# WIPP: Wi-Fi Compass for Indoor Passive Positioning with Decimeter Accuracy
## 4 Experimental Results
### AOA Measurement Estimation  
Figure 10 shows the geometrical relations of the target and an AP. In this section, we aim to demonstrate that the WIPP is able to accurately estimate the AOA measurements for both the direct and indirect signal paths, and, meanwhile, compare the WIPP with the conventional passive positioning system using the AOA solely.  
Figure 8: Layout of testbed.  
Figure 9: Photo of testbed.  
Figures 11-13 show an example of the result of AOA measurement estimation for test point P4. From these figures, we can find that the estimated AOA measurements with respect to AP1, AP2, and AP3 are \(58.5^{\circ}\), \(-19^{\circ}\), and \(-40^{\circ}\), which are very close to the real ones \(59.04^{\circ}\), \(-16.7^{\circ}\), and \(-39.8^{\circ}\), respectively. In addition, after the affinity propagation clustering, the AOA measurements corresponding to the direct signal path has a smaller variation compared with the ones corresponding to the indirect signal path.  
Figure 11: Result of AOA measurement estimation with respect to AP1. (**a**) result of joint estimation of AOA and TOA measurements; (**b**) result of affinity propagation clustering.  
Figure 10: Geometrical relations of the target and an Access Point (AP).  
Figure 12: Result of AOA measurement estimation with respect to AP2. (**a**) result of joint estimation of AOA and TOA measurements; (**b**) result of affinity propagation clustering.  
Figure 13: Result of AOA measurement estimation with respect to AP3. (**a**) result of joint estimation of AOA and TOA measurements; (**b**) result of affinity propagation clustering.  
The Cumulative Distribution Functions (CDFs) of errors in AOA measurement estimation by the WIPP and the conventional passive positioning system using the AOA solely are compared in Figure 14. From this figure, we can find that by using the WIPP, the mean of errors is about 3\({}^{\circ}\) and 70% errors are within 4\({}^{\circ}\), which are much smaller than the ones by the conventional passive positioning system using the AOA solely.  
We continue to examine the impact of the height of target on AOA measurement estimation. In the experiments, the target is placed with the height 120, 170, and 200 cm, respectively, while the APs are placed with the fixed height 200 cm. The CDFs of errors in AOA measurement estimation are shown in Figure 15. From this figure, we can find that the WIPP achieves the highest accuracy in AOA measurement estimation when the APs and target are placed with the same height. In this case, the mean of errors is about 2\({}^{\circ}\) and 90% errors are within 4\({}^{\circ}\). With the increase of the height difference between the APs and target, the performance of AOA measurement estimation deteriorates. For instance, the mean of errors increases to 3\({}^{\circ}\) and 5\({}^{\circ}\), and the percentages of errors within 4\({}^{\circ}\) decrease to 78% and 60% when the height difference is 30 and 80 cm, respectively.  
Figure 14: Cumulative Distribution Functions (CDFs) of errors in AOA measurement estimation.  
Figure 15: CDFs of errors in AOA measurement estimation under different heights of the target.