## 5 Experimental Setup  
An experimental setup has been set with the aim of validating the simulation results obtained previously. ZigBee technology has been chosen for emulating a WSN. Specifically, the wireless devices used for the measurements have been the XBee Pro motes from Digi International Inc., shown in Figure 16. These wireless communication devices operate in the unlicensed ISM 2.4 GHz band and the whip antenna mounted on it has an omnidirectional diagram with a gain of 1.5 dBi, which has been taken into account to calibrate the measured values. For transmitting or processing received data, the motes have been connected to a PC via USB cable after being plugged into an XBee explorer unit. Figure 14: _Cont._  
Figure 15: Channel capacity (bps) _vs._ the number of users for different data rates for five gateways considered. Two different measurement campaigns have been carried out. In first place, the radio propagation in the vegetation environment has been characterized setting a transmitting ZigBee more on the trunk of one of the trees, at the point indicated in Figure 4. The points of measurement are the rows of trees along the path, with the receiver placed inside the foliage of each tree. The received power level has been measured by means of an Agilent N9912 Field Fox portable spectrum analyzer. The omnidirectional antenna coupled to the analyzer operates at ISM 2.4 GHz band and has a gain of 5 dBi. The parameters of the transmitting ZigBee more can be seen in Table 4.  
The measurement results for received power in the locations previously described can be seen in Figure 17, where 3D ray launching simulation results have also been included for comparison. Figure 16: The ZigBee more used for wireless communications in the vegetation environment. Figure 17: Comparison of simulation _vs._ measurements. \begin{table}
\begin{tabular}{c c} \hline
**Transmitted Power** & **18 dBm (Maximum Default Value)** \\ \hline Transmission rate & 57,600 bps \\ Frequency & 2.41 (ZigBee Channel 12) \\ Measurement time & 5 min \\ \hline \end{tabular}
\end{table} 
Table 4: Configuration of the parameters of the XBee Pro wireless devices to measure the received power level. Good agreement is observed between the simulation results and the measurements. The resulting error mean for those measurement points is 1.67 dB, a low error that indicates that the proposed in-house 3D ray launching simulation method works properly, validating in the same way the simulation results shown in the previous sections of this work. Once the received power level for different positions within the vegetation environment has been measured and the simulation method has been validated, the second measurement campaign has been performed. The aim of these measurements is to deepen the analysis of the radio propagation in a highly complex environment such as the considered scenario. For that purpose, the quality of the ZigBee channel has been measured. Specifically, the value of Packet Error Rate (PER) has been used as quality parameter, which indicates the percentage of transmitted packets that has been lost and do not reach properly the receiver. The position of the transmitter and the measurement points have been the same as in the previous measurement campaign. But in this case, another XBee Pro more has been placed on the measurement points in order to implement ZigBee radio links. In addition, in this case, two different measurements have been done for each tree, one with the XBee Pro more placed at the trunk of the tree and the second one with the more placed in the foliage of the tree. Table 5 shows the configuration of the wireless mode's parameters used for measuring the PER. Notice that ACK options have been disabled to avoid packet retransmissions and the transmitted power level has been reduced to the minimum default value in order to analyze the worst case, in which the lost packet quantity will be the highest. The PER value has been calculated transmitting 100,000 packets.