Chih-Ning Huang and Chia-Tai Chan / Procedia Computer Science 5 (2011) 58–6561

The ZigBEACON system first surveys the RSSI values of the transmitted power and the distance between the sender and the receiver, and then uses the Euclidean distance to estimate the diversity of reference nodes and mobile node that could effectively choose the p-nearest reference nodes to calculate the mobile node’s position. The ZigBEACON system consists of three phases, offline path loss survey phase, real-time RSSI collection phase and online position calculation phase, as shown in Figure 1.

3.2.1.Offline path loss survey phase

The Path Loss (PL) model that means the relationship between received power and distance is formed as

·PL d PL d0 10nlog§¨dd¸ XV,

0¹©(1)

where n = 2 for free space. The PL(d0) is the received power from the transmitter at a known close distance d0 and X denotes a zero mean Gaussian random variable that reflect the interference from indoor environment [14]. In reality, the real received signal power would not conform to the PL model. In the offline path loss survey phase we survey the PL distribution of SuperNode to increase the accuracy of positioning. Figure 2 shows the experimental results of RSSI propagation at the distance between 0.5m, 1m, 2m, 3m, 4m, 6m, 7m and 10m separately in the indoor environment. For each distance, we took 480 measurements then calculated the means and standard variations. We find the PL distribution presents a non-log-linear relationship between RSSI and distance. According to the characteristics between RSSI and distance, the RSSI can be divided into four classes:

xClass I: the RSSI is above 144. Because RSSI value is over 144 within 0.5m transmission distances. In other words, the RSSI in this class means the distance between receiver and transmitter is over close.

xClass II: the RSSI is from 112 to 143. The RSSI versus distance is near linear relationship above 112. Class II is the fuzzy region between Class I and Class III that the signal will be shown between 0.5m to 2m.

xClass III: the RSSI is from 60 to 111. The RSSI in this class will be distributed at the distance from 2m to 7m. xClass IV: the RSSI is under 60. When the distance far from 7m and above, the RSSI values rapidly decay under to the RSSI value of 60.

Fig. 2. The path loss distribution of SuperNode

3.2.2.Real-time RSSI collection phase

In real-time RSSI collection phase, ZigBEACON system collects the real-time RSSI, and then transmits these RSSI value to the computer for the next phase. First, mobile node triggers the real-time RSSI collection phase by sending the positioning request to gateway. Then the gateway will ask RF generators by turns to send the broadcast message to all reference nodes and mobile node. As soon as mobile node and reference nodes receive the broadcast message, they will save the value of RSSI into its memory. After all RF generators sent broadcast message, mobile node and reference nodes will forward the RSSI values to gateway. Finally, the location of mobile node will be

calculated on the computer on the next phase.