## 3 Wireless Sensor Network and Wireless Sensor Node Deployment
### Analytical Network Model for Node Placement  
One of the first issues to take into account is the placement of the wireless sensor node. The position of each sensor is determined by two different coverage areas. The fist one is the radio coverage area, which allows the sensors to communicate. The second one is the sensing coverage area. Usually, the radio coverage area of a sensor node is quite different from the sensing area. Moreover, generally, there is no relationship between them. Both types of coverage areas could be affected by the field where the WSN is deployed, but just one of them could be affected. The factors that affect the sensing area are different from the factors that affect the radio coverage area. The goal is to have the physical space within the sensing range of at least one wireless sensor. The number of wireless sensors deployed is usually higher than optimum required for the physical space due to the lack of precise sensor placement. In a rural and agricultural zone, the radio coverage area for a wireless sensor mainly depends on the type of antenna, used by the wireless sensor and its gain, its height with respect to the top of the vineyard plants and the humidity of the location. In our model we take placement of the wireless sensor at the top of a post of 6 meters height (vineyard plants are not usually more than 2-2.5 meters high), so we will assume that the vineyard plants do not affect to the radio coverage because of the difference in height. In order to estimate the radio coverage we use the power balance formula [given  
Figure 1: Vineyard monitored by cameras. by Equation (1)]. This equation states that the received signal power, in dBm, is equal to the transmitted power plus the transmitter and receiver gain, minus the basic loss and minus other losses produced by other issues (such as vegetation or humidity) [33], but in this case we will only take into account the humidity. The value of humidity losses can be obtained from reference [34]:  
\[P_{rx}(dBm)=P_{rx}(dBm)+G_{rx}(dB)+G_{rx}(dB)-10\ n\ log\ d-L_{humidity}(dB) \tag{1}\]  
Here \(n\) is the attenuation variation index. \(n=2\) for air medium and \(d\) is the distance between the transmitter and the receiver. So, solving Equation (1) in \(d\), we obtain the coverage distance in Equation (2):  
\[d=10\frac{P_{rx}+G_{rx}+G_{rx}-L_{humidity}-P_{rx}}{20} \tag{2}\]  
In order to estimate the distance between devices we fix some parameters. The theoretical transmitted power is \(-40.2\) dBm for an IEEE 802.11g WLAN device at 1 meter (\(P_{rx}\)) and at 2.44 GHz (mean value in the 2.4 GHz WLAN range). We fixed to \(-80\) dBm the threshold power in order to have enough quality of signal (\(P_{rx}\)). So, the received power must be greater than or equal to this value, otherwise it is not in the radio coverage area. We will consider omnidirectional antennas for both the transmitter and the receiver both with the same _Gain_. Moreover, this study has been done in Spain, which has two main hydrometric areas: the H area and the K area [35], so losses due to rain, in the worst case, have a value of 0,026 dB for two kilometers. Equation 3 shows the formula needed to design the radio coverage of each wireless sensor:  
\[d=10\frac{39.77+2\ Gain}{20} \tag{3}\]  
Figure 2 shows the radio coverage for several typical 2.4 GHz antenna gains. The sensing coverage area of a node is also called sensing field (or region) of the node. The characteristics of sensing equipment are completely different and rely on the application requirements. The sensing region of an isotropic sensor in the ideal environment is circular (or elliptical). But a  
Figure 2: Radio coverage for several typical antenna gains. directional sensor has a finite angle of view and thus cannot sense the whole circular area, so its sensing region would be a sector in a two-dimensional plane or a 3D zone. The most commonly used model for sensing coverage is the disk model, which assumes that the sensing region for a sensor is a circular region centered on it [27].