# Performance Evaluation of Directional Antennas in ZigBee Networks under NLOS Propagation Conditions
## 3 Results
### Location 3--Vegetation  
The third location for measurements was an area with vegetation. The Wi-Fi network broadcast from the university building also affected the ZigBee channel. Unlike the other environments, the signal attenuation was mainly due to obstruction in the entire propagation path. With the transmitter in a fixed position, we measured the RSSI and the PDR for the nine positions shown in Figure 3. The transmitter and receiver directional antennas were oriented to provide the highest RSSI and PDR values. We used the line formed by the transmitter and receiver antennas for the angle count. \(\phi_{T}\) and \(\phi_{R}\) are the angles of the transmitter and receiver antennas, respectively. For distances up to 70 m, both antennas were oriented in the same direction, meaning that \(\phi_{T}=\phi_{R}=0^{\circ}\). For the other distances, we obtained \(\phi_{T}=30^{\circ}\) and \(\phi_{R}=-50^{\circ}\), \(\phi_{T}=20^{\circ}\) and \(\phi_{R}=-40^{\circ}\), \(\phi_{T}=0^{\circ}\) and \(\phi_{R}=-20^{\circ}\), and \(\phi_{T}=\phi_{R}=0^{\circ}\), for 80 m, 90 m, 100 m, and 110 m, respectively. The non-zero angles corresponded to rotating the antennas toward the university building. Figure 11 shows boxplots of the dispersion of the measured RSSI. The fading due to vegetation may introduce RSSI variations of 10 dB or 15 dB in a few cm. Figure 12 shows the average RSSI and PDR. Regarding the RSSI, the directional antenna received, on average, 7.2 dB more power than the omnidirectional one for distances up to 60 m. Above this distance, the RSSI was almost the same for both antennas. The attenuation increased with distance because of the vegetation density. Compared to location 1, Wi-Fi caused less interference, so the lowest measured RSSI value was \(-95\) dBm. Figure 12b shows the packet delivery ratio. The systems did not lose packets for distances up to 60 m. Above this distance, the packet loss was similar for both antennas. However, the directional antenna had 7.5 dB more gain than the omnidirectional one. To understand the degradation in the directional antenna performance compared to the omnidirectional one, we measured the received signal as a function of the azimuth angle. Figure 13a shows the results for the distance of 50 m. In this case, the higher components arrived at the receiver from the transmitter's direction, and the directional antenna provided a higher RSSI than the omnidirectional antenna. Figure 13b shows the results for the distance of 80 m. In this situation, the directional antenna did not improve the transmission performance compared to the omnidirectional one. For distances between 60 m and 100 m, the signal received by the system using omnidirectional antennas provided higher signal components for various azimuth angles, as seen in Figure 13b. The results for the directional antenna shown in Figure 13 also confirmed that the receiving antenna should be oriented around \(-50^{\circ}\) to provide the highest received power. This orientation was the same as that for XBee to produce the best results. We noted that this coincided with the direction of the university building, which was the source of the Wi-Fi interference. Figure 11: RSSI measurements: (**a**) omnidirectional antenna; (**b**) directional antenna. Figure 13c shows the results for the distance of 100 m. Observing Figure 12b, the receiver lost many packets above this distance. The signals represented in Figure 13c show that they were more dispersed in space. Once again, the system with the omnidirectional antennas provided higher received signal components for various angles. Figure 12: Measurements results of XBee for location 3: (a) average RSSI; (b) PDR. Figure 13: _Cont._  
The signal distribution also allowed us to estimate the narrow-band received power. The received power was determined through Equation (1), with \(g(\theta)\) being the radiation pattern of each receiver antenna. We addressed the quality of these estimates with auxiliary measurements.