## 2 Literature Review
Moreover, not only spatial properties of link have been observed, but also seasonal properties [21; 22; 23]. In [22; 23], the authors evaluated the impact of the weather on the link quality in a WSN. Their results show a daily and seasonal fluctuation of link quality. In [23], the authors measured over a period of six months the effects of weather on IEEE 802.15.4 links. By measuring the correlation between the variations in the packet reception ratio (PRR) and the signal strength indicator (RSSI) with four selected meteorological factors (temperature, absolute humidity, precipitation and sunlight), the authors showed that the PRR and the RSSI correlate the most with the temperature (better link quality when the temperature is lower). In [22], the authors designed a low-cost experimental infrastructure to vary the on-board temperature of sensor nodes and their results also show a correlation between link quality and temperature. In [21], the authors proposed a new platform and used it to estimate the parameters of an over-water radio link, through two experiments: one conducted in the morning and another in the afternoon. From the morning to the afternoon experiment, the temperature and the relative humidity reduced by about 2 \({}^{\circ}\)C and 25% respectively. Their results shown an increasing of the radio range from 40 m in the morning to 70 m in the afternoon. The results presented in [21; 22; 23] show a poor link quality in presence of high temperature. In our case, since temperature is higher at ground level, this brings a negative impact on link quality compared to links placed higher above ground. Experimenting at large scale requires a lot of hardware and is a fastidious and time-consuming task. Therefore, several testbed platforms have been deployed all around the world to allow faster experimentations, with various sizes, hardware, topologies, and degrees of flexibility [24; 25; 26; 27]. These testbeds are usually deployed in indoor environments, but allow conducting complex and easily reproducible experiments. In these testbeds, a user usually selects and configures the set and type of sensor it wants for its experiment. For some testbeds, it might be possible to select sensors in order to guarantee the main radio link property considered in this paper: radio link at ground level. However, in our work, we preferred conducting experiments in a real environment, which allowed us to evaluate, for example, the impact of the topography. To summarize, previous results show that link properties in WSN usually depend on hardware properties, radio communication parameters, meteorological conditions and deployment environment. This should be taken into account when designing communication protocols or mechanisms for WSN. In most experimental setups, the deployment choices usually depend on the desired objectives and the platform used: sensors are usually attached on some support, the nodes may be configured with an internal or external antenna. In our study, we considered an off-the-shelf WSN platform [28], with nodes configured with internal antennas and deployed outdoor on the ground surface. Our goal was to characterize the wireless links in such settings, taking into account radio parameters such as the packet size or the communication channel frequency, as well as environment properties such as its topography. Another parameter which may have an impact on the radio link properties, but not investigated in this study, is the transmission power [29].