睡眠模式在綠能無線網路之建模、設計、與最佳化

Abstract

最小化耗能是綠色節能無線通訊的一個基本目標，其中最有效且最有潛力的方法之一是根據網路流量來適應性的開關無線電收發器，這樣的想法可以被廣泛的運用在各種無線通訊設備或是裝置上。在本論文中，我們將探討睡眠模式在無線網路中的運作。我們對於一個網路中的節點功能性是可以互相被取代而且願意一起合作來達成整體目標的不自私(altruistic)網路感興趣，為了綠化傳統無時無刻都是主動運作的網路，我們利用睡眠模式所帶來的額外自由度(degrees of freedom)來節省網路的能量，使其成為一個在必要時刻才主動運作的網路。 在行動網路中，根據網路中的流量來開關基地台是一個節省網路能量的有效方式，然而這樣的運作可能會產生網路覆蓋漏洞(coverage holes)。在這樣的問題中，我們嘗試在保持整體網路覆蓋率的情況下，藉由動態的開關基地台來最小化整體網路的耗能。我們推導出在最小化每單位覆蓋面積的耗能下，每個基地台的最佳的覆蓋細胞大小(cell size)。在滿足網路覆蓋率的前提下，我們提出一個多項式時間複雜度的流量察覺(load-aware)的基地台開啟演算法。除此之外，我們展示了為了增加網路中熱點吞吐量(throughput)，藉由佈建的小細胞(small cell)達成的網路密致化(network densification)也可以在低流量的期間改善整體網路耗能。此外，我們探討異質網路(heterogeneous networks)下有可能的網路架構與多模(multi-mode)基地台運作方式並且建議非對稱的基地台與移動裝置的連線將有潛力在低流量時大幅降低網路耗能。 在無線多重跳躍中繼網路(wireless multi-hop relay network)中，睡眠排程是一 個有效的方法來降低網路耗能，但是通常會造成傳送訊息時的額外延遲。為了尋找在設計綠能多重跳躍中繼網路的洞見，我們建立了一個模型來分析和最佳化無線多重跳躍中繼網路在使用睡眠機制時，效能間的權衡(trade-offs)關係。進一步的說明，我們提出了一個隨機醒來的網路(random wakeup network)下使用投機式的路由(opportunistic routing)所建造的框架來分析。我們發現在最佳的參數設定 時，整個網路用來偵測訊息的能量消耗占整體網路的(α-1)/(α+2)，其中α是路徑損耗指數(path loss exponent)，在最佳的參數設定下，我們發現整個網路用來偵測訊息的 耗能應該要和整個網路用來做實際通訊的耗能是差不多的，並且我們發現整個網路在最佳參數設定下的最小耗能和傳送速度的(α-1)/(α+2)次方成正比。另一方面，我們研究在無線多重跳躍網路下訊息交換(information exchange)與訊息散播 (information dissemination)的耗能最小化。我們提出了一個隨機散播(random gossip)下週期性醒來的分析模型。我們發現在最佳的參數設定下，每個週期醒來 廣播的節點數量只和路徑損耗指數和網路維度有關，這說明了不論增加或減少網路的大小都不會影響整個網路的最佳參數設定，我們得到最佳的參數設定為擁有可拓展性(scalability)的性質。另外我們藉由模擬的方式展示我們提出的模型可以被很好的運用在效能的朔模(performance modeling)與最佳參數的預測。除了跨層最佳化之外，我們提出的模組可以在無線多重跳躍中繼網路下，藉由感知 (cognition)與最佳化來巧妙的調整軟體可重組化(software-configurable)的功能用 以達成整體網路的目標。

Energy consumption minimization is a fundamental goal for green wireless communications. One of the most effective and promising way to save energy is to power off radio transceivers adaptively according to network traffic conditions. This idea can be utilized in a wide spectrum of wireless communication devices and equipments. In this dissertation, we explore the “sleep mode” operations in wireless networks. We are interested in altruistic networks in which nodes are interchangeable in functionalities and are willing to cooperatively achieve network level goals. To “greenize” traditional networks from always-on networks to necessary-on networks, we exploit the additional degrees of freedom by sleep mode operations for network energy conservation. In cellular networks, switching on/off base stations (BSs) according to network traffic profile is an effective way to conserve network energy. However, such operations may create coverage holes in the network. We attempt to minimize the total power consumption of the network by switching on and off BSs adaptively while maintaining the network coverage. We derive the optimal cell size for minimizing BS power consumption per unit coverage area and propose a polynomial-time load- aware algorithm for energy-efficient BS activation while avoiding creating coverage holes. Besides, we demonstrate that network densification with small cells for bursting throughput in hot spot areas can also improve network energy savings during the low traffic load period. Furthermore, we explore potential network structures with multi-mode BS operations in heterogeneous networks (HetNets) and suggest that asymmetric BS-MS (mobile stations) connections with uplink by small cells and downlink by macro cells will be energy-efficient during the low traffic load periods for green HetNets. For wireless multi-hop relay networks, while applying sleep-awake scheduling is an effective way to reduce network energy consumption, it usually comes at a price of additional delay for message delivery. To seek insights for the design of green wireless multi-hop relay networks, we develop a model for the analysis and optimization of performance trade-offs in wireless networks while sleep-awake mechanisms are applied. Specifically, we propose a random wakeup network with opportunistic relay as a framework for our analysis. Under optimal settings, we find that the proportion of power for message detection in the entire network is (α-1)/(α+2), where α is the path loss exponent; the energy consumed for message detection should be of the same order as that for actual communications. Moreover, we find that the minimal network power consumption under optimal operations grows at (α-1)/(α+2) -th order of the delivery speed, meaning that the investment of network power can efficiently reduce delivery delay. Besides, we investigate network energy minimization for information exchange and information dissemination in wireless multi-hop relay networks. We propose a random gossip network with periodic listening as a framework for our analysis. We find simple rules govern the optimal settings for network-wide information exchange in the random gossip network. One key relationship is that the optimal number of nodes broadcasting messages in a time epoch within the transmission range depends only on the path loss exponent and the network dimensionality. It is shown that neither increasing nor decreasing the physical network size will affect the optimal value of these design parameters. The optimal setting for information exchange is a scalable solution. We show through simulation results that our proposed frameworks are well applicable for performance modeling and parameter optimization in wireless multi-hop relay networks when sleep-awake operations are adopted. Beyond cross-layer optimizations, our proposed frameworks can facilitate the cognition and optimization for ingeniously adapting software-configurable functionalities to achieve network goals in wireless multi-hop relay networks.