近年來，在 Wireless Sensor Networks (WSNs)的架構之下可以探討的主題，有非常多種不同的方向，如傳送路徑的規畫、佈點的方式、繞境處理、空洞偵測等…，其中有關sensor node的電源消耗，一直是一個非常重要而且值得深入研究的議題。為了要延長網路的有效存活時間，如何好好善用電源來延長網路壽命將是個重點。而其中Two-Tiered Wireless Sensor Network(TT-WSN)的就是用來改善sensor node在網路存活時間的一種網路架構；在TT-WSN的結構底下，會有sensor node(SN)跟Relay node (RN)兩種感測器，其中SN用來感測資料，並起把資料傳送給one hop鄰居內的RN，而RN蒐集完這些資料後傳送回BS，而其間的傳送只能透過RN來傳遞。而RN的成本是較為高昂的，因此我們希望能將RN的佈建數量降到最低，而每個RN還可以保持互相通訊。而我們現存的技術不是計算過於複雜，不然就是效率不夠明顯，因此本篇論文提供一個有效的方法，在佈置Relay node方面可以大幅降低所需要用到的數量，來有效降低花費在佈建RN上面的成本。 本論文主要概念是針對共享路徑方式，去建構佈點的方向，並使用此方法去找尋佈置RN的位置，因此在佈建用來與遠端RN達成通訊的新RN時，可以因此大幅降低佈建的個數。 In Wireless Sensor Networks (WSNs), energy is a scarce resource which must be utilized efficiently in order to enhance the network lifetime. Two-Tiered Wireless Sensor Network (TT-WSN) architecture is proposed to improve the lifetime longevity of the network. In TTWSN the lower tier consists of sensor nodes (SN), which are mainly responsible for sensing the environment and forwarding the data to its one hop neighbor relay node (RN). While the upper tier is constituent of more power affluent relay nodes (RNs), which deliver the data to the base station through potentially multiple connected relay nodes in a multi-hop fashion. As relay nodes are more expensive, it is therefore desirable to deploy a minimum number of such nodes so that every sensor node has at least one relay node as its one-hop neighbor and all the relay nodes form a connected network. Unfortunately the problem of finding such a minimum set of relay nodes is NP-Hard. Thus an approximation based algorithm is required to solve the problem in polynomial time. Existing solutions are either very complex or less efficient. In this paper we present a centralized deployment algorithm to solve the problem. The performance of the algorithm is compared with the existing algorithms through simulations. The extensive simulation results show that our algorithm outperforms the existing algorithms in terms of number of relay nodes deployed.
