目前在ZigBee定位系統中,定位用的訊號以量測無線接收信號強度(Received Signal Strength Indicator; RSSI)為主。因此在測量RSSI定位信號的強度來進行定位之前,必需先行建立定位信號強度與距離衰減之環境模型。從過去研究結果得知,環境中的同頻干擾會影響所建立之環境模型的正確性。當同頻干擾嚴重時,利用建立之環境模型進行定位常會產生很大的定位誤差,因而無法使用ZigBee系統進行有效的定位。因此,本論文首先針對測量之定位空間,提出一個評估標準用以判別環境模型是否適合用於ZigBee定位系統。實驗結果顯示,當環境模型具有類似對數反比(Log-normal)曲線的特性時,定位結果會有很高的準確度。反之,環境模型曲線無此特性時,定位結果時則會產生極大之誤差值,這反應出此種環境不適合使用ZigBee系統進行定位。除了定位信號強度與距離衰減之環境模型外,不同之定位方法亦會影響定位的準確度。本論文提出一種簡單且快速之區塊定位法,此法只需比對先行建立好的RSSI強度與區塊間的對照表,即可快速找出所要定位之區塊的位置。定位採用分階段的方式縮小定位範圍,進而達到定位之目的。此外,區塊定位方法將與三邊定位法的比較實驗結果指出,三邊定位法的估計誤差範圍較大,而區塊定位法不但簡單不需計算座標,而且誤差變動範圍較小。因此,本論文提出之方法極適合在低干擾環境進行快速定位之應用。
In a ZigBee-based positioning system, the positioning signals used to estimate the distance are mainly obtained by detecting the received signal strength indicator (RSSI). Before applying the RSSI values to estimate the distance, an environment model to represent the relation between RSSI and distance needs to be established by RSSI measurement methods. According to the previous studies, the accuracy of the environment model is susceptible to the interference around the target location. In serious interference environments, it will introduce large location errors for the positioning process. Obviously, the ZigBee-based positioning system is not appropriate for such environments. Therefore, it is necessary to propose a method to determine the usefulness of an environment model. The experiment results show that one kind of environment model exhibiting the Log-normal curve could result in better positioning performance. On the other hand, a worse positioning performance occurs when the environment model does not exhibits the same curve. This indicates that the ZigBee-based positioning system is not suitable for environment models without the Log-normal properties. In addition to the issue of environment models, the different positioning methods also result in different degrees to positioning errors. In this respect, a simple and rapid positioning method, named Zone-Positioning method, is proposed to keep small positioning errors. In this method, the located zone can be quickly determined through checking the RSSI database which is a function of location. The location is divided into three zones in order to reduce the located scope. As compared with the Trilateration method, the proposed Zone-Positioning method outperforms the Trilateration method in different locations. As a result, the proposed fast positioning method is more applicable to low interference environments and location positioning with fast movement.
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