# Flying Ad Hoc Networks: A New Domain for Network Communications
## 2 State of the Art
### Propagation Models  
The characteristics of the radio channel through which waves are transmitted are crucial for the design and planification of any communications system. These characteristics allow for mathematically modeling the channels and thus obtaining a propagation model that simulates the attenuation of the waves when they propagate. The simplest propagation model is the Friis free space model [53], which only takes into account the frequency of the signal and the distance, with the corresponding limitations. Planning and deployment in FANETs involve knowing in advanced the characteristic of the radio channel. Calculating the power received in an aerial network from an UAV to a BS (or vice versa) or between two UAVs requires an appropriate channel modelling for each case, considering the unique characteristics of these networks in terms of propagation, such as high possibility of a direct ray (LoS), ground reflection effects, motion effects in 3-D of the UAV (roll, pitch, and yaw), changes in environmental conditions, or variations in distances between nodes, among others. Due to these characteristics, the scientific community has developed a series of theoretical, empirical, and semi-empirical models that approximate a model of channel losses through which electromagnetic waves (EM) are attenuated. The most significant existing body of work on UAV channels modeling is focused on the U2G communication channel (e.g., References [54; 55; 16; 56; 57; 58; 59]), although there are a few works for U2U channels [13; 14]. Our categorization is hence divided into four classes: theoretical models, empirical models, semi-empirical models, and well-known models. Whereas the first three categories focus on developing new models, the adequacy of well-known models is studied in the last category. A summary of the reviewed channel models is included in Table 4.
* Theoretical models: Regarding the theoretical models, in Reference [16], the authors proposed a very detailed propagation model of the U2G channel for aerial small cells. This model considered a lot of factors that other theoretical models do not include such as the altitude of the UAV, the particular environments (urban, sub-urban, dense-urban, and urban high-rise), rain (light, moderate, and heavy) multipath fading, LoS probability, shadowing, wave propagation, gaseous absorption, Doppler spread, and other additional losses that may exist. However, this model did not take into account the impact that the UAV can have on the radiation pattern of the communication device. We will see this limitation in all similar studies. [PERSON] et al. [54] proposed another model of theoretical propagation losses for the U2G channel in different types of urban areas in the frequency band between 2 and 6 GHz (since these are some of the frequencies used for mobile systems). In addition, this model considered the elevation angle between the UAV and the terrestrial node, as well as the possibility of LoS. Finally, the model was compared with a set of real measurements showing that the applicability of this propagation model was likely. * Empirical models: On the other hand, there are empirical models obtained from a series of measurements made in different urban/rural scenarios. Indeed, there is a large number of articles related to obtaining a propagation loss model in an empirical way. For example, in References [55; 56] the U2G channel model was obtained from a set of measurements in the 800 MHz band. In Reference [57], the authors made a summary of a U2G measurements campaign. They showed the outcomes obtained and the propagation models calculated in a series of experiments conducted in conjunction with NASA. [PERSON] et al. [58] studied the ultrawideband (UWB) propagation models for U2G channels. These UWB models allow signals with a greater bandwidth, a lower attenuation by penetration, and a great co-existence with narrow band signals.