## 2 Indoor Scenario Simulation and Characterization
For a more in depth analysis of the results shown in Figure 5, in Figure 6 the estimated received power _vs._ linear distance for the aisle where antennas 5, 6 and 7 are placed (see Figure 4) is shown. The results show the typical signal strength variations due to the multipath propagation, as well as how a unique antenna cannot provide service for the whole scenario (the sensibility of the used wireless devices is \(-92\) dBm), making necessary the deployment of more wireless nodes. The result presented in Figure 5 is the consequence of testing different antenna emplacements and configurations and choosing the most suitable in terms of coverage and power consumption. Therefore, with the aim of prove the necessity of a correct characterization before the implementation of the system, two preliminary cases are depicted in Figure 7. This process is inherent to the radioplanning strategy in order to provide the final network configuration, proposed in last term by the network designer. Figure 5: RF power contribution of the simulated antennas for the plane at height = 1 m, individually as well as the total power distribution (bottom plane). Figure 6: Estimated received power _vs._ linear distance for the antennas transmitting separately and together. The theoretical study of the scenario allows the consideration of multiple configurations avoiding the tedious work of test different implementations of the system until the most optimum configurations is obtained. In the two cases presented in Figure 7 it is visible a coverage result that is unsuitable since the complete scenario does not exhibit an acceptable received power value level. One of the most limiting factors in WSN performance is given by receiver sensitivity, determined basically by hardware limitations (such as noise factor) and employed modulation and coding schemes, which in turn determine maximum tolerable interference levels. The possible interference produced between transmitters must be considered in order to reduce packet loss and optimize the communication. Based on the results extracted from mathematical simulations, an efficient sensor mesh can be constructed, therefore in Figure 8 a horizontal plane is depicted where the interference caused by all the antennas over third antenna, acting as potential valid transmitter is shown. Figure 7: Preliminary distribution of antennas where three (**a**) and four (**b**) antennas have been considered. Related with this, in Figure 9 estimations of SNR level at the point where antenna 4 is placed are shown. This is the case of antenna 3 acting as the transmitter which communicates with node 4 in the chain routing protocol, considering the rest of devices intra-system interference (transmitting at maximum power level, 0 dBm). The same SNR estimations have been calculated for inter-system interference, considering \(-55\) dBm of noise level, an average value extracted from a spectrogram measured in the scenario before the deployment of the ZigBee network. With the aim of determining if the communication between the two wireless devices is possible, five SNR values at position 4 have been calculated, one for each of the five transmitted power levels allowed by the devices used for measurements. The minimum required SNR value for a correct transmission between these two devices has been calculated using the following well known formula:  
\[[C=BW\times log_{2}\left(1+\frac{s}{N}\right)] \tag{5}\]  
where \(C\) is the channel capacity (250 Kbps, maximum value for ZigBee devices) and BW is the Band-Width of the channel (3 MHz). Figure 8: SNR map with the interference caused by all the antennas over the third antenna acting as valid transmitter. Figure 9: Estimated inter-system and intra-system SNR values for antenna 3. As can be seen, there is not an inter-system interference problem in the presented scenario for the communication between devices 3 and 4.