# Performance Evaluation of Directional Antennas in ZigBee Networks under NLOS Propagation Conditions  
[PERSON]  
1 Center for Research in Mathematics and Applications (CIMA), Faculty of Exact Sciences and Engineering, University of Madeira, 9020-105 Funchal, Portugal; [EMAIL_ADDRESS]  
[PERSON]  
2222112032222112032  
for the transmit powers of 3 dBm and \(-\)3 dBm, respectively. [PERSON] and [PERSON] [7] studied the performance of ZigBee networks based on XBee S2 modules. The active period also consisted of three time intervals. The activation and deactivation lasted 20 ms (8.1 mA), the listening and receiving ACK packets lasted 6 ms (40 mA), and the transmission of a 111-byte packet lasted 3.6 ms (38 mA). The radios could operate with an output power between \(-\)8 dBm (boost mode disabled) and \(+\)3 dBm (boost mode enabled). The authors did not show the power consumption for the different transmit powers. According to the component's datasheet, the current consumptions were 35 mA and 40 mA for the boost modes disabled and enabled, respectively. The average current consumption of the active period was 17.8 mA for the first mode and 18.4 mA for the second mode. As noted, reducing the transmit power lowers the power consumption of the sensor node. This conclusion has led several authors to propose the reduction in transmission power by using directional antennas [8; 9; 10]. In applications with directional antennas, the beam is oriented in the proper direction to increase the received power level. Beam control can be mechanical or electronic, but an electronically steerable beam is preferred [11]. Nevertheless, it should be considered that beam control in directional antennas also consumes energy. For instance, the typical control of reconfigurable antennas with PIN diodes requires a current of at least three milliamperes [12]. Directional antennas can also reject interference caused by multiple transmission technologies sharing the same channel. [PERSON] and [PERSON] [13] indicated that directional antennas may remarkably improve the transmission performance of a sensor node. However, the work assumed sensor nodes deployed in an open area. Other works studied the application of directional antennas in WSNs, but they obtained the performance results by simulation [14; 15]. [PERSON] et al. [16] studied the interference of the IEEE 802.11 g system in IEEE 802.15.4 sensor nodes. The beam selection with a four-beam patch antenna provided a higher Received Signal Strength Indicator (RSSI) when compared with the omnidirectional antenna. Nevertheless, the Wi-Fi source was close to the sink node. [PERSON] et al. [17] found that the correlation between RSSI and transmit power is approximately linear for different propagation environments. This correlation is not correct for the gain of the transmit antenna. Measurements carried out in zones with vegetation demonstrated that the power received by directional antennas was much lower than that obtained by an omnidirectional one for the receiver oriented to the transmitter. [18]. The best antenna orientation leading to the highest received signal may vary over time, particularly under Non-Line-Of-Sight (NLOS) conditions [19]. [PERSON] et al. [20] investigated WSN performance using directional antennas in an outdoor environment through measurements. The experiments included zones with trees, traffic, and moving people. They concluded that the transmit power could be decreased by several dB when using directional antennas. However, the outdoor environment considered in the experiments had a path loss exponent of \(\alpha\) = 2.41 and \(PL(1\) m\()\) = 40.28, which is close to the free-space conditions (\(\alpha\) = 2, \(PL(1\) m\()\) = 40.1). Directional antennas require proper Medium Access Control (MAC) protocols, and routing is more complex [21; 22]. Furthermore, most works consider the communication range of directional antennas modelled as a sector because of the complexity of a communication range derived from an actual directional antenna [23].