Wireless sensor networks (WSNs) have received tremendous attention in recent years due to their wide applications such as battlefield surveillance, target tracking, and scientific exploration in dangerous environments. Sensors are driven by batteries and have limited energy resources. Hence, the energy conservation is a very important issue in WSNs. In this dissertation, we consider the problem of event detection and propose energy-efficient sequential fusion schemes to reduce the transmission energy consumption of sensors.
First, we consider the homogeneous sensing environment where the observations at sensors are independent identically distributed. Motivated by Wald’s sequential probability ratio test (SPRT), the truncated SPRT (TSPRT) is applied for WSNs with finite number of sensors. We consider two fusion models, centralized and decentralized, and propose the corresponding centralized-TSPRT (C-TSPRT) and decentralized-TSPRT (D-TSPRT). The analytic expressions for the upper bounds of the false alarm and miss detection probabilities are derived for C-TSPRT and D-TSPRT, which are used to determine the upper and lower stopping thresholds. According the simulation results, it is shown that, with a little detection performance degradation, a significant amount of transmission energy can be saved.
Next, in the second portion of this dissertation, we address the issue of distributed detection in inhomogeneous sensing environments, where the observations at sensors are independent but not necessary identically distributed. The sensors transmit their messages one by one and the log-likelihood ratio (LLR) fusion rule is considered at the fusion center (FC). By treating the LLRs of the un-transmitted sensors as random variables, we propose the undistorted sequential fusion (UDSF) scheme. To reduce the computational complexity, the extreme value based-UDSF (EVB-UDSF) scheme is then proposed. We prove that, with a fraction of sensors, the EVB-UDSF can provide the same detection performance as the conventional fusion scheme where all sensors transmit to the FC. Besides, we propose the tolerable sequential fusion (TSF) scheme and the extreme value based-TSF (EVB-TSF) scheme, where the detection performance can be traded for the transmission energy. Since the sensors transmit their messages sequentially, the transmission order also plays an important role in energy conservation. We propose three transmission rules, including LLR-based, mutual information (MI)-based and channel quality-based transmission rules. The simulation results show that the MI-based transmission rule achieves the best energy saving performance.
In the last part of this dissertation, we consider the problem of network lifetime for distributed detection is WSNs, where the network lifetime is defined as the time span from the deployment to the instant when the network can not sustain the target detection performance. Considering the finite energy at each individual sensor, we propose the lifetime extension sequential fusion (LESF) scheme. Furthermore, two sensor selection strategies are proposed, maximum mutual information first (MMIF) and maximum residual energy first (MREF), to schedule the transmissions of sensors. With the balanced energy usage, the simulation results show that the LESF-MREF can actually extent the network lifetime.

近年來，由於微機電技術、無線通訊技術及嵌入式系統技術的迅速發展，使得微小的感測器可以內嵌感測、計算及無線通訊等功能。無線感測網路是由為數眾多的感測器以及一到數個資料融合中心所構成的網路系統，感測器被散佈在待感測區域來蒐集環境資料，並藉由無線傳輸的方式將蒐集的資訊傳送到資料融合中心。無線感測網路可以被應用在許多領域中，如環境監控、入侵偵測等。為了達到大量佈建的目的，感測器必須具備低成本、體積小等特性，並且通常是由電池所驅動。因此，在無線感測網路中，如何節省感測器的能量消耗便是一個非常重要的議題。在過去的文獻中指出，無線傳輸會比資料處理消耗更多的能量，因此在本論文中提出了循序融合的概念來減少感測器的傳送數目以達到節省能量消耗的目的。
在本論文的第一個主題中，我們考慮同質感測環境以及相同的通訊通道品質，並利用橫斷循序機率比值檢定來達到節省傳輸能量的目的。在集中式與分散式融合模型下，我們提出了相對應的集中式橫斷循序機率比值檢定與分散式集中式橫斷循序機率比值檢定，並且推導出假警報機率與遺失偵測機率的上界。當給定所要求的假警報機率與遺失偵測機率，我們可以利用此機率上界得到橫斷循序機率比值檢定的上閾值與下閾值。由模擬的結果可以發現，利用所提出的橫斷循序機率比值檢定，可以犧牲些微的偵測效能以節省非常多的傳輸能量。
在第二個主題中，我們考慮異質感測環境以及相異的通訊通道品質。藉由將尚未傳送感測器的對數概似率視為隨機變數，提出了無失真循序融合方法。為了降低計算複雜度，我們進一步提出了基於對數概似率極值的無失真循序融合方法，並且證明此方法可以在不犧牲偵測效能的情況下節省非常多的傳輸能量。為了節省更多的傳輸能量，我們提出了失真循序融合方法，藉由此方法，我們可以對偵測效能與傳輸能量進行取捨。此外，由於各個感測器的觀測品質以及通訊通道品質都不相同，因此傳送順序對所能節省的傳輸能量的也會造成非常大的影響。我們提出了三種不同的傳送順序規則並比較其差異。
在第三個主題中，我們探討如何延長無線感測網路的生命週期，其中生命週期定義為從感測器佈放直到感測網路無法達到所要求之偵測效能的時間。在考慮感測器的有限能量下，我們提出循序融合的概念來延長無線感測網路的生命週期。模擬的結果顯示，利用所提出的方法可以有效的延長網路的生命週期。

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