近幾年來，隨著無線網路的普及與微型感測器製造技術的提昇，人們開始關注到各式各樣的生理資訊且相關的應用也愈來愈多元化。因此，隨時監控與取得生理訊號就顯得更加重要。電機電子工程師學會制定一種適合在人體運作的網路協定，即IEEE 802.15.6[9]，又稱為無線人體區域網路。無線人體區域網路中的感測器節點被裝置在人體皮膚表面或嵌入至皮膚裡，當傳送端發送封包時，訊號會經由人體皮膚表面或皮膚內傳輸至接收端，而發送封包時所產生的電磁波輻射會對人體健康造成影響。特定吸收率(Specific Absorption Rate，SAR)是電磁波輻射對人體影響程度的重要指標，高的特定吸收率數值，表示人體吸收較多的電磁波輻射量，對人體健康造成的危害程度也愈大。 在單躍傳輸的情況下，封包傳送成功率普遍偏低，本論文希望在兩躍傳輸的情況下能夠擁有低的特定吸收率路徑以及高的封包傳送成功率。因此，本論文利用粒子群最佳化演算法尋找中繼節點合適的佈建位置，使得感測器節點透過此中繼節點轉傳封包到集中裝置這段路徑的特定吸收率較小。有別於藍芽使用大的功率運作，論文中以節點與節點之間的距離來調整節點的發送功率，讓節點以最小的功率即可完成封包傳輸。模擬結果顯示，本論文提出的方法能有效降低特定吸收率且對於封包的傳送成功率也有顯著提升。 In recent years, with the popularity of wireless networks and the enhancement of micro-sensor technology, people have paid more attention to physiological information and related applications also have become more diverse. Therefore, to monitor and obtain physiological signals becomes very important. IEEE 802.15.6, also known as wireless body area networks (WBAN), is a new network protocol developed by IEEE Standard Group for operation on, in or around human body. In WBANs, sensor nodes are deployed on the surface or embedded in the human skin. When the sender sends out packets, the signals will reach the receiver through human body or skin. Nevertheless, the step to send packets will generate electromagnetic radiation, which may affect human body negatively. The specific absorption rate (SAR) is an important index that measures the electromagnetic radiation absorbed by human body: the higher the level of SAR means the more harmful effects of the electromagnetic radiation on human body. In the single-hop case, success rate of packet transmission from the sensor nodes to the hub is quite low. The aim of this paper is not only to search out a path of low SAR level, but also to achieve high success rate of packet transmission in the two-hop case with relay node. Hence, this paper uses Particle Swarm Optimization (PSO) algorithm to search out the optimal position of relay node placement so that the sender can send packets to the receiver through relay node via the path of low SAR level. Unlike Bluetooth that uses high power for system operation, our proposed method regulates the power of nodes according to the distances between nodes and therefore allows a node to complete the packet transmission with the minimum power. The simulation results show that our proposed method can effectively reduce the SAR level and increase success rate of packet transmission with relay node.
