下一代無線網路增進技術之研究

Abstract

現今無線網路技術持續不斷地朝著更高的傳輸速度和更大的容量演進，然而由於頻譜資源的限制，下一代的無線網路必須採用各種先進且更具彈性的技術以達到更高的頻譜效率和使用率，這些技術包括：頻寬或無線資源的動態配置、多通道或多載波聚合、超微型基地台(femtocell)的佈建、多輸入多輸出(MIMO)傳輸、合作式多點傳輸等等，本論文針對其中前三項技術提出四個研究主題加以探討。 在第一個主題中，我們提出一個可供無線虛擬網路業者(MVNO)共享網路頻寬或無線資源之系統模型，同時針對此模型設計了一個高效能動態頻寬配置演算法，此演算法根據各虛擬業者之交通特性估計其可能帶來之收入，並利用動態規劃之技巧一次考慮多個時段之收入與可能的損失，以最大化無線頻寬提供者之收益。模擬結果顯示此演算法可有效地提高收益和頻寬使用率。 無線網狀網路的骨幹是由多重跳躍之無線轉運節點所組成，加以無線通訊協定中每傳輸一個封包就必須附加不少的控制虛耗，這容易造成通道使用效率低落，尤其是當網路中傳輸大量小封包時(例如：VoIP之語音封包)，無線骨幹將成為網路的瓶頸。為解決此問題，我們在第二個主題中為無線網狀網路之轉運節點設計了一個可同時使用多個平行通道的封包排程器，並加入封包串接功能，藉由良好的通道選擇和封包串接演算法，我們不僅提高了通道的使用效率和骨幹的容量，同時有效壓抑了可能發生之封包順序錯亂問題。此外，我們也針對此排程器架構推導出一個可廣泛適用於多種排程演算法之延遲上限公式，有助於各種允入控制之計算。 在第三個主題中，我們提出一個以無線虛擬網路業者推動超微型基地台佈建之全新模型，此模型探討了超微型基地台傳輸功率設定和原本大型基地台(macro base station)使用者行為之間的互動關係，並將此情境表示成一個雙方各自做最佳化決策的數學問題，透過賽局理論方法，我們求得此問題之奈許平衡解(Nash equilibrium)，同時藉由數值範例結果證明，此解法求得之傳輸功率設定可在不減損服務品質的情形下大幅降低對大型基地台使用者之干擾。此結果將有助於處理未來大規模佈建超微型基地台時之干擾問題。 最後一個主題所探討的情境和資源動態分配、多載波聚合(carrier aggregation)、超微型基地台三項技術均相關。在未來LTE-Advanced的環境中，基地台與使用者設備均可同時使用多個載波，未經規劃佈建的超微型基地台將可利用這項特性自動地挑選適當的載波以避免產生過多的干擾。目前3GPP所提出的自主式成分載波選擇(Autonomous Component Carrier Selection, ACCS)演算法在典型家庭上網情境中，很可能因為部分超微型基地台長期佔用資源造成效能低落。為此我們提出透過適當地排序重選載波和功率配置方法來改進其效能，模擬結果證明這兩種方法均可在不需要多增加量測負擔的前提下提升細胞邊緣使用者的流量與頻譜效率。

In the evolution of wireless network technologies, the pursuit of higher efficiency and larger capacity never stops. The next generation of wireless networks aims at achieving these goals via those techniques which can make use of the spectrum with more flexibility and higher order of diversity. These techniques include, to name a few, dynamic allocation of bandwidth/radio resources, aggregation of multiple channels/carriers, deployment of femto base stations, MIMO transmission with up to eight antennas, cooperative multipoint transmission, etc. In this dissertation, we focus on the first three techniques and present results for four research topics. For the first topic, we present a model for the bandwidth/radio resource sharing in a wireless access network among multiple mobile virtual network operators (MVNOs). Within this model, we propose an efficient scheme to adaptively allocate the bandwidth in accordance with the traffic fluctuation. The proposed bandwidth allocation scheme adopts the technique of dynamic programming to maximize the expected total revenue across multiple stages from the perspective of the facility provider. The design is evaluated by simulation results and proved to be efficient and beneficial. Concerning the second topic, we consider a scenario of aggregating multiple channels in order to serve as the wireless backhaul. Both the multi-hop nature and the large per packet channel access overhead of the wireless backhaul networks can lead to low channel efficiency. The problem may get even worse when there are many applications transmitting packets with small data payloads, e.g. Voice over Internet Protocol (VoIP). In order to cope with this issue, we propose a scheduler that concatenates small packets into large frames and sends them through multiple parallel channels with an intelligent channel selection algorithm between neighboring nodes. Besides the expected capacity improvement, delay bounds and call admission control principles of a broad range of scheduling algorithms for this scheduler have also been derived. With respect to the third topic, we proposes a detailed model for the femtocell-based MVNO and analyzes the dynamics between the femto base station transmit power and its absorbing effect on the macrocell users via game theoretic techniques. The numerical results show that the power settings given by the Nash equilibrium maintain the required QoS level without causing excessive interferences. Such results should support the feasibility of large-scale deployment of femtocell-based MVNOs. The last study of this dissertation is a mixture of the ideas of the three techniques. It proposes a set of low-complexity schemes to improve the spectral efficiency of the femtocells in LTE-Advanced by intelligently allocating multiple carriers. The introduction of carrier aggregation in LTE-Advanced provides the uncoordinatedly deployed femtocells with the opportunity to autonomously mitigate the interferences. In order to address the inefficiency of the autonomous component carrier selection (ACCS) scheme proposed by 3GPP in a typical home Internet access scenario, we propose a scheme to improve the total throughput of a cluster of femtocells by reselecting the carriers according to a well-planned order. Additionally, we also propose to ameliorate the performance of the cell edge users with the fractional power suppression technique. Both the proposed schemes are shown to improve the original ACCS without requiring extra measurements and with only limited additional signaling overhead.