### Channel Models  
Path loss channels are considered in most studies to model an air-to-ground (ATG) wireless link, along with a network architecture that depends solely on aerial wireless access elements, such as UAVs. In [22], the deployment of an aerial cloud radio access network (cloud-RAN) is considered to minimize the energy consumption of UAVs. The location, coverage radius, and functional split option of each UAV are jointly optimized, and a path loss channel model is considered. A cloud-RAN in which devices or user terminals (UTs) on the ground are served by UAVs to offload their tasks is considered in [23] with a path loss channel. The aim is to maximize the sum rate of all the UTs by jointly optimizing the UT association, transmit power, and UAV placement. In [24], mobile flying base stations (BSs) mounted on UAVs with a path loss channel are exploited to maximize data throughput by controlling the travel directions of UAVs. Whereas standard stochastic channel models have been widely used to study terrestrial networks, some studies have extended this model to ATG cases, as in [25; 26; 27], to study coverage probability rates when UAVs are used for the purpose of providing cellular connectivity. The advantage of the stochastic channel model is its ability to capture spatial randomness and provide an analytical model that applies, on average, to all cellular networks' realizations [28]. In [29], the coverage probability of UAV-assisted cellular networks is evaluated in urban and rural areas using a stochastic geometry-based model. Study [30] focuses on providing cellular connectivity to offshore areas for search and rescue operations. It considers a simple two-ray channel model to compare the performance of UAVs acting as relays or BSs in terms of data rate and latency. Hybrid network architectures typically use UAVs to complement terrestrial BSs. The authors in [31] considered a path loss channel in studying the use of UAVs to complement terrestrial cloud-RAN in scenarios where a subset of terrestrial BSs breakdown. Interestingly, it is found that using UAVs to compensate for the failed BSs can deteriorate performance in some cases, as compared to using only the remaining working BSs. This is because the presence of UAVs can increase the interference level. With a path loss channel, the feasibility and performance of a multi-tier drone architecture are investigated in [13] in terms of spectral efficiency, optimal drone proportion, and altitude. Load conditions in which the deployment of UAVs can be beneficial are identified. The authors in [32] proposed a drone-cell deployment framework, considering a path loss channel, to alleviate overload conditions caused by flash crowd traffic and boost the capacity of the ground network. The interference caused by proximate drone cells and ground small cells is not considered. A clustering method is employed to identify the required number of drone cells and their locations. Table 1 summarizes existing frameworks in UAV-assisted communication systems. Several channel models have been developed by standards organizations, academia, and research groups to characterize a wireless medium at mmWave and sub-6 GHz bands in different propagation environments. Table 2 summarizes these channel models according to [33]. The 3 GPP specified 5G channel models for frequencies from 0.5 to 100 GHz in various scenarios, such as urban micro (UMi) and urban macro (UMa), with outdoor-to-outdoor