## 2 UAV in Wireless Communication Networks
### Channel Characteristics
#### 2.1.1 Air-to-Ground Channel  
An air-to-ground channel includes a strong LoS link, but not always due to shadowing by obstacles such as buildings and trees. The mobility of UAV also causes the shadowing when it flies, therefore, it is essential to model an air-to-ground channel properly to evaluate the performance of UAV wireless networks. Ref. [14] introduces the LoS probability as  
\[\mathbf{P}_{LoS}(\theta)=\frac{1}{1+a\text{exp}(-b[\theta-a])} \tag{1}\]  
to reflect a LoS component on channel modeling, where \(a\) and \(b\) are the environmental parameters such as suburban, urban, dense urban, and high-rise urban; \(\theta\) is an elevation angle between UAV and ground node. The evaluation angle \(\theta\) is defined as \(\theta=arctan(h/r)\), where \(h\) is the UAV altitude and \(r\) is the horizontal distance between UAV and ground node. Note that \(\mathbf{P}_{LoS}(\theta)\) increases as \(\theta\) becomes large. The non-LoS probability is \(\mathbf{P}_{NLoS}(\theta)=1-\mathbf{P}_{LoS}(\theta)\). The pathloss \(PL_{LoS}\) for LoS link and \(PL_{NLoS}\) for non-LoS link can be expressed in the dB scale as [14]  
\[PL_{LoS} =20\text{log}(d)+20\text{log}(f)+20\text{log}\bigg{(}\frac{4\pi} {c}\bigg{)}+\eta_{LoS} \tag{2}\] \[PL_{NLoS} =\underbrace{20\text{log}(d)+20\text{log}(f)+20\text{log}\bigg{(} \frac{4\pi}{c}\bigg{)}}_{\text{PSPL}}+\eta_{NLoS} \tag{3}\]respectively, where FSPL represents the free space pathloss, \(d\) is the distance between UAV and ground node, \(f\) is the system frequency, \(c\) is the speed of light, and \(\eta_{LoS}\) and \(\eta_{NLoS}\) are the excessive pathloss for LoS and non-LoS links, respectively. The resulting average pathloss is  
\[PL=\mathbf{P}_{LoS}(\theta)\times PL_{LoS}+\mathbf{P}_{NLoS}( \theta)\times PL_{NLoS} \tag{4}\]  
and the air-to-ground channel gain \(G\) can be defined as  
\[G=\frac{G_{0}}{PL}, \tag{5}\]  
where \(G_{0}\) is the channel gain when the distance between UAV and ground node is same as the reference distance.  
Based on the probabilistic channel model, the air-to-ground channel has been expressed in a different way to reflect the link characteristics more accurately. Ref. [15] presents the impact of shadowing effects by buildings on non-LoS links along with the LoS probability. Ref. [16] utilizes the two-ray ground reflection model to construct a base model of data transmission from UAV to ground station and to allocate radio resources efficiently. A link budget calculation between UAV and ground station is considered in [17] to determine the feasible frequency band, propagation loss, antenna gain, and so on. Ref. [18] proposes an height-dependent small-scale fading and pathloss exponent model to reflect the combination of LoS and multipath scatters along with a Rician model. A statistical propagation model to predict an air-to-ground pathloss between UAV and terrestrial terminal is proposed in [19], and an air-to-ground propagation channel model for an ultra-wideband (UWB) is introduced in [20]. The air-to-ground channel model varies with environments and system parameters, hence, comprehensive measurements and calculations should be conducted for various UAV scenarios.