# Performance Evaluation of Directional Antennas in ZigBee Networks under NLOS Propagation Conditions
## 4 Discussion  
The results indicate that directional antennas did not improve the performance of omnidirectional antennas under NLOS conditions. Several authors have proposed the use of directional antennas to suppress interference. The study carried out in the three zones showed that the directional antennas were oriented in the direction of the strongest signal to provide the best results, as expected. However, these signals often came from directions with high Wi-Fi interference. The directional antenna used in this work can increase the link budget by 7.5 dB. This gain allows for double the range under conditions close to free space. The performance of the directional antenna was better when the received signal was more concentrated in a specific direction. On the other hand, when higher-signal components reached the receiving antenna from various directions, omnidirectional antennas had similar results to directional antennas. Furthermore, it is only feasible to control the receiving antenna to increase the received power in practical situations. Control of the transmitting antenna requires feedback from the receiver. However, there is no active communication link yet. Therefore, the performance of directional antennas can be lower than that described in this work. In this study, the systems operated with the same EIRP. The reason for choosing this condition was to obtain the maximum range using the maximum transmit power allowed by the regulations. In this case, the transmit power of the directional antenna system was 7.5 dB below the transmit power of the omnidirectional antenna system. Under these conditions and for a PDR greater than 90%, the system range at location 1 was about 30 m. The reached distance for the systems at locations 2 and 3 was about 90 m.  
The system with the omnidirectional antenna used a higher transmit power. Higher transmit power means more power consumption. That is why many authors have proposed directional antennas, as they can reduce the power consumption of battery-operated networks. However, the control of directional antennas also consumes energy. Therefore, a comparison of power consumption between systems is required. Considering the transmit power, we measured the current consumption of the XBee S2C by inserting a shunt resistor  
Figure 13: Received signal after deconvolution for different distances from the transmitter: (**a**) 50 m; (**b**) 80 m, (**c**) 100 m.  
of 1 \(\Omega\) between the battery and the supply pin of the sensor node. The XBee S2C module uses the Silicon Labs EM357 chipset [34]. An oscilloscope connected to the resistor terminals allowed us to obtain the current results. Figure 14 shows the measured transmit current for each power level. There is a difference of 12 mA between the highest and the lowest power levels with the boost mode disabled. Activating the boost increased the current consumption, on average, by 5.5 mA. We measured the current consumption with the XBee operating as an end device and transmitting 111-byte packets. Figure 15 shows the current consumption for the highest power level and two operating conditions. The graphs are similar to those represented in [40] for the radio CC2530. Figure 15a refers to the case in which the system uses application acknowledgements (ACK). In this case, the active period lasted 98.5 ms. The duration of the transmit mode and the idle/receive modes were 3.6 ms and 5.7 ms, respectively. For a processing current of 9 mA, the system consumed, on average, 11.6 mA during the active period. The current consumption would be 11 mA for the lowest transmit power level. The difference between both cases was 0.6 mA (1 mA with boost mode enabled). Figure 15b describes the current consumption for transmissions without application acknowledgements. The active period lasted 18.6 ms. The difference in current consumption between the highest and lowest transmit levels was, on average, 2.3 mA (3.5 mA with boost mode enabled).