and complexity, while the efficient frequency domain scheduling provides significant improvement in both system sum-rate and user fairness over previous methods under a fixed feedback overhead. In the second part, we propose an efficient user scheduling method for interference alignment (IA) based macrocell/small cells hierarchical overlaying wireless networks. The proposed method has been done from two aspects: Firstly, to address the interference problem in macrocell/ small cells hierarchical overlaying networks, we design an IA scheme in small cell group to eliminate the inter-cell interference (ICI) between macrocell/small cells and among coordinated small cells. Then the user scheduling method is applied to IA-based small cell group in efficiently pairing a group of users with coordinated small cells that maximizes the system throughput. Comparing to the conventional IA scheme without user scheduling method, the proposed method greatly improve the system sum-rate for both the users at the cell edge and those within a whole cell on average. In the third part, with the aim for a more practical implementation, the opportunistic IA (OIA) is investigated. In contrast to second part, the OIA is to harvest the multi-user diversity to construct asymptotically perfect IA when the number of users is large, without global CSI of all the wireless links or beamforming vector reporting. We propose a physical/media access control (MAC) cross-layer scheduling for OIA-based multi-cellular orthogonal frequency division multiple access (OFDMA) wireless system. Unlike previous OIA schemes that rely on the channel randomness among users to achieve asymptotic IA for all cells, the proposed scheme employ an active alignment approach to increase the efficiency for IA in the multi-cellular network. Specifically, in different sub-channels, each user adaptively adjusts the transmit beamforming vector to mitigate the user-generated interference to a specific cell according the predefined cell-specific interference sub-space. The proposed OIA scheme greatly improve the cell edge users’ performance, as well as improving the fairness among coordinated cell. Moreover, among the coordinated cells, we propose a physical/MAC cross-layer scheduling to the OIA-based multi-cellular OFDMA system. Where physical-layer scheduling allocates each sub-channel to a group of users, and MAC-layer scheduling aims for pairing a user with a base station. We design a genetic-algorithm (GA) to facilitate OIA-based cross-layer scheduling for sub-channel allocation and user scheduling, which provide good tradeoff between system performance and computational complexity. In summary, in this dissertation, we investigate the frequency-domain and user-domain scheduling for cooperative multiple antennas and base stations to optimize the system throughput. Advanced multiple antenna techniques provide huge gains in the communication reliability and the channel capacity,while efficient scheduling algorithms ensure data streams from the base station to various users fairly and efficiently, and vice versa. This dissertation provides important insights into the designs of multiple antenna techniques, user information feedback, and scheduling algorithms to improve the system performance in both system sum rate and user fairness.