# Simultaneous Indoor Tracking and Activity Recognition Using Pyroelectric Infrared Sensors
## 3 Sensor Node Design
### Reference Structure Specification  
The sensor node design is based on the sensing model mentioned above. In our design, there are nine PIR sensors on one sensor node, as shown in Figure 2. To enhance the sensibility of the PIR sensor array, each PIR sensor is equipped with a hemisphere Fresnel lens array. Before visibility modulation, the FOVs of all of the sensors are multiplexed, forming a cone-shaped monitored space. The opaque mask plays its role as the reference structure. There are two types of masks in our design for FOV segmentation; they are used for bearing segmentation and radial segmentation, respectively. Figure 1: Our sensing model: reference structure, measurement plane, monitored space. \(\Omega_{1}\)...\(\Omega_{7}\) are sampling cells. Each sampling cell has the same visibility to the sensor array. The Type I mask is used for bearing segmentation, as shown in Figure 3a. In our node design, four out of nine sensors are equipped with the Type I mask. The sweep angle \(\phi\) of the FOV is \(120^{\circ}\), as shown in Figure 3b. All of the FOVs of these four PIR sensors are overlapping and then are rotated \(90^{\circ}\) one by one. As a result, such a multiplexing scheme segments the monitored region of the sensor node into eight sampling cells. The coding scheme of corresponding angle \(\theta\) for each sampling cell is listed in Table 1. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sampling Cells** & **PIR 6** & **PIR 2** & **PIR 4** & **PIR 8** & **Angle \(\theta\)** \\ \hline
01 & 1 & 0 & 0 & 1 & 67.5\({}^{\circ}\) \\
02 & 1 & 0 & 0 & 0 & 22.5\({}^{\circ}\) \\
03 & 1 & 1 & 0 & 0 & 337.5\({}^{\circ}\) \\
04 & 0 & 1 & 0 & 0 & 292.5\({}^{\circ}\) \\
05 & 0 & 1 & 1 & 0 & 247.5\({}^{\circ}\) \\
06 & 0 & 0 & 1 & 0 & 202.5\({}^{\circ}\) \\
07 & 0 & 0 & 1 & 1 & 157.5\({}^{\circ}\) \\
08 & 0 & 0 & 0 & 1 & 112.5\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The coding scheme of angle \(\theta\). Figure 3: Type I mask: (**a**) bearing segmentation; (**b**) multiplexing of four PIRs forms eight sampling cells. FOV: field of view. Figure 2: The pyroelectric infrared (PIR) sensor node. The sensor node consists of nine PIR sensors arranged in a grid shape. Four of them using the Type I mask, the rest using the Type II mask. CC2530 is used to sample the PIR signals and communicate with the sink node. The Type II mask is used for radial segmentation, as shown in Figure 4a. After being masked, the sensing region of each PIR sensor is still a full cone, but the cone angle \(\beta\) is modulated by the radius of the mask. There are five sensors on the sensor node equipped with the Type II mask. The multiplexing of these five sensors segments the monitored region of the sensor node into five sampling cells, which correspond to different radii \(r\). The coding scheme of radius \(r\) is listed in Table 2, where the preferential coding strategy is employed. By integrating these two kinds of masks, the monitored space is segmented into several sampling cells, as shown in Figure 5a. The center of the sampling cell \((x,y)\) is represented by polar coordinates \(r\angle\theta\), as shown in Figure 5b:  
\[\left\{ \begin{array}{l}x=r\times\cos\theta\\ y=r\times\sin\theta\end{array} \right. \tag{2}\]  
When the human object moves within the monitored space, several PIR sensors will be triggered. According to the coding scheme, we could decode the location of the human. The center of the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sampling Cells** & **PIR 1** & **PIR 5** & **PIR 9** & **PIR 3** & **PIR 7** & **Radius \(r\) (m)** \\ \hline
1 & 1 & x & x & x & 0.0 \\
2 & 0 & 1 & x & x & 0.8 \\
3 & 0 & 0 & 1 & x & 1.3 \\
4 & 0 & 0 & 0 & 1 & x & 2.3 \\
5 & 0 & 0 & 0 & 0 & 1 & 3.0 \\ \hline \hline \end{tabular} \({}^{*}\)x\({}^{\prime}\) denotes 0 or 1.  
\end{table} 
Table 2: The coding scheme of radius \(r\). Figure 4: Type II mask: (**a**) radial segmentation; (**b**) multiplexing of five PIRs forms five sampling cells.